High frequency large signal characterization of heterojunction bipolar transistors

A Ka-band active load pull measurement system is described. Its use for measuring the dependence of output power and gain on bias, input power, and frequency of heterojunction bipolar transistors (HBTs) from 26.5 to 40 GHz is reported. To help explain the experimental results a detailed numerical simulation has been used to develop a simple HBT device model. This simplified model includes transit time effects and is easily implemented in commercially available harmonic balance software such as Libra. Measured and simulated results are compared. Both set of results indicate that the primary source of gain compression is caused by the device entering the ohmic region. Device mismatch is an additional compression mechanism for fixed load impedances at high drive levels