P-N Junction diode
A p-n junction diode is formed when a p-type semiconductor is fused with an-type semiconductor. It is a device with single p-n junction as shown in Figure 9.11(a). The circuit symbol is shown in Figure 9.11(b).
Biasing means providing external energy to charge carriers to overcome the barrier potential and make them move in a particular direction. The charge carriers can either move towards the junction or away from the junction. The external voltage applied to the p-n junction is called bias voltage. Depending on the polarity of the external source to the p-n junction we have two types of biasing
1. Forward bias
2. Reverse bias
If the positive terminal of the external voltage source is connected to the p-side and the negative terminal to the n-side, it is called forward biased as shown in Figure 9.12. The application of a forward bias potential makes the electrons move into the n-side and the holes into the p-side. This initiates the recombination with the ions near the junction which in turn reduces the width of the depletion region and hence the barrier potential.
The electron from the n-side is now accelerated towards the p-side as it experiences a reduced barrier potential at the junction. In addition, the accelerated electrons experience a strong attraction by the positive potential applied to the p-side. This results in the movement of electrons towards the p-side and in turn, holes towards the n-side. When the applied voltage is increased, the width of the depletion region and hence the barrier potential are further reduced. This results in a large number of electrons passing through the junction resulting in an exponential rise in current through the junction.
If the positive terminal of the battery is connected to the n-side and the negative potential to the p-side, the junction is said to be reverse biased as shown in Figure 9.13.
As the positive potential is connected to the n-type material, the electrons in the n-type material are attracted towards the positive terminal in turn, the holes in the p-type material move towards the negative terminal (both away from the junction). It increases the immobile ions at the junction. The net effect is the widening of the depletion region. This leads to an increase in the barrier potential. Consequently, the majority charge carriers from both sides experience a great barrier to cross the junction. This reduces the diffusion current across the junction effectively.
Yet, a small current flows across the junction due to the minority charge carriers in both regions. The reverse bias for majority charge carriers serves as the forward bias for minority charge carriers. The current that flows under a reverse bias is called the reverse saturation current. It is represented as Is .
The reverse saturation current is independent of the applied voltage and it depends only on the thermally generated minority charge carriers. Even a small voltage is sufficient enough to drive the minority charge carriers across the junction.
The reverse saturation current of a silicon diode doubles for every 10 ¬įC rise in temperature.