  We report here a general theory describing photoelectron transportation
dynamics in GaAs semiconductor photocathodes. Gradient doping is incorporated
in the model through the inclusion of directional carrier drift. The
time-evolution of electron concentration in the active layer upon the injection
of an excitation pulse is solved both numerically and analytically. The
predictions of the model are compared with experiments via carrier-induced
transient reflectivity change, which is measured for gradient-doped and
uniform-doped photocathodes using femtosecond pump-probe reflectometry.
Excellent agreement is found between the experiments and the theory, leading to
the characterization of key device parameters such as diffusion constant and
electron decay rates. Comparisons are also made between uniform doping and
gradient doping for their characteristics in photoelectron transportation.
Doping gradient is found to be able to accelerate electron accumulation on the
device surface. These results offer new insights into the dynamics of III-V
photocathodes and potentially open a new avenue toward experimental
characterization of device parameters.
