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 de
signs and at various bias points, loads, and frequencies, Calculations from
an analytical model reveal a strong bias and load dependence of third-orde
r 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 HB
T model with an accurate description of the base-collector charge is shown
to account for the measured trends. The base-collector charge function acco
unts for the modulation of base-collector capacitance with current, electro
n velocity modulation, and Kirk effect (base pushout) for GaAs-based HBTs,
A detailed study of the influence of collector design on linearity is also
presented.