PN Junction in Semiconductor Diodes

(Last Updated On: March 28, 2022)

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PN Junction in Semiconductor Diodes- In this article, we will discuss Junction in semiconductors in very detail.


PN Junction in Semiconductor Diodes

If a piece of P-type semiconductor is connected/ jointed with an N-type semiconductor in a fitting manner, the point of connection of both the pieces is known as a junction and the entire piece as a PN junction. In other words, it is possible to construct such a piece of semiconductor, the first half of which has P-type impurity while the second half of which has N-type impurity. The object, which is prepared by means of combining P-type and N-type semiconductors, is called PN junction(As shown in figure 2.1). The place where P and N types of junction congregate is called junction.

PN junction has the following advantages as compared to a vacuum tube:

  1. Smaller in size
  2. Occupies small place
  3. Less heavy
  4. Extremely trustworthy
  5. Do not require to be heated or provide heat
  6. Its lifespan and efficiency is greater
  7. Internal resistance is low
  8. Can be easily installed or removed
  9. Simple in structure and strong
  10. Easily available and cheap
PN Junction
Figure 2.1

Remember, that PN or semiconductor junction diodes are prepared through a PN junction. PN junction is not accomplished by combining two pieces or welding it, rather an exacting fabrication method is used for its formation, which is called alloying.



In order to understand the operation of diodes, transistors, and other solid-state devices (anti grand circuits, etc.), it is very important to understand the PN junction. That’s the operation of these electronic devices fundamentally depends on the PN junction. The following two exceptional conditions happen during the formation of the PN junction.

  1. A thin depletion layer builds up around both sides of the junction, which is called the depletion region, space charge region, or transition region. This name has been given to it because free charge carriers are depleted (finish) at this spot. The thickness of this region or layer is about 1 micrometer (1 µm) or 10 -11
  2. A barrier potential or junction potential pops up parallel to the junction, the value of which for germanium is 0.3 volts and silicon 0.7 volt.

The details of above two points is as below:


Formation of PN Junction Depletion Layer

The region around the PN junction, which consists of ions, is called the depletion region or space region. Or the region around the PN region, where positive and negative ions are formed, is called the depletion region or depletion zone.

Suppose that a PN junction is building, at this point holes still exist in the P region whereas electrons are at hand in the N region. However, the number of holes in the P region (where they exist as minority carriers) is greater than the N region. Exactly, in the same way, the number of electrons in the N region (where they are present as minority carriers) is greater than the P region. As soon as the PN junction is being made, some holes of the P region and some electrons of N region, diffuse in each other and disappear as a result of recombination. The process of diffusion of holes and electrons near the junction has been explained through figure 2.2 (a).

PN Junction
Figure 2.2

Holes start moving from P to N region and electrons from N to P region (i.e. holes and electrons move in opposite directions). Remember, that the flow of charge from a high-density region to a low-density region is called diffusion. A narrow region builds upon junction due to the recombination of free and mobile holes and electrons, which is called the depletion layer. This region has been given this name because free and mobile carriers e.g. electrons and holes are depleted of or devoid after recombination. In other words, the region which consists of both negative and positive ions is called the depletion region, because it is free of mobile and free charge carriers.  (In case an electron emits from a neutral atom, a positive charge comes on it and it is called a positive ion). On the contrary, if an electron is entered into a neutral atom, a negative charge appears on it, it is called a negative ion) However, this region consists of fixed and immovable positive and negative ions.


During the operation, it seems that entire holes of the P side will diffuse in the N side and whole electrons of N side will diffuse on the P side, but this does not happen due to the formation of conflicting ions on both sides of the junction. When a free electron enters the P region, it becomes a minority carrier. As there are so many holes around this minority carrier, therefore, the lifetime (average quantity of time required to produce free electrons and holes along with their recombination, is called lifetime) of this minority carrier is very small. And soon, it drops into hole and when such a situation arises, holes disappear and free electrons become valence electrons.

As free electrons cross junction and recombine with holes, negative ions are produced on the left side of the junction, while on the right side, positive ions are left behind (because a positive charge produces, wherefrom electron emits) As is clear from figure 2.2 (a). Therefore, the region consisting of negative and positive ions is called the depletion region.

After the founding of the depletion region, if a hole tries to enter into the N region, the positive charge on the right side of the N region repels it (because two similar charges fend off each other). Similarly, if an electron tries to diffuse in the P region, the negative charge on the left side of the depletion region pushes it back. Therefore, further diffusion of electrons and holes stops parallel to the junction. As fixed ions inside the depletion region, exist in the form of two inversely charged paralleled lines or plates, therefore, this region or layer works as an insulator and is capable of capacitance due to fixed charged lines.

The depletion layer depends on the doping level of impurity in P and N-type semiconductors. The greater the doping level, the thinner the depletion layer.

