vi characteristics of pn junction diode experiment theory

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V-I Characteristics of p-n-Junction Diode

V-I characteristics of p-n-Junction Diode

Objectives:

• To understand the basic concepts of semiconductors.
• To study p type and n type semiconductor and potential barrier.
• To understand forward and reverse biasing.
• Perform the experiment on bread board and the trainer kit and plot the graph of V-I characteristics of PN junction diode.

Components and equipments required: single strand cable, diode, resistors, bread board, multimeter, connecting wires, CRO, voltage source.

General Instructions: You will plan for Experiment after self study of Theory given below, before entering in the Lab.

PN Junction Diode The effect described in the previous tutorial is achieved without any external voltage being applied to the actual PN junction resulting in the junction being in a state of equilibrium. However, if we were to make electrical connections at the ends of both the N-type and the P-type materials and then connect them to a battery source, an additional energy source now exists to overcome the barrier resulting in free charges being able to cross the depletion region from one side to the other. The behavior of the PN junction with regards to the potential barrier width produces an asymmetrical conducting two terminal device, better known as the Junction Diode.

A diode is one of the simplest semiconductor devices, which has the characteristic of passing current in one direction only. However, unlike a resistor, a diode does not behave linearly with respect to the applied voltage as the diode has an exponential I-V relationship and therefore we cannot described its operation by simply using an equation such as Ohm's law. If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased. By applying a negative voltage (reverse bias) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased. This has the effect of increasing or decreasing the effective resistance of the junction itself allowing or blocking current flow through the diode.

Then the depletion layer widens with an increase in the application of a reverse voltage and narrows with an increase in the application of a forward voltage. This is due to the differences in the electrical properties on the two sides of the PN junction resulting in physical changes taking place. One of the results produces rectification as seen in the PN junction diodes static I-V (current-voltage) characteristics. Rectification is shown by an asymmetrical current flow when the polarity of bias voltage is altered as shown below.

But before we can use the PN junction as a practical device or as a rectifying device we need to firstly bias the junction, ie connect a voltage potential across it. On the voltage axis above, "Reverse Bias" refers to an external voltage potential which increases the potential barrier. An external voltage which decreases the potential barrier is said to act in the "Forward Bias" direction.

There are two operating regions and three possible "biasing" conditions for the standard Junction Diode and these are:

• Reverse Bias - The voltage potential is connected negative, (-ve) to the P-type material and positive, (+ve) to the N-type material across the diode which has the effect of Increasing the PN-junction width.
• Forward Bias - The voltage potential is connected positive, (+ve) to the P-type material and negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the PN-junction width.

Forward Biased Junction Diode When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material. If this external voltage becomes greater than the value of the potential barrier, approx. 0.7 volts for silicon and 0.3 volts for germanium, the potential barriers opposition will be overcome and current will start to flow. This is because the negative voltage pushes or repels electrons towards the junction giving them the energy to cross over and combine with the holes being pushed in the opposite direction towards the junction by the positive voltage. This results in a characteristics curve of zero current flowing up to this voltage point, called the "knee" on the static curves and then a high current flow through the diode with little increase in the external voltage as shown below.

The application of a forward biasing voltage on the junction diode results in the depletion layer becoming very thin and narrow which represents a low impedance path through the junction thereby allowing high currents to flow. The point at which this sudden increase in current takes place is represented on the static I-V characteristics curve above as the "knee" point.

This condition represents the low resistance path through the PN junction allowing very large currents to flow through the diode with only a small increase in bias voltage. The actual potential difference across the junction or diode is kept constant by the action of the depletion layer at approximately 0.3v for germanium and approximately 0.7v for silicon junction diodes. Since the diode can conduct "infinite" current above this knee point as it effectively becomes a short circuit, therefore resistors are used in series with the diode to limit its current flow. Exceeding its maximum forward current specification causes the device to dissipate more power in the form of heat than it was designed for resulting in a very quick failure of the device.

Reverse Biased Junction Diode When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material. The positive voltage applied to the N-type material attracts electrons towards the positive electrode and away from the junction, while the holes in the P-type end are also attracted away from the junction towards the negative electrode. The net result is that the depletion layer grows wider due to a lack of electrons and holes and presents a high impedance path, almost an insulator. The result is that a high potential barrier is created thus preventing current from flowing through the semiconductor material.

Procedure:-

• Make the connections as shown in fig.:
• Switch on the power supply.
• Now vary in small step the forward bias voltage and current readings on multimeter. Draw the graph between current and voltage.
• Make the connection as shown in fig:

Observation:

Forward biasing

Observation Table

Reverse biasing

Do and Don’ts to be strictly observed during experiment:

Do (also go through the General Instructions):

• Before making the connection, identify the components leads, terminal or pins before making the connections.
• Before connecting the power supply to the circuit, measure voltage by voltmeter/multimeter.
• Use sufficiently long connecting wires, rather than joining two or three small ones.
• The circuit should be switched off before changing any connection.
• Avoid loose connections and short circuits on the bread board.
• Do not exceed the voltage while taking the readings.
• Any live terminal shouldn't be touched while supply is on.

Outputs: Submit the graph as per observation table.

