A photoconductor intrinsically has no gain

TL;DR

This paper challenges the traditional theory of photoconductor gain, showing that intrinsic gain is impossible due to carrier confinement effects, and that observed high gain results from extrinsic factors like defects and surface states.

Contribution

It reveals that the common assumption of uniform excess carrier distribution is invalid, demonstrating that intrinsic photoconductors have no gain regardless of device length or carrier lifetime.

Findings

Intrinsic photoconductors have no gain due to carrier confinement.

High gain arises from extrinsic effects such as defects and surface states.

A universal equation for photogain based on Ohm's Law is established.

Abstract

In the past 50 years, the high gain in quantum efficiency of photoconductors is often explained by a widely accepted theory in which the photogain is proportional to the minority carrier lifetime and inversely proportional to the carrier transit time across the photoconductor. It occasionally misleads scientists to believe that a high-speed and high-gain photodetector can be made simply by shortening the device length. The theory is derived on the assumption that the distribution of photogenerated excess carriers is spatially uniform. In this Letter, we find that this assumption is not valid for a photoconductive semiconductor due to the metal-semiconductor boundary at the two metal electrodes inducing carrier confinement. By solving the continuity equation and performing numerical simulations, we conclude that a photoconductor intrinsically has no gain or at least no high gain, no matter how short the transit time and how long the minority lifetime is. The high gain observed in experiments comes from other extrinsic effects such as defects, surface states and surface depletion regions that localize excess minority carriers, leaving a large number of excess majority carriers accumulated in the conduction channel for the photogain. Following the Ohm's Law, a universal equation governing the photogain in a photoconductor is established at the end of this Letter.