HEMT and HBT for RF Applications

The two-dimensional electron gas (2DEG) principle, already introduced for the GaN power HEMT in the GaN Power Devices chapter, also underlies high-frequency HEMTs based on GaAs and InP: a heterostructure of two semiconductors with different band gaps creates an undoped, high-mobility channel at the interface, whose electrons are spatially separated from the ionized impurities of the donor region.

For high-frequency applications, GaAs is typically used in the form of a pseudomorphic HEMT (pHEMT) or a metamorphic HEMT (mHEMT) with an InGaAs channel, since InGaAs, with mobilities of 10,000 to 12,000 cm²/Vs, offers even higher electron mobility than pure GaAs. Parameters such as the transit frequency f_T and the maximum oscillation frequency f_max of modern GaAs pHEMTs reach 100 to 150 GHz, while highly scaled InP HEMTs with gate lengths below 50 nm have demonstrated f_T values above 600 GHz in the laboratory, making these devices suitable for applications approaching the terahertz-adjacent millimeter-wave range.

The heterojunction bipolar transistor (HBT) applies the heterostructure principle to the bipolar transistor by using an emitter material with a wider band gap than the base, such as InGaP or AlGaAs on a GaAs base. The wider band gap of the emitter suppresses the back-injection of holes from the base into the emitter, which allows the base to be doped much more heavily than in a conventional bipolar transistor (typically 10¹⁹ to 10²⁰ cm^-3) without sacrificing current gain, which for GaAs HBTs typically reaches values of 50 to 150.

The heavier base doping reduces base resistance and allows thinner base layers, often only 50 to 100 nm, which substantially improves carrier transit time and therefore cutoff frequency – commercial GaAs HBTs reach f_T and f_max values in the range of 150 to 300 GHz. HBTs are also known for a very linear transfer characteristic and low phase noise, which makes them particularly well suited to power amplifiers.