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VI Characteristics of a P-N Junction Diode

Semiconductors is a kind of material that has resistivity and conductivity in between metals and insulators. On the basis of purity semiconductors are of two types: Intrinsic Semiconductors is a kind of pure semiconductor without any significant dopant (impurities) species present. An intrinsic semiconductor is also called an undoped semiconductor and Extrinsic Semiconductors is a kind of semiconductor they are doped with an impurity, it is known as an extrinsic semiconductor.

p-n Junction Diode

Junction generally means the area or point that bounds two different parts, similarly in diodes junction is a boundary of two semiconductor materials i.e. the p-type and the n-type, semiconductor.

The p-side in the p-n junction has a positive side of the semiconductor and it has an excess of holes whereas the n-side has an excess of electrons, therefore, it is the negative side. The p-n junction in semiconductors is developed by the method of doping. adding impurities in a semiconductor is known as doping.

Formation of p-n Junction

Let us understand for an example, consider a thin p-type silicon semiconductor sheet. If we add a small amount of pentavalent impurity(having valency five) to this, a part of the p-type Si will get converted to n-type silicon. This sheet will now contain both regions i.e. p and n-type region and a junction is created between two regions. 

• There are two types of processes that follow after the formation of a p-n junction – diffusion and drift. As we know, diffusion is the process that follows the flow of particles from higher concentration to lower concentration, due to difference in the concentration of electrons and holes at the two sides of a junction, the electrons from the n-side diffuse to the p-side and the holes from the p-side diffuse to the n-side. this leads to raise in diffusion current.
• Also, there is an ionized donor is left behind on the n-side, which is an immobile charge this develop when an electron diffuses from the n-side to the p-side. As the result of this process, a layer of positive charge is developed on the n-side of the junction. 
• Similarly, An ionized acceptor is left behind in the p-side when a hole goes from the p-side to the n-side, resulting in the formation of a layer of negative charges in the p-side of the junction. This region of negative (-) and positive charge (+) on either side of the junction is termed the depletion region. 
• An electric field direction from a positive charge towards the negative charge is developed, Due to this positive charge region on either side of the junction, Due to this electric field, the flow of electrons and holes takes place. This is termed the drift motion. generally, the direction of the drift current is opposite to that of the diffusion current.

Forward Bias

The forward bias of p-n  junction

In biasing semiconductor is connected to an external source. when the p-type semiconductor is connected to the positive terminal of the source or battery and negative terminal to the n-type, then this type of junction is said to be forward-biased. In forward bias the direction of built-in electric field near the junction and applied electric field are opposite in direction. this means that the resultant electric field has a magnitude lesser than the built-in electric field. due to this there is less resistivity and therefore depletion region is thinner. In silicon, at the voltage of 0.6 V, the resistance of the depletion region becomes completely negligible.

Reverse Bias

The reverse bias of p-n  junction

In the reverse biasing, the n-type is connected to the positive terminal and the p-type is connected to the negative terminal of the battery . In this case, the applied electric field and the built-in electric field are in the same direction and the resultant of electric field has higher magnitude than the built-in electric field creating a more resistive, therefore depletion region is thicker. if the applied voltage becomes larger, then the depletion region becomes more resistive and thicker.

p-n Junction Formula The potential difference created by the electric field in the p-n junction is given by: E o = V T ln [N d N a / n i 2 ]  where E o  junction voltage at no bias, V T is the thermal voltage at room temperature i.e. 26mv, N d and N a are the concentrations of impurity and n i is intrinsic concentration.

V-I Characteristics of p-n Junction Diode

V-I characteristics of p-n junction diode

• In forward bias condition p-type is connected to positive terminal of battery and the n-type to the negative terminal of the battery, there is a reduction in the potential barrier, in this condition. For germanium diodes, when the voltage is 0.3 V, and for silicone diodes, when the voltage is 0.7 V the potential barriers decrease and there is a flow of current.
• When the diode is in forward bias , as the voltage applied to the diode is overcoming the potential barrier, the current increases slowly and the curve obtained is non-linear. Once the potential barrier is crossed by the diode, the diode behaves normally and the curve rises sharply as further external voltage increases and the curve obtained is linear.
• When the PN junction diode is under reverse bias , this results in an increase in the potential barrier and resistance also increases. Minority carriers are present in the junction which creates reverse saturation current flows in the beginning.
• If the applied voltage increases rapidly, there is increased kinetic energy due to minority charge carriers which affect the majority charges. In this stage the diode breaks down. or the voltage is called breakdown voltage, This may also destroy the diode.

Sample Questions

Question 1: When silicon is doped with indium it leads to which type of semiconductor?

As we know, Valency of Indium is 3 therefore it is Trivalent in nature, when it is doped in Silicon it has majority of holes, that’s why it is of p-type semiconductor.

Question 2: A transistor has a current gain of 30 Ampere. If the collector resistance is 6 kΩ, the input resistance is 1 kΩ, calculate its voltage gain.

Given, R in =1 kΩ  and R out = 6k Ω   ∴ R gain =  R out /R in   =  6/1 = 6   Voltage gain = current gain × Resistance gain                        = 30 × 6 =180

Question 3: Write characteristics of holes.

Following are the characteristics of holes: A hole is equivalent to a positive electric charge. The mobility of a hole as compare to that of an electron is less.

Question 4: Name the kind of biasing which leads the following result:

a) Increase in resistance,

b) Decrease in resistance and 

c) Increase in width of the depletion region.

a) In reverse bias resistance increases. b) In forward bias resistance decrease. c) In reverse bias there is increase in the width of depletion region take place.

Question 5: What is the ratio of electrons and holes in the intrinsic semiconductor?

