Lecture -1: p-n Junction Diode
A pure silicon crystal or germanium crystal is known as an intrinsic semiconductor. There are not enough free electrons and holes in an intrinsic semi-conductor to produce a usable current. The electrical action of these can be modified by doping means adding impurity atoms to a crystal to increase either the number of free holes or no of free electrons.
When a crystal has been doped, it is called a extrinsic semi-conductor. They are of two types
• n-type semiconductor having free electrons as majority carriers
• p-type semiconductor having free holes as majority carriers
By themselves, these doped materials are of little use. However, if a junction is made by joining p-type semiconductor to n-type semiconductor a useful device is produced known as diode. It will allow current to flow through it only in one direction. The unidirectional properties of a diode allow current flow when forward biased and disallow current flow when reversed biased. This is called rectification process and therefore it is also called rectifier.
How is it possible that by properly joining two semiconductors each of which, by itself, will freely conduct the current in any direct refuses to allow conduction in one direction.
Consider first the condition of p-type and n-type germanium just prior to joining fig. 1. The majority and minority carriers are in constant motion.
The minority carriers are thermally produced and they exist only for short time after which they recombine and neutralize each other. In the mean time, other minority carriers have been produced and this process goes on and on.
The number of these electron hole pair that exist at any one time depends upon the temperature. The number of majority carriers is however, fixed depending on the number of impurity atoms available. While the electrons and holes are in motion but the atoms are fixed in place and do not move.
As soon as, the junction is formed, the following processes are initiated fig. 2.
Holes from the p-side diffuse into n-side where they recombine with free electrons.
Free electrons from n-side diffuse into p-side where they recombine with free holes.
The diffusion of electrons and holes is due to the fact that large no of electrons are concentrated in one area and large no of holes are concentrated in another area.
When these electrons and holes begin to diffuse across the junction then they collide each other and negative charge in the electrons cancels the positive charge of the hole and both will lose their charges.
The diffusion of holes and electrons is an electric current referred to as a recombination current. The recombination process decay exponentially with both time and distance from the junction. Thus most of the recombination occurs just after the junction is made and very near to junction.
A measure of the rate of recombination is the lifetime defined as the time required for the density of carriers to decrease to 37% to the original concentration
The impurity atoms are fixed in their individual places. The atoms itself is a part of the crystal and so cannot move. When the electrons and hole meet, their individual charge is cancelled and this leaves the originating impurity atoms with a net charge, the atom that produced the electron now lack an electronic and so becomes charged positively, whereas the atoms that produced the hole now lacks a positive charge and becomes negative.
The electrically charged atoms are called ions since they are no longer neutral. These ions produce an electric field as shown in fig. 3. After several collisions occur, the electric field is great enough to repel rest of the majority carriers away of the junction. For example, an electron trying to diffuse from n to p side is repelled by the negative charge of the p-side. Thus diffusion process does not continue indefinitely but continues as long as the field is developed.
This region is produced immediately surrounding the junction that has no majority carriers. The majority carriers have been repelled away from the junction and junction is depleted from carriers. The junction is known as the barrier region or depletion region. The electric field represents a potential difference across the junction also called space charge potential or barrier potential . This potential is 0.7v for Si at 25o celcious and 0.3v for Ge.
The physical width of the depletion region depends on the doping level. If very heavy doping is used, the depletion region is physically thin because diffusion charge need not travel far across the junction before recombination takes place (short life time). If doping is light, then depletion is more wide (long life time).
The symbol of diode is shown in fig. 4. The terminal connected to p-layer is called anode (A) and the terminal connected to n-layer is called cathode (K)
If positive terminal of dc source is connected to cathode and negative terminal is connected to anode, the diode is called reverse biased as shown in fig. 5.
When the diode is reverse biased then the depletion region width increases, majority carriers move away from the junction and there is no flow of current due to majority carriers but there are thermally produced electron hole pair also. If these electrons and holes are generated in the vicinity of junction then there is a flow of current. The negative voltage applied to the diode will tend to attract the holes thus generated and repel the electrons. At the same time, the positive voltage will attract the electrons towards the battery and repel the holes. This will cause current to flow in the circuit. This current is usually very small (interms of micro amp to nano amp). Since this current is due to minority carriers and these number of minority carriers are fixed at a given temperature therefore, the current is almost constant known as reverse saturation current ICO.
In actual diode, the current is not almost constant but increases slightly with voltage. This is due to surface leakage current. The surface of diode follows ohmic law (V=IR). The resistance under reverse bias condition is very high 100k to mega ohms. When the reverse voltage is increased, then at certain voltage, then breakdown to diode takes place and it conducts heavily. This is due to avalanche or zener breakdown. The characteristic of the diode is shown in fig. 6.
When the diode is forward bias, then majority carriers are pushed towards junction, when they collide and recombination takes place. Number of majority carriers are fixed in semiconductor. Therefore as each electron is eliminated at the junction, a new electron must be introduced, this comes from battery. At the same time, one hole must be created in p-layer. This is formed by extracting one electron from p-layer. Therefore, there is a flow of carriers and thus flow of current.