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Gallium arsenide (GaAs) heterojunction bipolar transistor (HBT) has become the dominant technology for handset power amplifier (PA) applications due to its linearity and efficiency. Consequently, GaAs HBT with the emitter made of indium gallium phosphide (InGaP) is widely used in handset PAs, and is gaining acceptance in infra-structure amplifier designs. To address base station applications with power levels above multi-Watt, operation voltage higher than the common 3 V to 7 V now used must be adopted. The base station PA application commonly requires 24 V to 28 V operation, and occasionally even above 30 V^[1,2]. An InGaP/GaAs HBT technology was developed in our labs for 28 V power amplification^[3-5], which is compatible with standard MMIC technology.

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To achieve high breakdown voltage in InGaP/GaAs HBT, a thick collector is required: 3 µm thickness is commonly used. The thick collector presents challenges to semiconductor processing in lithography and step coverage of metal interconnection. Power transistors demand large total device size. Multiple fingers are arranged into a single building block. Such building blocks can deliver 2 W RF power in the 1 GHz to 3 GHz frequency band. Multiple building blocks can be arrayed into a single-power HBT for higher power levels. The f_T and f_max of the basic HBT finger are 6.4 GHz and 25 GHz respectively.

In a typical lineup of power amplifiers, the power stage is often a class B circuit for the best efficiency, and the driver stage a class AB for a trade-off of linearity and efficiency, and the pre-driver stage may be a class A amplifier. Working with InGaP/GaAs HBT, the goal is to operate the driver and pre-driver stages in near-class B operation for better efficiency, while achieving a superior linearity at the back-off power level.

Class B operation with high linearity can be achieved by a low-frequency low-source impedance, or a dynamic bias circuit^[7]. The ruggedness is improved by increasing the ballasting to withstand high output mismatch of 10:1 VSWR with typical input power at 1 dB gain compression and overdrive condition of typical 8 dB gain compression into 50 Ω load at 30 V.

Accelerated lifetime test at 315 °C junction temperature and 28 V bias was repeated. More than 3000 hours lifetime test on HBTs was achieved; the Gummel plot before and after the 3000 hours lifetime test shows no increase of the leakage current. The high linearity power performance in class B condition in the back-off power level, the ruggedness under mismatch and overdrive condition, and the long lifetime of the InGaP/GaAs HBT technology makes it a contender for the 28 V power amplifier application.