A Composite Transistor to Suppress Kink Phenomenon in HBTs for Broadband Design

The kink phenomenon in heterojunction bipolar transistors (HBTs) is introduced and mathematically analyzed. A novel composite transistor based on feedback theory is proposed to suppress the kink phenomenon with negligible increase in chip area and power consumption. According to the small-signal analysis, the output impedance of the composite transistor can be represented by an RC series circuit. Thus, the output resistance is a constant value over a wide frequency range. A test die using 2-μm InGaP/GaAs HBT technology from AWSC is fabricated, and a constant real part of S22 is measured from 0.1 to 20 GHz (approaching the HBT's unity-gain frequency). The proposed composite transistor can significantly ease the design for output matching and broadband amplifiers.