Calculating the photocurrent and transit-time-limited bandwidth of a heterostructure p-i-n photo-detector

We have calculated the photocurrent and transit-time-limited bandwidth of a heterostructure p-i-n photodetector. The effective heights of potential barriers at the heterojunction interfaces in the valence band and conduction band have been calculated at different bias voltage and grading lengths for InP-In_0.53Ga_0.47As and Al_0.2Ga_0.8As-GaAs systems. The rates of thermionic emission from the trap can then be easily estimated for each type of material system at an applied bias and for a particular thickness of the grading layer. An expression for current through the photodetector in the presence of traps has been derived by solving rate equations for an arbitrary distribution of photogenerated carriers in the absorption region. Frequency-domain calculations are used to find the transit-time-limited 3-dB bandwidths of the photodetector. It has also been indicated how the results could be used to estimate the bandwidth of the photodetector without performing the exact calculations in the presence of interface trapping. The results from the present model show good agreement with experimental data already reported in the literature for conventional and resonant-cavity-enhanced p-i-n photodetectors