Bias of PN Junctions
There are three modes of operation in semiconductor devices:
- Thermal Equilibrium: At the state of thermal equilibrium there are no inputs from the outside as applied voltage or light. There is no net current within the device as the currents balance out each other.
- Steady State: At steady state external inputs like applied voltage or light exist, but the conditions are constant with time. Devices normally operate under steady state and are in either forward or reverse bias.
- Transient: When the applied voltage changes quickly, there will be a delay which is short before the response of solar cell. Since solar cells are not designed for high speed operation there are some more transient effects that should be taken into consideration.
Diodes under Forward Bias
Forward bias refers to applying a voltage across the device for reducing the electric field at the junction. An electric field with a direction opposite to that in the depletion region is applied across the device by the application of a positive voltage to p-type material and a negative voltage to n-type material. Because the depletion region’s resistivity is much greater than the resistivity otherwise in the device – due to of the restricted number of carriers in the depletion region- almost all of the electric field applied is dropped across the depletion region.
The difference between the existing field in the depletion region and the applied field is the net electric field, therefore decreasing the depletion region’s net electric field. Decreasing the electric field disrupts the equilibrium present at the junction, reducing the barrier to the carriers diffusion from one side of the junction to the other side and increasing the diffusion current. Though the diffusion current increases, the drift current is basically not changed because it depends on the number of carriers produced inside a diffusion length of the depletion region or in the depletion region. Since the depletion region is reduced in width only by a small amount, the number of minority carriers swept across the junction is basically not changed.
Carrier Injection and Forward Bias Current Flow
Minority carrier injection at the depletion region’s edge is caused by the increased diffusion from one side of the junction to the other. Diffusion makes these carriers move away from the junction and will ultimately recombine with a majority carrier. The majority carrier is supplied from an external circuit and therefore a flow of net current occurs under forward bias. In recombination’s absence, the minority carrier concentration would climb to a higher equilibrium concentration and the diffusion of carriers from a single side of the junction to the other would stop, similar to the introduction of two different gasses.
At first, gas molecules have a net movement from the region of high carrier concentration to the lower carrier concentration region, but when reaching a constant concentration, there is no more net gas molecule movement. However, in a semiconductor the injected minority carriers recombine and therefore more carriers can possibly diffuse across the junction. Accordingly, the diffusion current flowing in forward bias is a recombination current. The higher the recombination events rate, the greater the current flowing across the junction.
I0 – the dark saturation current– is a really important factor that differentiates one diode from another. I0 measures of the recombination in a device. A diode that has a larger recombination will have a larger I0.
Reverse bias refers to applying a voltage is across the device to increase the electric field at the junction. The depletion region’s higher electric field decreases the probability of carriers diffusing from one side of the junction to the other, thus decreasing the diffusion current. Similar to forward bias, the drift current is restricted by the number of minority carriers on each side of the pn junction and is relatively not affected by the increased electric field. A slight increase in the drift current is noticed as a result of the small increase in the depletion region’s width, but this is basically an effect of second order in silicon solar cells