Photoelectric measurement techniques are ideally suited to investigate
barrier and bandgap energies in pseudomorphic heterostructure FETs. I
n these devices the bandgap in the channel is smaller than that of the
substrate and hence there exists a wavelength window in which the sub
strate is transparent and the channel is absorbing. Consequently, phot
oelectric measurements can be performed on fully fabricated PHEMTs usi
ng backside illumination. In this paper we describe photocurrent measu
rements for two distinct processes: photoemission of carriers between
the Schottky gate and the channel, and photoconduction through the cha
nnel. We have successfully modeled the observed shift in the onset of
photoconduction using a self-consistent Schrodinger-Poisson solution.
The peak photocurrent magnitude shows non-monotonic dependence on gate
bias, possibly suggesting amplification of the photocurrent.