We report dramatic differences in the impulse response and wavelength
dependence of back versus top illuminated In0.53Ga0.47As planar metal-
semiconductor-metal devices. Via direct measurement of transit-time li
mited devices we identify the mechanisms involved and thereby allow th
e optimum design of multi-Gbit, high responsivity back-illuminated dev
ices. We show that responsivities greater than 0.8 A/W are achievable
with >8 GHz bandwidth for 50-mu m-diameter devices.