The PN junction diode

2.01 The PN junction diode

Understanding the operation of the semiconductor diode is the basis for an understanding of all semiconductor devices. The diode is actually manufactured as a single piece of material but it is much easier to explain the operation if we imagine producing two separate pieces of N type and P type material and then “sticking” them together.

Consider a piece of N type material. It contains mobile charge carriers in the form of free electrons. These electrons will be in motion due to thermal energy. (It is important to realise that this motion does not result in an electrical current because the motion is random and there is not net movement of charge from one area of the material to another. This is similar to the way that even in a perfectly still glass of water the individual molecules will be moving randomly on a microscopic scale.) The net result is that the random motion of the electrons results in them being evenly distributed throughout the N type material. In the P type material it is the positively charged holes that are mobile and for identical reasons to those previously described the holes are evenly distributed throughout the P type material.

p_type_2n_type_2

Now consider what will happen if these two separate pieces of P and N type material are joined together. The random motion of the mobile electrons in the N type material and the holes in the P type material would tend to cause an even distribution of electrons and holes throughout the semiconductor. And in fact this is what begins to happen.

Consider the electrons in the N type material. The electrons start to migrate across the junction of the two materials. When they cross into the P type material they recombine with the holes (ie they fill in the holes in the valence band by filling in the vacant electron positions around the trivalent donor atoms). This means that the number of holes near to the junction becomes depleted. Also as the electrons leave the previously neutral N type material a positive charge builds up at the junction. (This is because the positive charge from the nucleus of atoms near to the junction is now greater than the negative charge of the electrons in that region. This is due to the reduction in the number of electrons due to those which have moved across the junction.)

Similarly as holes migrate from the P to N type material they recombine with electrons (the free electron from the pentavalent atoms completes the fourth covalent bond around the trivalent atom). This leaves a depletion of free electrons near the junction in the N type material. Also a negative charge builds up near the junction in the P type material due to the loss of positively charged holes.

pn_diode

The net result is that the migration of electrons from N to P type material and the migration of holes from P to N has two effects. It results in a depletion of mobile charge carriers near the junction ( a depletion of electrons in the N type material and a depletion of holes in the P type material). This depletion layer is typically about 1 micrometre wide ( 1 millionth of a meter!). Also a voltage is produce across the junction which is called a barrier voltage. The N type material develops a positive charge close to the junction and the P type develops a negative charge. This prevents any further migration of mobile charge carriers.

The effect of the barrier voltage
The positive charge at the N side of the junction repels any positively charged holes that would tend to migrate across the junction from the P type material. It also attracts free electrons and therefore to prevents them moving out of the N type material. Similarly the negative charge in the P type material close to the junction repels electrons which would tend to migrate from the N type material and it attracts the holes and prevents them moving out of the P type material. The migration of mobile charge carriers across the junction would stop when the barrier voltage had built up to a sufficient level to prevent any further migration. For Silicon this is about 0.6 to 0.7 volts for Germanium it is about 0.2 to 0.3 volts.

2.02 Reverse Bias

Consider applying an external voltage to the diode as shown below with the positive terminal connected to the N type material and the negative terminal connected to the P type material. The external voltage would tend to cause the movement of electrons from the negative terminal of the supply through the diode and back to the positive terminal (electron flow).

To do this the negative terminal would tend to inject electrons into the P type material causing a further depletion of holes. This would produce a widening of the depletion layer and an increase in the negative charge at the junction until it was equal in magnitude to the applied voltage. The negative charge at the junction would oppose the negative terminal of the external voltage and this would prevent any further injection of electrons into the P type material.

reverse_bias

Similarly, the positive terminal would tend to pull electrons from the N type material. This would further deplete the N type material of electrons, widening the depletion layer and increasing the positive charge at the junction until it was equal to the magnitude of the applied voltage. This would then prevent any further loss of electrons.

The net effect is that when an external voltage is connected this way the effect of the barrier voltage opposes the external voltage. Any initial movement of charge due to the external voltage will just increase the barrier voltage until it is equal to the applied voltage and therefore no current will flow through the diode. When an external voltage is connected to a diode with this polarity we say that it is reverse biased.

Note as holes are the majority current carriers in P type material it is more common to consider the movement of holes rather than electrons in the P type material. Therefore we can say that the negative terminal tends to remove holes rather than injecting electrons in the same way that we considered the positive terminal removing electrons from the N type material. The effect is the same, the removal of holes from the P type material would increase the depletion layer and increase the barrier voltage.

2.03 Forward bias

Consider applying an external voltage to the diode as shown below with the positive terminal connected to the P type material and the negative terminal connected to the N type material. The external voltage would tend to cause the movement of electrons from the negative terminal of the supply through the diode and back to the positive terminal (electron flow).

The negative terminal would tend to inject electrons into the N type material. This would increase the number of electrons and therefore reduce depletion layer. This would reduce the positive charge at the junction. Similarly the positive terminal would tend to pull electrons from the P type material. This would increase the number of holes, reducing the depletion layer and reducing the negative charge at the junction.

The net effect is that when the external voltage is connected this way it reduces the barrier voltage and if the applied voltage is greater than the barrier voltage it will overcome it and produce a current flow through the diode. When an external voltage is connected to a diode with this polarity we say that it is forward biased.

forward_bias

Note as holes are the majority current carriers in P type material it is more common to consider the movement of holes rather than electrons in the P type material. Therefore we can say that the positive terminal injects holes rather than removes electrons in the same way that we considered the negative terminal injecting electrons into the N type material. The effect is the same the injection of holes would reduce the depletion layer and reduce the barrier voltage.

04 Diode Characteristic

A diode characteristic is simply a graph of the voltage applied to a diode and the current it produces. The negative part of the voltage axis corresponds to when the diode is reverse biased and the positive part is when the diode is forward biased. The negative part of the current axis shows current flowing in the reverse direction through the diode.

characteristic
  • The main features of the characteristic are:
  • No current flows when the diode is forward biased until the barrier voltage is overcome (0.6V – 0.7V for silicon 0.2V – 0.3V for germanium)
  • The forward characteristic is non linear (not a straight line). This shows that the resistance is not constant.
  • the gradient of the forward characteristic quickly becomes very steep. This shows that the forward resistance is very low
  • The negative current axis is on a different scale (showing millionths of an amp rather than thousandths) this is so we can indicate the very small leakage current which flows due to electron hole pair generation (ie due to the natural conduction properties of the pure silicon). The leakage current flows in both directions but is too small to indicate on the current scale used on the forward part of the characteristic
  • If a large enough reversed bias voltage is applied the diode will eventually conduct due to zener then avalanche breakdown (ie due to the natural conduction properties of the pure silicon). The actually voltage that breakdown occurs varies for individual diodes and can be determined by the manufacturing process