INTRODUCTION:A p–n junction is formed at the boundary between a p-type and n-type semiconductor created in a single crystal of semiconductor by doping . p–n junctions are elementary elementary "building blocks" of most semiconductor electronic devices such as diodes, transistors, solar cells, LEDs, and integrating circuits; they are the active sites where the electronic action of the device takes place. For example, a common type of transistor, the bipolar junction transistor, transistor, consists of two p–n junctions junctions in series, in the form n–p–n or p–n–p. The discovery of the p–n junction is usually attributed to American physicist Russell Ohl of Bell Laboratories.
ABOUT SEMICONDUCTORS:SEMICONDUCTORS:A semiconductor, semiconductor, such as silicon, has properties somewhere between those of a conductor and an insulator. The ability of a semiconductor to conduct electricity can be changed dramatically by adding small numbers of a different element to the semiconductor crystal. This process is called doping. Early experiments showed that an electric current through a semiconductor was carried by the flow of positive positive charges as well as negative charges (electrons).
DOPED SEMICONDUCTORS:SEMICONDUCTORS:A semiconductor semiconductor crystal is called n-type if the addition of an impurity element results in a large number if free electrons (negative charge carriers) available for conduction. Each impurity atom is called a donor donor atom atom since it donates an electron. The electron is free to move m ove and can contribute to an electric current. The positive ion left behind is fixed and cannot take part in conduction .
A semiconductor crystal can be made p-type by doping it with a different element so that there are a large number of positive charge carriers available for conduction. The positive charge carriers actually correspond to vacancies or deficiencies of electrons in the bonds holding the atoms in the crystal lattice. The positive charges are called holes. These holes can move through the lattice. The dotted lines represent the crystal lattice. Note that the movement of a hole is due to the movement of a bound electron from one bond to another. It is not due to the motion of free electrons. In a p-type semiconductor, most of the mobile charge carriers are holes. A hole moving away from its host impurity atom is equivalent to the atom gaining or accepting an electron into its bonding structure. The host atom gains an excess negative charge and is then called an acceptor ion. Note again that the ions are locked in the crystal lattice and therefore reperesent fixed charges and cannot contribute to current. On the other hand, the holes are mobile charge carriers and can contribute to current flow. Even in a highly doped p-type semiconductor there will always be some free electrons. This very small number of free electrons have been omitted in Figure 4 for clarity. Similarly, n-type semiconductors always contain some holes. The predominant mobile charge carriers are called majority carriers, whilst those in the minority are called minority carriers. For example, the majority carriers in n-type material are free electrons. The terms majority carriers and minority carriers have meaning only if the type of semiconductor (n- or p-) is specified. A pure or undoped semiconductor is said to be intrinsic. Such material has equal numbers of holes and free electrons. These carriers are produced as a result of thermal agitation of the atoms, even at room temperature. Some bound electrons can acquire sufficient energy to escape from their atoms, becoming free electrons and leaving holes behind., This process of producing hole-electron pairs is called thermal generation. It is possible for a free electron and a hole to come near each other in the course of their random wandering through the crystal. The free electron can then occupy the vacant position represented by the hole. The hole and electron are said to recombine. There is then no mobile charge carrier at that point. The rate of recombination depends upon the number of carriers present. Thermal generation and recombination occurs in both doped and undoped
semiconductor material. When a semiconductor material is in thermal equilibrium, the rate of generation of hole-electron pairs equals the r ate of recombination. The density or concentration of both holes and electrons then remains constant.
Drift:Applying an electric field across a semiconductor will cause holes and free electrons to drift through the crystal in the directions. The total current is equal to the sum of hole current and electron current.
Diffusion:A drop of ink in a glass of water diffuses through the water until it is evenly distributed. The same process, called diffusion, occurs with semiconductors. For example, if some extra free electrons are introduced into a p-type semiconductor, the free electrons will redistribute themselves so that the concentration is more uniform. Note that when a few minority carriers are diffusing through a sample, they will encounter a large number of majority carriers. Some recombination will occur. A number of both types of carrier will be lost.