Junction Barrier or Potential Barrier

Junction potential generates due to the formation of negative and positive charges on both sides of the depletion region, which is called potential barrier. It is reflected by VB. In other words, electrical force consumed on charge carriers the by depletion region is called potential barrier or barrier potential.

The moment depletion layer is generated; a potential difference emerges parallel to the junction. As negative ions are positioned towards the left and positive ions to the right of the junction, a difference of this potential ultimately becomes too huge and the diffusion of free electrons parallel to the junction also stops.


Now, if an electron enters into the depletion layer, it has to confront a wall built up by the negative ions, which repels it back to the N region. In the beginning, this wall of free electrons is diminutive, and due to possession of excessive free energy, free electrons easily cross it. Every electron crossing the junction creates an additional junction of positive and negative ions. Thus, when the wall of negative ions becomes too large, free electrons do not have the required energy to overcome the repulsion of negative ions, due to which diffusion stops and potential is produced parallel to the junction. The potential difference produced on the junction is called barrier potential (as shown in figure 2.3). The further flow of carriers parallel to junction stops due to potential barrier until it is provided energy via some external source. At room temperature (300⁰K) the value of barrier potential or barrier voltage of a silicon diode is 0.7 volts and germanium diode 0.3 volt. Barrier voltage depends on doping, density, electronic charge, and temperature. First two factors in every junction are constant; therefore, VB depends solely on temperature. More minority charge carriers are produced due to an increase in temperature, due to which its drift increases parallel to the junction. Thus, balance develops on the low barrier potential.

PN Junction
Figure 2.3


Biasing of P-N Junction or Diode

A PN junction operates as a diode i.e. it passes current in one direction and halts passing of current from in another direction. This function of PN junction can be explained with the help of the following biasing methods.

Forward Bias PN Junction

If a battery is linked with the PN junction in such a way that the positive terminal of the battery is connected with P and the negative terminal of the battery with the N side of the junction, such type of PN junction is called forward bias junction (figure 2.4)

In other words, if the P side of a PN junction is connected with the positive terminal of the battery, and N side with the negative terminal of the battery, this process of connecting the junction with the supply is called forward bias.

Under forward bias, the positive terminal of the battery (connected with the P side of the battery) repels positive holes present in the P side of the junction towards the junction (because similar charges repel each other). Similarly, the negative terminal of the battery (connected with N side of the battery) repels negative electrons present in the N side of the junction, towards the junction. Due to the pressure of the battery, some holes and electrons recombine after entering into the depletion layer or space charge region. As a result, the width of the depletion layer and height of potential layers get less (shown in the diagram)

PN Junction
Figure 2.4


In other words, as a result of battery pressure, ions in the donor and acceptor region get neutralized via the movement of these electrons and holes towards the junction. After the barrier potential becomes zero, movement of holes and electrons (i.e. majority carrier) becomes consistent or continuous and they cross the junction easily without any obstacles. Therefore, electrons entering from the N region and drifting towards P, recombine with holes in the P region, and holes entering from the P region and drifting towards N, combine with electrons in N region. Thus, free electrons and holes inside a crystal, move continuously towards the junction. When they start moving from their place, new holes are produced towards the right side of the junction, therefore N side keeps full of electrons whereas the P side remains filled with holes. Free electrons attempting to cross junction, combine with the holes which have reached the junction. Thus, due to the movement of majority carriers parallel to the junction,a large forward current (IF) transmits through the junction without any resistance, (shown in figure 2.5)

PN Junction
Fig 2.5

PN Junction

It is apparent from the figure that the space charge region or depletion region eliminates completely due to a suitable forward biasing.A small increase in forwarding bias leads to a speedy increase in the value of forwarding current. For germanium, the value of this forward bias is 0.3 volts and silicon 0.7 volts. After crossing the junction, the majority carriers become minority carriers reaching another side after crossing the junction and recombining forthwith. Therefore, the forward current flowing in the PN junction drifts/flows due to majority carriers. Thus, when PN junction is supplied external voltages in the direction, they eliminate potential barrier thoroughly and pass too much current through the junction, such a process is called forward bias.


Reverse Bias PN Junction

If the P side of a PN junction is linked with the negative terminal of a battery and the N side is connected with the positive terminal, this process of connecting the junction with supply is called reverse bias. (shown in figure 2.6)

PN Junction
figure 2.6
PN Junction
Figure 2.7

Due to reverse biasing of junction, holes present in P side of junction will drift towards negative terminal of the battery and electrons present in N side (due to attraction of battery terminal), will move towards the positive terminal of the battery. (contrasting charges mutually attract each other). Thus, majority carriers (i.e. holes and electrons) will distance from each other due to away movement from the junction, which will increase the number of positive ions in the N region and the number of negative ions in the P region. Thus, the width of the depletion layer increases and there is also an increase in potential barrier equivalent to the external bias voltage. Thus, the flow of current stops due to the impossibility of recombination between electrons and holes, and the junction becomes highly resistant (figure 2.7). Therefore, when PN junction is supplied with an external voltage from such a direction,that space charge or potential barrier increase, it is called reverse bias.


