Abstract

Staircase heterostructures have recently been proposed as a means of achieving noise-free photomultiplierlike avalanche gain and repeated velocity overshoot. No experimental study of transport in such structures has so far been reported. In this paper we report the first investigation of carrier transport in a staircase potential configuration. The latter is obtained by appropritely biasing an MBE compositionally graded multilayer AIGaAs structure contacted by two n⁺ regions. The graded gap undoped region consists of three layers each graded from x = 0 to x = 0.2 over a distance of 5000 Å. A new photoconductive gain phenomenon which is peculiar to these graded materials is found. Dramatic differences in the photoconductivity are observed as the band diagram changes from a sawtooth to a staircase configuration, with increasing bias. In the staircase conduction band configuration electrons have high velocity while holes are slowed down and partially trapped by virtue of the valence band steps. This leads to high current gain (≥ 10⁴). In the sawtooth configuration, on the other hand, the opposite occurs and the gain dramatically decreases (≤ 10). By engineering the staircase band diagram the performance of the photoconductor (gain and gain-bandwidth product) can be tuned over a very wide range. Other device applications are also discussed.

© 1986 Optical Society of America