Investigations of the high frequency performance of impact avalanche transit-time diodes

The high frequency performance of impact avalanche transit time (IMPATT) diodes is investigated. Theoretical studies of the IMPATT performance were made by simulating the operation of the device numerically. Simulations were made of the IMPATTs operating at 100 to 400 GHz. GaAs abrupt structure diodes were found to operate up to 400 GHz, which extends the upper frequency limit of 200 GHz for GaAs devices established previously. Above 200 GHz, the efficiencies of the IMPATTs fall below one percent. Diffusion and distributed generation effects degrade the efficiency of the diodes as the frequency of operation is increased. The generation of carriers becomes less localized in millimeter frequency diodes as the ionization coefficient becomes saturated. Read-type structure IMPATTs are not able to operate above 200 GHz because the generation region in those devices is too narrow for adequate avalanching current. Diffusion effects cause the carrier pulse to spread as it drifts across the device. Diffusion current at the boundary of the active region of the IMPATT causes the carriers to diffuse in and out of the active layer. Heterostructure IMPATTs were shown to have higher efficiencies than comparable conventional structures.