Heterojunction bipolar transistors

Abstract

AlGaAs/GaAs heterojunction bipolar transistors (HBTs) are used for digital and analog microwave applications with frequencies as high as Ku band. HBTs can provide faster switching speeds than silicon bipolar transistors mainly because of reduced base resistance and collector-to-substrate capacitance. HBT processing requires less demanding lithography than GaAs FETs, therefore, HBTs can cost less to fabricate and can provide improved lithographic yield. This technology can also provide higher breakdown voltages and easier broad-band impedance matching than GaAs FETs.In comparison with Si bipolar junction transistors (BJTs), HBTs show better performance in terms of emitter injection efficiency, base resistance, base-emitter capacitance, and cutoff frequency. They also offer good linearity, low phase noise and high power-added efficiency. Table 3-3 shows a comparison of typical device characteristics between AlGaAs/GaAs HBTs and Si BJTs. HBTs are used in both commercial and high-reliability applications, such as power amplifiers in mobile telephones and laser drivers. Table 3-3. Comparison of AlGaAs/GaAs HBT and Si bipolar transistors. Parameter AlGaAs/GaAs HBT Si BJT Forward transit time, τ F 4 ps 12 ps Early voltage, V a 800 V 25 V Collector-substrate capacitance, C cs~0~1 5 fF Base resistance, R b 70 W 200 WFor NPN BJTs, a useful figure of merit that is important in determining the current gain is the ratio, electron current injected from the emitter into the base hole current injected from the base into the emitterThis ratio is called the injection efficiency, and it is usually optimized in BJTs by highly doping the emitter and lightly doping the base. High injection efficiency is obtained in an HBT by using a material with a larger energy band gap for the emitter than that used for the base material. The large energy band-gap emitter blocks injection of holes from the base. Therefore, the doping concentration in the base and emitter can be adjusted over a wide range with little effect on injection efficiency. In the normal operation of a bipolar transistor, the collector-base junction is reverse biased or at least not forward biased enough to cause appreciable injection current. The collector and base material are the same in most HBTs, although some use wide band-gap collector materials to improve the collector base breakdown voltage.It follows that AlGaAs/GaAs HBTs benefit from the following advantages:(1) Lower forward transit time along with a much lower base resistance (due to the much higher base doping concentration), giving increased cutoff frequency f t .