  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.