Number of electrons = n e Number of holes = n h In intrinsic semiconductor, n e = n h n e /n h = 1 

Question 6: Define the term breakdown voltage of p-n junction.

In reverse bias condition, when the applied voltage increases gradually at a certain point there is increase in reverse current noticed, this is junction breakdown, corresponding applied voltage is known as breakdown voltage of p-n junction diode.

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PN Junction Diode Characteristics – Explained in Detail with Graphs

In this article, we learn about PN junction diode characteristics in detail – like how to bias a PN junction (Forward & Reverse bias methods), behavior of PN junction during forward & reverse bias setups, how to plot the VI characteristics, what is reverse breakdown and many other essential concepts regarding a PN junction diode. So let’s begin.

In chapter 1 – Understanding the PN junction , we have seen how a PN junction is formed from a p-type and n-type semiconductor. We have also learned about diffusion current, depletion region, drift current and barrier potential. If you find these terms foreign, just read the chapter about “ understanding the pn junction ” once more. Lets just make some questions. What is the use of a PN junction? Why have scientists created a pn junction device? What kind of problem it solves ? Learning anything is really fun when we question it. So these are our questions. Why there exists a pn junction in this world! ?;)

To get an answer to all these questions, lets first try to understand the characteristics of a PN junction. We know a pn junction has a “barrier potential”. Only if we overcome this “barrier potential” by applying an external voltage to the pn junction, we would be able to make it conducting. This simply means, current will pass through the pn junction only if we apply an external voltage higher than the “barrier potential” of pn junction. In chapter 1, we have seen that  net current inside a pn junction is zero. Inorder to understand the behavior of a pn junction we need to make it conducting by applying an external voltage over a range (say from 0 volts 5 or 10 volts ), and then we study how the current passed through the pn junction varies with increasing voltage levels. To apply an external voltage, we usually connect 2 metallic contacts at the two ends of the pn junction ( known as terminals ); one on the p-side and other on the n-side. A PN junction with two metallic contacts is known as a pn junction diode or a semiconductor diode. 

Note:- I have written an interesting article which tells the story behind invention & discovery of PN Junction diode. If you like to read the story, follow here:- Story behind Invention & Discovery of PN Junction

PN junction diode is symbolically represented as shown in picture. The direction of arrow is the direction of conventional current flow (under forward bias). Now lets try applying an external voltage to the pn junction diode. The process of applying an external voltage is called as “biasing” . There are two ways in which we can bias a pn junction diode.

1) Forward bias and 2) Reverse bias  

The basic difference between a forward bias and reverse bias is in the direction of applying external voltage. The direction of external voltage applied in reverse bias is opposite to that of  external voltage applied in forward bias.

Forward biasing a PN Junction diode

Forward biasing a pn junction diode is very simple. You just need to take a battery whose values can be varied from (o to V volts), connect its positive terminal to the p-side of pn junction diode and then connect the negative terminal of battery to the n-side of the pn junction diode. If you have done upto this, the forward bias circuit of pn junction diode is complete. Now all we need to do is understand how the pn junction diode behaves when we increase the voltage levels from 0 to say 10 volts or 100 volts. We have learned that if we apply an external voltage higher than the barrier potential of pn junction diode, it will start conducting, which means it will start passing current through it. So how we are going to study the behavior of pn junction diode under forward biased condition? Lets get a voltmeter and ammeter and connect it to the forward biased circuit of pn junction diode.A simple circuit diagram is shown below, which has a pn junction diode, a battery (in picture it is not shown as variable. keep in mind we are talking about a variable power source), an ammeter (in milli ampere range) and a voltmeter.

Note:- Assume that the pn junction diode is made from Silicon. The reason is difference in barrier potential for a diode made from Germanium and Silicon. (For a silicon diode – barrier potential is 0.7 volts where as for a Germanium diode barrier potential is low ~ 0.3 volts)

How to plot the characteristics of a pn junction ?

What we are going to do is, vary the voltage across diode by adjusting the battery. We start from o volts, then slowly move 0.1 volts, 0.2 volts and so on till 10 volts. Lets just note the readings  of voltmeter and ammeter each time we adjust the battery (in steps of 0.1 volts). Finally after taking the readings, just plot a graph with voltmeter readings on X-axis and corresponding Ammeter readings on Y axis. Join all the dots in graph paper and you will see a graphical representation as shown below. Now this is what we call “characteristics of a pn junction diode” or the “behavior of diode under forward bias” 

How to analyse the characteristics of a pn junction diode ?

Its from the “characteristics graph ” we have just drawn, we are going to make conclusions about the behavior of pn junction diode. The first thing that we shall be interested in is about “barrier potential” . We talked a lot about barrier potential but did we ever mention its value ? From the graph, we observe that the diode does not conduct at all in the initial stages. From 0 volts to 0.7 volts, we are seeing the ammeter reading as zero! This means the diode has not started conducting current through it. From 0.7 volts and up, the diode start conducting and the current through diode increases linearly with increase in voltage of battery. From this data what you can infer ? The barrier potential of silicon diode is 0.7 volts 😉  What else ? The diode starts conducting at 0.7 volts and current through the diode increases linearly with increase in voltage. So that’s the forward bias characteristics of a pn junction diode. It conducts current linearly with increase in voltage applied across the 2 terminals (provided the applied voltage crosses barrier potential).

What happens inside the pn junction diode when we apply forward bias ?

