On the optimization of heterostructure avalanche photodiodes

We use the modified dead-space multiplication theory (MDSMT) that incorporates both the initial energy effect and the heterostructure effect to predict the optimal performance of two-layered-multiplication APDs. We first consider an Al/sub 0.6/Ga/sub 0.4/As homojunction APD with a multiplication layer (i-layer) of width 140 nm, and then a device with a multiplication region consisting of two layers: Al/sub 0.6/Ga/sub 0.4/As and GaAs for which carriers are injected into the Al/sub 0.6/Ga/sub 0.4/As layer. From a design perspective, maximizing the initial energy is clearly the key in reducing noise. Unfortunately, the ability to control the initial energy in the fabrication stage is apparently not yet well established at this point of time. However, for a given initial energy of the injected electrons, the heterostructure APD would result in improved noise performance if the Al/sub 0.6/Ga/sub 0.4/As layer is either 30 nm or 110 nm wide. Moreover, the 30 nm Al/sub 0.6/Ga/sub 0.4/As case would result in less uncertainty in the excess noise factor than the 110 nm case (/spl Delta/F=0.7 vs. /spl Delta/F =2.3), which is attributed to the unknown initial energy.