The p-n Junction:Imagine that a p-type block of silicon can be placed in perfect contact with an n-type block. Free electrons from the n-type region will diffuse across the junction to the p-type side where they will recombine with some of the many holes in the p-type material. Similarly, holes will diffuse across the junction in the opposite direction and recombine. The recombination of free electrons and holes in the vicinity of the junction leaves a narrow region on either side of the junction that contains no mobile charge. This narrow region which has been depleted of mibile charge is called the depletion layer. It extends into both the p-type and n-type regions. Note that the diffusion of holes from the p-type side of the depletion layer leaves behind some uncovered fixed negative charges (the acceptor ions). Similarly, fixed positive charges (donor ions) are uncovered on the n-type side of the depletion layer. There is then a separation of charges: negative fixed charges on the p-type side of the depletion layer and positive fixed charges on the n-type side. This separation of charges causes an electric field to extend across the depletion layer. A potential difference must therefore exist across the depletion layer.
Junction Diode Behaviour:- The most important property of a junction diode is its ability to pass an electric current in one direction only. If the diode is connected to a simple circuit consisting of a battery and a resistor, the battery can be connected in either of two ways. When the p-type region of the p-n junction is connected to the positive terminal of the battery, current will flow. The diode is said to be under forward bias. However, when the battery terminals are reversed, the p-n junction almost completely blocks the current flow. This is called reverse bias. If the diode is not connected at all, it is said to be open-circuited and of course no current can flow through the diode.
Forward bias
The application of a forward bias voltage V to a junction diode reduces the built-in potential from V_i_ to V_i_ - V. The reduction in the built-in potential is due to the applied voltage forcing more electrons into the n-type region and more holes into the p-type region, thus covering some of the fixed charges and narrowing the depletion layer. Since the total uncovered charge is reduced, the built-in potential must be lower. Remembering that the built-in potential opposes the flow of majority carriers across the junction, a reduction in that potential makes it easier for holes in the p-type region to cross the junction and for electrons in the n-type region to cross the junction in the opposite direction. As the forward bias voltage is increased, the current through the junction becomes greater. When the applied voltage V approaches V_i_, the potential hill is almost removed. There is then little opposition to the flow of carriers across the junction and a large current can flow through the diode.
Reverse Bias
The application of a reverse voltage V_R_ extracts holes from the p-type region and free electrons from the n-type region and so uncovers more bound charges near the junction. The depletion layer therefore widens and the height of the potential hill is increased to (V_i_ + V_R_ ) volts. Majority carriers are thereby firther inhibited from crossing the junction. As the reverse voltage is increased, the current is reduced to almost zero. However, a very small reverse current does flow. This reverse saturation current depends only on the thermal generation of holes and electrons near the junction, not on the height of the potential barrier. In practice, this reverse saturation current is quite small but it increases with increasing temperature.
V-I Characterstics of a p-n junction diode:-
I-V Characterstics of a p-n junction diode:-
Junction Breakdown:- The large increase in reverse current is the result of junction breakdown. It occurs when the reverse voltage reaches a critical value.
How a p-n junction diode works? Bring one n-type & one p-type semiconductor together and join them to make one piece of semiconductor which is doped differently either side of the junction. Free electrons on the n-side and free holes on the p-side can initially wander across the junction. When a free electron meets a free hole it can 'drop into
it'. So far as charge movements are concerned this means the hole and electron cancel each other and vanish.As a result, the free electrons and holes near the junction tend to eat each other, producing a region depleted of any moving charges. This creates what is called the depletion zone. Now, any free charge which wanders into the depletion zone finds itself in a region with no other free charges. Locally it sees a lot of positive charges (the donor atoms) on the n-type side and a lot of negative charges (the acceptor atoms) on the p-type side. These exert a force on the free charge, driving it back to its 'own side' of the junction away from the depletion zone. The acceptor and donor atoms are 'nailed down' in the solid and cannot move around. However, the negative charge of the acceptor's extra electron and the positive charge of the donor's extra proton (exposed by it's missing electron) tend to keep the depletion zone swept clean of free charges once the zone has formed. A free charge now requires some extra energy to overcome the forces from the donor/acceptor atoms to be able to cross the zone. The junction therefore acts like a barrier, blocking any charge flow (current) across the barrier. The holes behave a bit like balloons bobbing up against a ceiling. On this kind of diagram you require energy to 'pull them down' before they can move from the p-type side to the n-type side. The energy required by the free holes and electrons can be supplied by a suitable voltage applied between the two ends of the pn-junction diode. Notice that this voltage must be supplied the correct way around, this pushes the charges over the barrier. However, applying the voltage the 'wrong' way around makes things worse by pulling what free charges there are away from the junction! This is why diodes conduct in one direction but not the other.