We have seen the characteristics of pn junction diode through its graph. What really happens inside the diode during the forward bias ? We know a diode has a depletion region with a fixed barrier potential. This depletion region has a predefined width, say W . This width will vary for a Silicon diode and a Germanium diode. The width highly depends on the type of semiconductor used to make pn junction, the level of doping etc. When we apply voltage to the terminals of diode, the width of depletion region slowly starts decreasing. The reason for this is, in forward bias we apply voltage in a direction opposite to that of barrier potential. We know the p-side of diode is connected to positive terminal and n-side of diode is connected to negative terminal of battery. So the electrons in n-side gets pushed towards the junction (by force of repulsion) and the holes in p-side gets pushed towards the junction. As the applied voltage increases from 0 volts to 0.7 volts, the depletion region width reduces from ‘ W’ to zero. This means depletion region vanishes at 0.7 volts of applied voltage.  This results in increased diffusion of electrons from n-side to p-side region and the increased diffusion of holes from p-side to n-side region. In other words, “ minority carrier ” injection happens on both p-side (in a normal diode (without bias) electrons are a minority on p-side) and n-side (holes are a minority on n-side) of the diode.

How current flow takes place in a pn junction diode ?

This is another interesting factor, to explain. As the voltage level increases, the electrons from n-side gets pushed towards the p-side junction. Similarly holes from p-side gets pushed towards the n-side junction. Now there arises a concentration gradient between the number of electrons at the p-side junction region and the number of electrons at the region towards the p-side terminal. A similar concentration gradient develops between the number of holes at the n-side junction region and the number of holes at region near the n-side terminal. This results in movement of charge carriers (electrons and holes) from region of higher concentration to region of lower concentration. This movement of charge carriers inside pn junction gives rise to current through the circuit.

Reverse biasing a PN junction diode

Why should we reverse bias a pn diode ? The reason is, we want to learn its characteristics under different circumstances. By reverse biasing, we mean, applying an external voltage which is opposite in direction to forward bias. So here we connect positive terminal of battery to n-side of the diode and negative terminal of the battery to p-side of the diode. This completes the reverse bias circuit for pn junction diode. Now to study its characteristics (change in current with applied voltage), we need to repeat all those steps again. Connect voltmeter, ammeter, vary the battery voltage, note the readings etc etc. Finally we will get a graph as shown.

Analysing the revere bias characteristics

Here the interesting thing to note is that, diode does not conduct with change in applied voltage. The current remains constant at a negligibly small value (in the range of micro amps) for a long range of change in applied voltage. When the voltage is raised above a particular point, say 80 volts, the current suddenly shoots (increases suddenly). This is called as “ reverse current ” and this particular value of applied voltage, where reverse current through diode increases suddenly is known as “ break down voltage “.

What happens inside the diode ?

We connected p-side of diode to negative terminal of battery and n-side of diode to positive terminal of battery. So one thing is clear, we are applying external voltage in the same direction of barrier potential. If applied external voltage is V and barrier potential is Vx , then total voltage across the pn junction will be V+Vx . The electrons at n-side will get pulled from junction region to the terminal region of n-side and similarly the holes at p-side junction will get pulled towards the terminal region of p-side.  This results in increasing the depletion region width from its initial length, say ‘W’ to some ‘W+x’. As width of depletion region increases, it results in increasing the electric field strength.

How reverse saturation current occurs and why it exists ?

The reverse saturation current is the negligibly small current (in the range of micro amperes) shown in graph, from 0 volts to break down voltage. It remains almost constant (negligible increase do exist) in the range of 0 volts to reverse breakdown voltage. How it occurs ? We know, as electrons and holes are pulled away from junction, they dont get diffused each other across the junction. So the net “ diffusion current ” is zero! What remains is the drift due to electric field. This reverse saturation current is the result of drifting of charge carriers from the junction region to terminal region. This drift is caused by the electric field generated by depletion region.

What happens at reverse breakdown ?

At breakdown voltage, the current through diode shoots rapidly. Even for a small change in applied voltage, there is a high increase in net current through the diode. For each pn junction diode, there will be a maximum net current that it can withstand. If the reverse current exceeds this maximum rating, the diode will get damaged.

Conclusion about PN junction characteristics

To conclude about pn junction characteristics, we need to get an answer to the first question we have raised – What is the use of pn junction? From the analysis of both forward bias and reverse bias, we can arrive at one fact – a pn junction diode conducts current only in one direction – i.e during forward bias. During forward bias, the diode conducts current with increase in voltage. During reverse bias, the diode does not conduct with increase in voltage (break down usually results in damage of diode). Where can we put this characteristics of diode into use ? Hope you got the answer! Its in conversion of alternating current to direct current (AC to DC). So the practical application of pn junction diode is rectification!

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Hey! That’s so helpful

Thickness of diplition layer depend on which factor?

DESC: Diode forward biased 24VDC QTY: 20pcs

DESC: Diode Reverse biased 24VDC QTY: 20pcs

Faith N. Dolorito Procurement Specialist MANILA OVERSEAS INC. TEL:6328004227 FAX:6328004172

thank you so very much…. I am clearly understood to read it……. ……..

As width of depletion region increases, it results in increasing the electric field strength.Why?

What is zener effect and avalanche effect.?

Utmost/extremly thanks ….. For this crystal clear explanation….. I really got something from it…. But sir what is Zener effect.and avalenche effect.?

