Frequency response characterisation of superlattice avalanche photodiodes

Recently, interest has been expressed in superlattice avalanche photodiodes (SAPDs) as low noise detectors for long wavelength optical communication systems. In the paper we discuss the speed of response of these structures, and the effect on the performance of multigigabit per second lightwave receivers. An expression for the frequency response of single carrier multiplication SAPDS is presented, providing an estimate for the maximum gain bandwidth (GB) product which can be achieved with ideal devices. The existence of residual hole ionisation degrades GB product, mainly due to avalanche build-up time, arising from the regenerative nature of the avalanche process. In this case, the GB product is found to be inversely proportional to the width of the multiplication region W and directly proportional to the number of stages N, especially when N is large. Sensitivity calculations for equalised GB-limited SAPD receivers are presented which take into account the effects of both residual hole ionisation and the width of the avalanche region. The analyses should provide useful guidelines for the design of high bit rate optical receivers employing SAPDs.