Types of p-n Junction Diodes:Light Emitting Diode (LED):-
LEDs or light emitting diodes are the most popularly known diodes today. They are p-n junction diodes that permit transfer of electrons between the electrodes and produce light. However, not all LEDs emit visible light. There are those that emit infrared light, which cannot be seen by human eyes. Such LEDs are used in remote controls of television, DVD players, etc. When the diode is switched on or forward biased, the electrons recombine with the holes and release energy in the form of light. This means electronic excitation spearheads photon emission, which in turn results in light emission or electroluminescence. Aluminium gallium indium phosphide or aluminum gallium arsenide are generally the conducting materials used in LEDs. The color emitted by the LED, will depend on the combination of semiconductor material used.
Avalanche Diode:-
Slightly doped p-n junctions at times encounter avalanche breakdown, which is the sudden multiplication of voltage (voltage transients) across a diode. This sudden increase often destroys diodes. However, avalanche diodes, available with breakdown voltages of 4000V are built in such a way, that they can break down the voltage and permit passage of reverse bias voltage. Thus, these diodes are used to protect circuits (especially high voltage circuits) from transient voltage. They are often used along with Zener diodes and often confused with the same.
Zener Diode:-
Zener diode is a type of diode that not only allows current to flow through it in one direction like any other diode, but in reverse bias, also allows current to flow in the reverse direction, if the voltage exceeds a certain limit. This voltage limit is known as Zener voltage and is fixed for Zener diodes with breakdown voltage ranging from 1.8V to 200V. Thus, Zener diodes protect circuits from damages. Unlike, avalanche diodes, such diodes have heavilydoped p-n junctions and the doping is done differently to achieve different Zener breakdown voltage. These diodes are mostly used to control voltage in electrical circuits.
Laser Diode:-
This type of diode is different from the LED type, as it produces coherent light, which is nothing but radiation in which the waves are of the same frequency and in the same phase. These diodes are small in size, however, compared to their size, the output is commendable. Laser diodes can again be divided into two types: low power diodes and high power diodes. The coherent light produced by these diodes make them perfect for devices such as DVD and CD drives, laser pointers, high-definition TVs, barcode readers, etc. Laser diodes are more expensive than LEDs. However, they are cheaper than other forms of laser generators. Moreover, these laser diodes have limited life.
Photodiode:-
Photodiodes are used to detect light and convert light falling on it into electric current. They feature wide, transparent p-n junctions and work on the mechanism of photoelectric effect. These diodes operate in reverse bias, wherein even small amounts of current flow, resulting from the light, can be detected with ease. Photodiodes can also be used to generate electricity, used as solar cells and even in photometry. Some photodiodes feature an undoped layer sandwiched between the p and n layers, and such diodes are called PIN photodiodes. This kind of photodiode is more popularly used today, because of its higher efficiency. These diodes are used to rectify alternating power inputs in power supplies. This means they convert alternating current (AC) to direct current (DC). They are extremely useful in DC devices which require continuous flow of direct current. Rectifier diodes can be further classified into two types: full wave and half wave rectifiers. The half wave rectifier allows the flow of only the positive half of the wave through it. The full wave rectifier on the other hand features a combination of two diodes that rectify not only the positive half of the wave, but also the negative half of the wave. Diodes are used widely in the electronics industry, right from electronics design to production, to repair. Besides the above mentioned types of diodes, the other diodes are point contact diode, signal diode, step recovery diode, tunnel diode and gold doped diodes. The type of diode to transfer electric current depends on the type and amount of transmission, as well as on specific applications. ********************************
