Abstract

High speed optical detectors in the 1.3-1.55 μm wavelength range are required for a new generation of microwave-optical instruments to characterize high data rate systems, microwave transmission via lightwave, and other applications requiring true replication of the modulation signal. Front-side illuminated photodiodes have an advantage over back-side illuminated devices in simplicity of packaging. In addition, the transit time of the photocarriers is determined by electrons rather than by holes as for backside illuminated devices fabricated on n-type substrates. The speed of response of the double heterostructure photodiode is wavelength independent since photocarriers can only be generated in the i-layer which is fully depleted. For single heterostructure backside illuminated photodiodes with p-GaInAs, the speed response degrades as the wavelength increases since photocarriers can be generated in the low field p-layer resulting in carrier diffussion. For high speed operation of the device, the i-layer thickness is selected such that the device is transit time limited and is of the order of one absorption length. However one potential obstacle exists that could degrade the speed of a front-side illuminated device, and that is the trapping of holes at the discontinuity in the valance band between the InP and GalnAs. The effect of hole trapping was not evident in the results of Susa et al[1] which utilized a separate absorption and multiplication (SAM) structure for a front-side illuminated avalanche photodiode. This suggests that the quality of interface could play a dominant role in the transit time at heterojunction interfaces.

© 1987 Optical Society of America