Why internal electric field generate after diffusion process in pn junction

i hve a question. why the arrow in pn junction thicker????

explain the working of PN junction diode in forward and reverse biasing configuration please ?

why the battery in reverse bias is greater than in forward bias

I think I missed something. You say that the PN junction only starts to conduct current after the voltage aplied on the diode (Vd) reaches 0.7V, the barrier potential as you call it, but all the graphics and equations shows us that there is current through the diode for values of Vd smaller than 0,7V. I mean, even considering the current for Vd near zero negligible, with Vd~0.60V there is current.

As I see it, we just consider 0.7V as a practival value for a conducting diode, where any variation of the current will cause a small variation on Vd, keeping it around the same 0.7V. It would me consistent with the diode current equation Id=Is(exp(Vd/nVt)-1), cause in 0.7V for a regular diode, de slope in the curve is too large to see any change in Vd as the current varies.

I don’t know if I made myself clear, but thats a point that is not really clear in many books about semiconductors physics and it’s annoying me. If you could clarify that for me I would be glad.

Why the forward voltage values are almost constant for source voltage from 5V to 1V during forward-biased?

what is the difference between the connections of forwardbias and reverse bias in pn junction…?

in forward biasis -VE terminal of battery is connected to pentavelent group N and +ve is connected to trivalent group P but in reverse biasis the connection is opposite …

can I get a pdf of this chapter??

very clear presantation if you were around i would offer you a cup of tea or coffee good work

why is the voltmeter connected across the ammeter and reverse biased diode..?

Can a diode work on ac voltage or not

@Anuj – A diode is basically a PN Junction. It is used to convert AC to DC.

diode worked on ac voltage but it will give output is DC why because ac has two half cycles in that case,it will conduct only positive half cycle….do not allow -ve cycles…

it’s working on ac voltage

The junction information is clearly understand so nice of it thanx

for eachelectron hole combination that take place near the junction a covalent bond breaks in the p section near the +ve pole of the battery how it is formed?

it is so helpful and it clears all the confusion…….plz answer meone question thatis why in CB mode the emitter current increases with increase of V(CB)

this is a exellent article……….sir plz letme know about base width modulation

It is very short notes It is very useful i am very happy after read that notes thank u very much

thanks 4 the good explanation. will you please show the one connected image source circuit of both forward and reverse biased a pn-junction

Please see Fig.10

wow it is very much helpful to me. Thanks the author

yes, its very great answer that i want. Thanks.

I really appreciate. Got a clearer explanation that i did in class… Kudos. Thanks Admin

a great work with full clearification. thanx !

Really interesting and clear clarification of every aspect of a junction diode characteristics.Very nice

Brilliant! Very helpful article. It’s clearly explaind and easy to understand. Bravo for the person who has put so much work to make it!!

Thanq So Much 🙂 this helped me a lot 🙂 Is there explanation for Transistor as a Switch and Amplifier?

explanation is little bit invalid

thaks very much for the good explanation.can you describe the current voltage characteristics of a photodiode when light is incident on it?

veryyyy goood explanation, i got it perfectly, please tell me about bridge wave rectifier, we connect 4 diodes in bridge but when the d1 and d2 are forward biased then haw the d3 and d4 are reversr biased

@Nayan – Read this article:- https://www.circuitstoday.com/full-wave-bridge-rectifier

It will help you understand bridge rectifier perfectly.

when we talk about reverse bias ,thn the width of depletion layer increases thn after more reverse voltage(greater than reverse breakdown voltage) how current flow through dide?

At break down, what happens really is that the diode gets damaged. It loses its junction & characteristics associated with the junction. The “diode” almost behaves like a shorted wire & hence current flows through it easily. Theoretically, internal resistance of a diode at breakdown is zero. But in practice, there exists a small internal resistance and hence the current increases with a deviation factor (and not a perpendicular graph).

Hope this helps!

Really helpfull , Thanks sir..

good explanation with neat a diagrams

its very simple to understand ……i like to read a lot in webpage…thank u to author who wrote this.

well explained. really enyoyed.

sir please add the curve charcterstic found when we use ge semiconductor as pn junction diode due to the this experiment

it was very useful and was written in a readble mannar

I like this and I enjoy

its a rely nuc explanation abt pn junctoin m a net qualified scientist

Thank you Pintu 🙂 It was very nice words 🙂

the difference between depletion barrier’s height and width . i mean why they are different and what they indicate?

If depletion region’s width indicates the area covered by defused electrons/holes then read further.

In forward bias condition external electric field ( produced by battery) will be opposite to the internal electric field ( produced depletion barrier ). in this case the external electric field will cancel the internal electric field and more electron will flow from n type to p type material(assumed external voltage is greater than depletion barrier) which increases the depletion region but in real, in forward bias condition the depletion region’s width decreases. And in reverse bias condition the depletion region increases instead of decreasing. (I am familiar with the increase/decrease of potential of depletion barrier and agree with the books)

I am very confused with this question. so please help me. Thank you

What really matters is the “barrier potential” of a diode. In a Silicon diode, the “barrier width” is higher than a Germanium diode. So “barrier potential” of a Silicon diode is higher than Germanium diode. I hope you understood.

cool great approach. hoping that 2 give more information about electronics

Please help me out.. In forward bias if battery voltage is 2v , drop across si diode cant be more than 1v i.e. Vd<1v… So now my qusetion is where this remaining 1v of battery is if no resistor is in series with diode?

In that case, 1 volt will be dropped across the wires with the help of a very large current.