Flow of Reverse Leakage Current During Reverse Biasing

During reverse biasing, as the positive terminal of the battery is joined to the N side of the junction and negative terminal with P side of the junction, therefore, free electrons and holes depart from junction momentarily. This increases the width of the depletion layer and its potential becomes equal to the applied voltage. After which the flow of majority carriers stop. The whole process is completed within a Nanosecond, and thereafter, the flow becomes zero.

We know that at absolute zero, only holes exist in P material and free electrons in N material. Thus, extremeDC current transmits as a result of forwarding bias while a zero DC current generates during reverse bias. Therefore, a PN junction or diode functions as a conductor in the forwarding direction while as an insulator in the reverse direction.

At absolute zero temperature, some free electrons on the P side and some free holes on the N side are generated by means of thermal energy, which are known as minority carriers. When junction diode is reverse biased, majority current becomes zero due to reverse bias. However, these minority carriers flow towards the junction and recombine after reaching a junction. Therefore, a very little current called reverse leakage current produces due to these minority carriers. As soon as the free electron and hole are generated through thermal energy, recombine at the junction, an electron-emitting from the negative terminal of the battery enters the left end of the crystal. In the same way, a valence electron after emitting from the right end of the crystal enters into the positive terminal of the battery. As minority carriers are produced constantly due to thermal energy, thus, a regular flow of reverse leakage current (IR) starts due to a reverse voltage (VR). If we fix an exceptionally sensitive D.C ampere meter between junction diode and battery series, it reflects flow of a very small DC current towards the outer circuit.

PN Junction
Figure 2.8

Therefore, an increase in potential barriers due to reverse voltage (VR), has an inverse impact on minority carriers present in the space charge region. That’s they cause an increase in minority carrier current passing easily through a junction.  As VR is increased, minority carrier current also increases. If a further increase in reverse current is desired, it is possible only when to reverse bias is increased. Thus, only a flow of minority carriers occurs through reverse biasing a PN junction. Holes, drifting from N towards P and electrons from P to N via junction, enable the flow of reverse current (IR) in an outer circuit (shown in the figure). Remember, as IR depends on minority carriers present in the region of the PN junction, therefore, temperatures’ effect on it is minimal. As junctions’ temperature increases, more electrons produce holes pairs, as a result of which P and N regions possess more minority carriers. Therefore, the value of IR also increases due to an increase in junction temperature. Practically, the value of this reverse current is extremely small (few nano amperes i.e. 10-9nA in the case of silicon and few microamperes i.e. 10-6 µA in the case of germanium.



Diode 

PN junction is also called a diode. It is a device consisting two terminals, which transmits current quite easily in one direction (in case of forward bias). And when it is reverse biased, passes very little saturation current or reverse current in the other direction, the value of which is equal to nothing. A PN junctions and symbols of diode, have been displayed in figure 2.9. Whether diode is of germanium or silicon nature, same symbol is used for all. Diode has two terminals. P type material terminal is called anode while the terminal made of N type material, is called cathode. Arrow in a diode symbol, describes the following three things:

PN Junction
Figure 2.9
  1. When diode is forward bias, it shows the direction of a conventional current
  2. It reflects anode or P type material
  3. It points towards N type material

PN Junction Diode

Construction

It is a device comprising two terminals, which is composed of germanium (Ge) type PN junction or Silicon (Si) crystal. PN junction has been shown by a diagram 2.10 (a) and PN junction diode symbol in diagram (b). P and N type regions are called anode and cathode, respectively. In the fig (b), arrow symbol displays conventional direction of current, when it is forward biased. It is direction, in which the flow of holes occurs.

PN Junction
Fig 2.10 (a)
PN Junction
Fig 2.10 (b)

From commercial point of view, diode is normally denoted by a symbol, which indicates which one of the leads of a diode is P and which one is N. Standard procedure for enlightening the description of a diode is via numbering or using some color band on it. For examples, IN240 and IN1250 etc. 240and 1250 depicts color bands. In the fig 2.11 (a), various diodes have been shown from physical construction point of view, while in fig (b) the identification of terminal has been shown. Every diode has a unique identity, which is clarified through coloring or putting some color band on its body. Bands have been made on one end of a small germanium diode enclosed in a glass. The end on which there is a black band, is cathode while the other one is anode.

