The origin of current gain under illumination in amorphous silicon n-i-p-i-n structures

Abstract

Detailed computer modeling in conjunction with experiments has been used to explain the current gain sometimes observed in amorphous silicon n-i-p-i-n structures under illumination and to specify under what conditions such a gain is possible. Calculations reveal that the origin of the current gain at a given applied voltage, is excess injection of electrons from the contacts as a result of the lowering of the i-layer/n-layer barrier in the forward biased diode on account of photogenerated hole trapping. However to actually obtain this gain, the thickness and the density of states of the partitioning p layer must be small enough, so that all the excess injected electrons do not recombine in the p layer before being collected in the reverse biased diode. We also find that under conditions for which the gain varies linearly with voltage, it is almost independent of the incident light flux. On the other hand, when the barrier height at the p layer (Ec−EF0)max, where Ec is the conduction band edge and EF0 the thermodynamic equilibrium Fermi level, approaches its maximum value corresponding to a thick or highly doped p layer, there is no gain. This is the condition to be fulfilled for a color sensitive device.