Comments on InP/Ga0.47In0.53As superlattice avalanche photodiode [and reply]

For the original article see ibid., vol.24, no.15, p.964-5 (1988). In their recent letter, Batra et al. computed the performance of a superlattice APD at an electric field of 3×10⁷ V/m in a structure having 50 alternate layers of InP (550 Å) and Ga_0.47In_0.53As (450 Å). In doing so, the commenter points out that they appear to have ignored the fact that this electric field is about a factor of two higher than can be tolerated in GaInAs if tunnel currents are to be kept to an acceptable low value. It is noted that at this field the voltage drop in each GaInAs layer is 1.35 V, or almost twice the bandgap, thus the band structure is not what the authors have sketched in their paper. At a field of 3×10⁷ V/m the unmultiplied tunnel current will be large (on the order of 10^-2 A/cm² or more) and the commenter considers the device virtually useless as a sensitive photodetector. In reply Batra et al. state that the effective ionisation rate ratio (β_e/α_e) will not be affected much, even at such a high field. With regard to the potential of the structure as a photodetector, the authors explain that it can be operated efficiently at a much lower voltage (corresponding to a field less than 2.0×10 ⁷ V/m). For such a low field, the tunnelling will be insignificant, while β_e/α_e will remain reasonably high