Linearity characteristics of GaAs HBTs and the influence of collector design

The linearity characteristics of GaAs heterojunction bipolar transistors (HBTs) are studied through measurement and analysis. Third-order intermodulation distortion behavior of HBTs is examined on devices with various epilayer designs and at various bias points, loads, and frequencies. Calculations from an analytical model reveal a strong bias and load dependence of third-order intercept point (IP3) on the nonlinearities from transconductance and the voltage dependence of base-collector capacitance. However, a simple model is not able to predict the fine details of IP3 with bias. A large-signal HBT model with an accurate description of the base-collector charge is shown to account for the measured trends. The base-collector charge function accounts for the modulation of base-collector capacitance with current, electron velocity modulation, and the Kirk effect (base pushout) for GaAs-based HBTs. A detailed study of the influence of collector design on linearity is also presented.