Awesome explanation.thank you

Crystal Clear approach, awesome!!

it’s very useful thank you

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Keep adding more and more info….

owsam… PERFECT …!!

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oh thank u..i am very confused to read my text book but now every thing is clear….thank you very much ..

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VI Characteristics of a Diode            

Structure of p-n junction diode.

The diode is a device formed from a junction of n-type and p-type semiconductor material. The lead connected to the p-type material is called the anode and the lead connected to the n-type material is the cathode. In general, the cathode of a diode is marked by a solid line on the diode.

Function of a P-N junction diode in Forward Bias

The positive terminal of battery is connected to the P side(anode) and the negative terminal of battery is connected to the N side(cathode) of a diode, the holes in the p-type region and the electrons in the n-type region are pushed toward the junction and start to neutralize the depletion zone, reducing its width. The positive potential applied to the p-type material repels the holes, while the negative potential applied to the n-type material repels the electrons. The change in potential between the p side and the n side decreases or switches sign. With increasing forward-bias voltage, the depletion zone eventually becomes thin enough that the zone's electric field cannot counteract charge carrier motion across the p–n junction, which as a consequence reduces electrical resistance. The electrons that cross the p–n junction into the p-type material (or holes that cross into the n-type material) will diffuse into the nearby neutral region. The amount of minority diffusion in the near-neutral zones determines the amount of current that may flow through the diode.

Function of a P-N junction diode in Reverse Bias

The positive terminal of battery is connected to the N side(cathode) and the negative terminal of battery is connected to the P side(anode) of a diode. Therefore, very little current will flow until the diode breaks down.

The positive terminal of battery is connected to the N side(cathode) and the negative terminal of battery is connected to the P side(anode) of a diode, the 'holes' in the p-type material are pulled away from the junction, leaving behind charged ions and causing the width of the depletion region to increase. Likewise, because the n-type region is connected to the positive terminal, the electrons will also be pulled away from the junction, with similar effect. This increases the voltage barrier causing a high resistance to the flow of charge carriers, thus allowing minimal electric current to cross the p–n junction. The increase in resistance of the p–n junction results in the junction behaving as an insulator. The strength of the depletion zone electric field increases as the reverse-bias voltage increases. Once the electric field intensity increases beyond a critical level, the p–n junction depletion zone breaks down and current begins to flow, usually by either the Zener or the avalanche breakdown processes. Both of these breakdown processes are non-destructive and are reversible, as long as the amount of current flowing does not reach levels that cause the semiconductor material to overheat and cause thermal damage.

Forward and reverse biased characteristics of a Silicon diode

In forward biasing , the positive terminal of battery is connected to the P side and the negative terminal of battery is connected to the N side of the diode. Diode will conduct in forward biasing because the forward biasing will decrease the depletion region width and overcome the barrier potential. In order to conduct, the forward biasing voltage should be greater than the barrier potential. During forward biasing the diode acts like a closed switch with a potential drop of nearly 0.6 V across it for a silicon diode. The forward and reverse bias characteristics of a silicon diode. From the graph, you may notice that the diode starts conducting when the forward bias voltage exceeds around 0.6 volts (for Si diode). This voltage is called cut-in voltage.

In reverse biasing , the positive terminal of battery is connected to the N side and the negative terminal of battery is connected to the P side of a diode. In reverse biasing, the diode does not conduct electricity, since reverse biasing leads to an increase in the depletion region width; hence current carrier charges find it more difficult to overcome the barrier potential. The diode will act like an open switch and there is no current flow.

Forward and reverse biased characteristics of a Germanium diode

In forward biasing , the positive terminal of battery is connected to the P side and the negative terminal of battery is connected to the N side of the diode. Diode will conduct in forward biasing because the forward biasing will decrease the depletion region width and overcome the barrier potential. In order to conduct, the forward biasing voltage should be greater than the barrier potential. During forward biasing the diode acts like a closed switch with a potential drop of nearly 0.3 V across it for a germanium diode. The forward and reverse bias characteristics of a germanium diode. From the graph, you may notice that the diode starts conducting when the forward bias voltage exceeds around 0.3 volts (for Ge diode). This voltage is called cut-in voltage.

Diode Equation

In the forward-biased and reversed-biased regions, the current I f , and the voltage V f , of a semiconductor diode are related by the diode equation:

$$I_f=I_s(exp^{\frac{V_f}{nV_T}}−1)$$

where, I s is reverse saturation current or leakage current, I f is current through the diode(forward current), V f is potential difference across the diode terminals(forward voltage) V T is thermal voltage, given by

$$V_T=\frac{kT}{q}$$

and k is Boltzmann’s constant = 1.38x10−23 J /°Kelvin, q is the electronic charge = 1.6x10−19 joules/volt(Coulombs), T is the absolute temperature in °Kelvin(°K = 273 + temperature in °C), At room temperature (25 °C), the thermal voltage is about 25.7 mV, n is an empirical constant between 0.5 and 2

The empirical constant, n, is a number that can vary according to the voltage and current levels. It depends on electron drift, diffusion, and carrier recombination in the depletion region. Among the quantities affecting the value of n are the diode manufacture, levels of doping and purity of materials.

If n=1, the value of $$\frac{kT}{q}$$ is 26 mV at 25°C. When n=2, the value of $$\frac{kT}{q}$$ becomes 52 mV. For germanium diodes, n is usually considered to be close to 1. For silicon diodes, n is in the range of 1.3 to 1.6.

