A self-aligned structure AlGaAs/GaAs HBT´s using silicon nitride sidewall technique

A self-aligned fabrication process for AlGaAs/GaAs heterojunction bipolar transistors (HBT´s) is presented. The advantage of this process is that selfaligned structure and device passivation can be realized simultaneously using silicon nitride sidewall technique. The silicon nitride sidewall functions both as an isolation layer to prevent shorting between the base metal and the emitter mesa and as an etching mask to prevent AlGaAs passivation layer to be removed. A current gain cutoff frequency f_T of 30 GHz and a maximum oscillation frequency f_max of 50 GHz have been obtained from the device with 3 μm×15 μm emitter size