FREQUENCY-RESPONSE OF AVALANCHE PHOTODETECTORS WITH SEPARATE ABSORPTION AND MULTIPLICATION LAYERS

We present analytical expressions for the frequency response of avalan che photodetectors (APD's) with separate absorption and multiplication regions (SAM). The effect of the electric held profile in the multipl ication layer on frequency response is considered for the first time. Previous theories have assumed that the multiplication layer is very t hin and the peak electric field, which corresponds to the effective mu ltiplication plane, is positioned away from the absorption layer. This is a poor assumption for many devices, and in particular for silicon hetero-interface photodetectors (SHIP's). We present a theoretical mod el in which the thickness of the multiplication layer is arbitrary and the peak electric held may be positioned arbitrarily in relation to t he absorption layer, We also consider the effects of parasitics, trans it-time, and avalanche buildup time. Both front and back illumination from either multiplication layer or absorption layer are considered. T he calculated results are compared with experimental results for exist ing SHIP's and performance predictions are also made for optimized SHI P structures, SHIP APD's with gain-bandwidth product in excess of 500 GHz are possible.