• Ideal Diode Model:Diode is a simple switch that is either closed (conducting) or open (non conducting). Specifically, the diode is a short circuit, like a closed switch, when voltage is applied in the forward direction, and an open circuit, like an open switch, when the voltage is applied in the reverse direction.
• Offset Voltage Model:The offset voltage model adds the barrier potential to the ideal switch model. When the diode is forward biased it is equivalent to a closed switch in series with a small equivalent voltage source equal to the barrier potential (0.6 V for Silicon, 0.2 for germanium) with the positive side towards the anode. When the diode is reverse biased, it is equivalent to an open switch just as in the ideal model.
• Complete diode Model:It is the most accurate of the diode models. The Complete diode model of a diode consists of the barrier potential, the small forward dynamic resistance and the ideal diode. The resistor approximates the semiconductor resistance under forward bias. This diode model most accurately represents the true operating characteristics of the real diode.
• When a diode is reverse biased a leakage current flows through the device. This current can be effectively ignored as long as the reverse breakdown voltage of the diode is not exceeded. At potentials greater than the reverse breakdown voltage, charge is pulled through the p-n junction by the strong electric fields in the device and large reverse current flows. This usually destroys the device. There are special diodes that are designed to operate in breakdown. Such diodes are called zener diodes and used as voltage regulators.

When is each Model used ?

Ideal Diode Model: This is primarily used in troubleshooting. Is the diode working or not. The greatest utility of the ideal diode model is in determining which diodes are on and which are off in a multi-diode circuit.

Offset Voltage Model: This is used when a more accurate determination of load current or voltage is required.

Complete Diode Model: This is use during the actual design of circuits using diodes.

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PN Junction Diode

A PN-junction diode is formed when a p-type semiconductor is fused to an n-type semiconductor creating a potential barrier voltage across the diode junction

The PN junction diode consists of a p-region and n-region separated by a depletion region where charge is stored. The effect described in the previous tutorial is achieved without any external voltage being applied to the actual PN junction resulting in the junction being in a state of equilibrium.

However, if we were to make electrical connections at the ends of both the N-type and the P-type materials and then connect them to a battery source, an additional energy source now exists to overcome the potential barrier.

The effect of adding this additional energy source results in the free electrons being able to cross the depletion region from one side to the other. The behaviour of the PN junction with regards to the potential barrier’s width produces an asymmetrical conducting two terminal device, better known as the PN Junction Diode .

A PN Junction Diode is one of the simplest semiconductor devices around, and which has the electrical characteristic of passing current through itself in one direction only. However, unlike a resistor, a diode does not behave linearly with respect to the applied voltage. Instead it has an exponential current-voltage ( I-V ) relationship and therefore we can not described its operation by simply using an equation such as Ohm’s law.

If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased.

By applying a negative voltage (reverse bias) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased. This has the effect of increasing or decreasing the effective resistance of the junction itself allowing or blocking the flow of current through the diodes pn-junction.

Then the depletion layer widens with an increase in the application of a reverse voltage and narrows with an increase in the application of a forward voltage. This is due to the differences in the electrical properties on the two sides of the PN junction resulting in physical changes taking place. One of the results produces rectification as seen in the PN junction diodes static I-V (current-voltage) characteristics. Rectification is shown by an asymmetrical current flow when the polarity of bias voltage is altered as shown below.

Junction Diode Symbol and Static I-V Characteristics

But before we can use the PN junction as a practical device or as a rectifying device we need to firstly bias the junction, that is connect a voltage potential across it. On the voltage axis above, “Reverse Bias” refers to an external voltage potential which increases the potential barrier. An external voltage which decreases the potential barrier is said to act in the “Forward Bias” direction.

There are two operating regions and three possible “biasing” conditions for the standard Junction Diode and these are:

• 1. Zero Bias – No external voltage potential is applied to the PN junction diode.
• 2. Reverse Bias – The voltage potential is connected negative, (-ve) to the P-type material and positive, (+ve) to the N-type material across the diode which has the effect of Increasing the PN junction diode’s width.
• 3. Forward Bias – The voltage potential is connected positive, (+ve) to the P-type material and negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the PN junction diodes width.

Zero Biased Junction Diode

When a diode is connected in a Zero Bias condition, no external potential energy is applied to the PN junction. However if the diodes terminals are shorted together, a few holes (majority carriers) in the P-type material with enough energy to overcome the potential barrier will move across the junction against this barrier potential. This is known as the “ Forward Current ” and is referenced as I F

Likewise, holes generated in the N-type material (minority carriers), find this situation favourable and move across the junction in the opposite direction. This is known as the “ Reverse Current ” and is referenced as I R . This transfer of electrons and holes back and forth across the PN junction is known as diffusion, as shown below.

Zero Biased PN Junction Diode

The potential barrier that now exists discourages the diffusion of any more majority carriers across the junction. However, the potential barrier helps minority carriers (few free electrons in the P-region and few holes in the N-region) to drift across the junction.

Then an “Equilibrium” or balance will be established when the majority carriers are equal and both moving in opposite directions, so that the net result is zero current flowing in the circuit. When this occurs the junction is said to be in a state of “ Dynamic Equilibrium “.

The minority carriers are constantly generated due to thermal energy so this state of equilibrium can be broken by raising the temperature of the PN junction causing an increase in the generation of minority carriers, thereby resulting in an increase in leakage current but an electric current cannot flow since no circuit has been connected to the PN junction.