PN Junction
Fig 2.11

Low current diode, body of which is about 3 millimeters in length, can transmit forwarding current of approx. 100Ma. While on room temperature (25⁰C), its saturation current is about 5µA. This diode can withstand reverse current up to 75 volts without a breakdown. Medium current diode can pass forward current up to 500 millimeters approximately and can sustain reverse voltage of about 250 volts. High current diodes or power diodes, can pass several amperes current, and can withstand hundreds of reverse voltages without any breakdown.

Diode Mounting

Low and medium current diodes are fitted by means of soldering its leads through the connecting terminals. When these diodes are operated, its generated heatusually is very low, which evaporates/emits through convection and conduction of air. However, high current or power diodes emit large quantity of heat, for which air becomes insufficient. It needs metals (e.g. copper or aluminum) made heat sinks for cooling it down, which are best conductors of heat (i.e. heat can pass through it easily without any obstacles). It absorbs heat from sink device and spread it in the surrounding air through convection and radiation. Therefore, the area of heat sink is very large. Such type of a heat sink has been shown in the fig 2.12. Its shape is usually finned type for enhancing its cooling efficiency and increasing its surface area.

PN Junction
Figure 2.12

Working

A PN junction diode is a one-way device, which offers very little resistance when it is forward biased (i.e. it works as conductor in case of forward bias) and when it is reverse biased, it offers too much resistance (i.e. it functions as an insulator in case of reverse bias). Thus, such diodes are mostly used as rectifiers i.e. they are used for converting alternating current to direct current.

V/I Characteristics

In the fig 2.13, low powered PN junction diode’s voltages/ current characteristics have been shown.

PN Junction
Fig 2.13

Forward Characteristics

When diode is forward biased and its applied voltage is exceeding zero, initially, current can hardly pass through the diode. Its reason is that barrier’s potential voltage (its value for silicon being 0.7 and germanium 0.3) opposes applied voltage. As soon as applied voltage’s value exceeds junctions’ potential barrier, it gets neutral and current passingthrough the diode, increases very briskly with an increase in its applied voltage.Practically, 50mA forward current generates by making just one-volt increase.If forward voltage is further increased beyond a safe limit, diode become useless.

Reverse Characteristics

When diode is reverse biased, majority carriers are blocked and due to minority carriers, minimum current flows through diode (its detail already mentioned in previous pages). As soon as reverse voltage increases to zero, reverse current very quickly reaches its saturation or maximum value (In). Reverse current is also called leakage current. The value of this current in a silicon brand diode is measuredas nano amperes current (nA) and in germanium, its value measured as micro amperes (µA).In or Iscurrent value has nothing to do with reverse voltage supplied, rather it depends on

  • Temperature (b) Doping Level (c) Physical size of junction

When reverse voltages increase from a certain value (which is called breakdown), leakage current increase very sharply and quickly. At this point, curve offers zero resistance. Increasing voltage above breakdown level, can render diode futile. Therefore, it is safeguarded via fixing a current limiting resister.

Applications

The basic applications of a semiconductor diode in modern electronic circuits, are as follows:

  1. As a rectifier or power diodes. It converts AC current to DC, in the DC power supplies being used in electronic circuits.
  2. As signal diodes for modulation and de-modulation of diodes. For this end, they are applied in communication circuits.
  3. As Zener diodes in voltage stabilizing circuits
  4. As Varactor diode. For controlling voltages in radio and tv receivers tuning circuits
  5. As signal detector. It is a circuit which separates audio signals from a carrier signal
  6. As an automatic switch in digital logic gates
  7. Logic circuits applied in computers (circuits which are given one or more inputs and which produce an output signal as a result of cumulative effect of these inputs)
  8. Wave shaping. (The method through which signal waves of one kind are converted to other form of signal wave, is called wave shaping e.g. converting a sign wave to a square wave with the help of a diode)
  9. For safety purposes. Thus, circuit components can be saved from destruction through the application of such types of diodes.
  10. These are applied for demodulating MF or S.S.B signals or signals of any other form.

Summary

  1. P side holes are diffused in the N side, when a PN junction is produced, where they combine with free electrons.
  2. N side’s free-electron diffuse in P side and produces P side P holes
  3. Due to the recombination of holes and electrons, a region consisting of negative and positive ions parallel to a junction, builds up, which is called the depletion layer.
  4. This depletion layer consists of fixed or immobile ions instead of mobile charge carriers.
  5. These immovable ions generate a potential barrier parallel to a junction, due to which the diffusion of free electrons parallel to a junction, stops immediately.
  6. The width of the depletion layer depends on the doping level. By making doping levels heavier, a thin depletion layer is produced, resultantly diffusing charge carriers cannot drift far away from the junction, due to their short lifetime. The situation will be contrary in case the doping level is maintained low.
  7. The barrier potential of a silicon diode at room temperature approximately is 0.7 volt and germanium diodes’ 0.3 volt

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