Frequency response of multilayer avalanche photodiodes: structural effects

The frequency response of SAM-APD devices is calculated from the response of each layer using matrix algebra. Most of the results apply to devices with absorption region of InGaAs and avalanche region of InP and they assume uniform carrier ionization coefficients and velocities. The effect of the width of each layer, carrier ionization ratio and velocities on the multilayer structure frequency response has been investigated. A change of the absorption region width changes the 3-dB bandwidth at low avalanche gains whereas a change in the avalanche region width only affects the frequency response at high avalanche gains. When the ionization ratio decreases an increase of the 3-dB bandwidth is observed at high avalanche gains. The frequency response seems to be very sensitive to the carrier velocities mainly the hole velocity. In order to include the strong dependence of the ionization coefficients on the electrical field, the avalanche region was modeled piecewise uniform by breaking it into three layers. The frequency response of this structure is seen to be similar to the one obtained when uniform ionization coefficients are considered assuming they are assigned the mean value of the corresponding ionization coefficients in the three layers.