Reverse Biased PN Junction Diode

When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material.

The positive voltage applied to the N-type material attracts electrons towards the positive electrode and away from the junction, while the holes in the P-type end are also attracted away from the junction towards the negative electrode.

The net result is that the depletion layer grows wider due to a lack of electrons and holes and presents a high impedance path, almost an insulator and a high potential barrier is created across the junction thus preventing current from flowing through the semiconductor material.

Increase in the Depletion Layer due to Reverse Bias

This condition represents a high resistance value to the PN junction and practically zero current flows through the junction diode with an increase in bias voltage. However, a very small reverse leakage current does flow through the junction which can normally be measured in micro-amperes, ( μA ).

One final point, if the reverse bias voltage Vr applied to the diode is increased to a sufficiently high enough value, it will cause the diode’s PN junction to overheat and fail due to the avalanche effect around the junction. This may cause the diode to become shorted and will result in the flow of maximum circuit current, and this shown as a step downward slope in the reverse static characteristics curve below.

Reverse Characteristics Curve for a Junction Diode

Sometimes this avalanche effect has practical applications in voltage stabilising circuits where a series limiting resistor is used with the diode to limit this reverse breakdown current to a preset maximum value thereby producing a fixed voltage output across the diode. These types of diodes are commonly known as Zener Diodes and are discussed in a later tutorial.

Forward Biased PN Junction Diode

When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material. If this external voltage becomes greater than the value of the potential barrier, approx. 0.7 volts for silicon and 0.3 volts for germanium, the potential barriers opposition will be overcome and current will start to flow.

This is because the negative voltage pushes or repels electrons towards the junction giving them the energy to cross over and combine with the holes being pushed in the opposite direction towards the junction by the positive voltage. This results in a characteristics curve of zero current flowing up to this voltage point, called the “knee” on the static curves and then a high current flow through the diode with little increase in the external voltage as shown below.

Forward Characteristics Curve for a Junction Diode

The application of a forward biasing voltage on the junction diode results in the depletion layer becoming very thin and narrow which represents a low impedance path through the junction thereby allowing high currents to flow. The point at which this sudden increase in current takes place is represented on the static I-V characteristics curve above as the “knee” point.

Reduction in the Depletion Layer due to Forward Bias

This condition represents the low resistance path through the PN junction allowing very large currents to flow through the diode with only a small increase in bias voltage. The actual potential difference across the junction or diode is kept constant by the action of the depletion layer at approximately 0.3v for germanium and approximately 0.7v for silicon junction diodes.

Since the diode can conduct “infinite” current above this knee point as it effectively becomes a short circuit, therefore resistors are used in series with the diode to limit its current flow. Exceeding its maximum forward current specification causes the device to dissipate more power in the form of heat than it was designed for resulting in a very quick failure of the device.

Tutorial Summary

The PN junction region of a Junction Diode has the following important characteristics:

• Semiconductors contain two types of mobile charge carriers, “Holes” and “Electrons”.
• The holes are positively charged while the electrons negatively charged.
• A semiconductor may be doped with donor impurities such as Antimony (N-type doping), so that it contains mobile charges which are primarily electrons.
• A semiconductor may be doped with acceptor impurities such as Boron (P-type doping), so that it contains mobile charges which are mainly holes.
• The junction region itself has no charge carriers and is known as the depletion region.
• The junction (depletion) region has a physical thickness that varies with the applied voltage.
• When a diode is Zero Biased no external energy source is applied and a natural Potential Barrier is developed across a depletion layer which is approximately 0.5 to 0.7v for silicon diodes and approximately 0.3 of a volt for germanium diodes.
• When a junction diode is Forward Biased the thickness of the depletion region reduces and the diode acts like a short circuit allowing full circuit current to flow.
• When a junction diode is Reverse Biased the thickness of the depletion region increases and the diode acts like an open circuit blocking any current flow, (only a very small leakage current will flow).

We have also seen above that the diode is two terminal non-linear device whose I-V characteristic are polarity dependent as depending upon the polarity of the applied voltage, V D the diode is either Forward Biased , V D  > 0 or Reverse Biased , V D  < 0 . Either way we can model these current-voltage characteristics for both an ideal diode and for a real silicon diode as shown:

Ideal and Real Characteristics

In the next tutorial about diodes, we will look at the small signal diode sometimes called a switching diode which is used in general electronic circuits. As its name implies, the signal diode is designed for low-voltage or high frequency signal applications such as in radio or digital switching circuits.

Signal diodes, such as the 1N4148 only pass very small electrical currents as opposed to the high-current mains rectification diodes in which silicon diodes are usually used. Also in the next tutorial we will examine the Signal Diode static current-voltage characteristics curve and parameters.

Read more Tutorials inDiodes

• 1. Semiconductor Basics
• 2. PN Junction Theory
• 3. PN Junction Diode
• 4. The Signal Diode
• 5. Power Diodes and Rectifiers
• 6. Full Wave Rectifier
• 7. The Zener Diode
• 8. The Light Emitting Diode
• 9. Bypass Diodes in Solar Panels
• 10. Diode Clipping Circuits
• 11. The Schottky Diode

406 Comments

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Much helpfull

Basic Electronic Devices answer

Good Notes👍👍

Nice and wonderful explained

Send me as a PDF

Understandable

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I love the book and the information.Thanks

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It’s very interesting and want to get more information

Notes are okay

P n junction diode

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