2.3 Heterojunction Bipolar Transistors (HBTs)

Heterojunction Bipolar Transistors (HBTs) are the HEMT's main competitors for replacing the MESFET in evolving RF technologies on a large scale. Based on different device approaches, they offer a considerable number of advantages for specific applications. Firstly, high current gain (which means high-power amplifications for low bias) and good low power RF-properties are provided. This makes them extremely suitable for low frequency cell phone applications. At the same time very good RF properties for low bias voltages make them suitable for low power high-speed integration. In general, lower levels of phase noise are another advantage of the HBT, which is important for oscillator circuits. A big breakthrough for the introduction of the mass production of InGaP/GaAs HBTs was demonstrated by RF-Microdevices founded as a spin-off of TRW, see [279]. The excellent current amplification and normally-off condition provided by the HBT are very suitable for cellular phone applications. This introduction followed by other vendors has significantly challenged MESFETs and HEMTs in the L-band [165,188]. Compiled data can be found in [287]. HBTs also demonstrated excellent MMIC power performance up to 30 GHz with 1.59 W maximum output power, 6.5 dB linear gain, and 35% PAE. A comparison with regard to output power as a function of frequency is given in Fig. 2.9. For high-speed optical communication links, InP based single heterojunction HBTs (SHBTs) as well as double heterojunction HBTs (DHBTs) are actively developed. They are especially interesting for low power (dissipation) high frequency operation [201]. Using InGaP/InGaAs SHBTs [296] demonstrated oscillators up to 134 GHz frequency of operation taking advantage of the low phase noise. A 68 GHz frequency divider was shown by Tang in [288] using DHBTs. A wide band tunable voltage controlled oscillator (VCO) was realized in [24] allowing for 80 Gbit/s operation. Using a transferred substrate concept, see [233], extremely high values of the current gain cut-off frequency ${\it f}_\mathrm{T}$ and maximum frequency of oscillation ${\it f}_\mathrm{max}$ values near 1 THz were reported. For analog applications, InP based HBTs can be efficiently used in MMICs in order to realize circuits with competitive PAE [29]. Recently, extremely promising prototypes have been shown by adding Sb as a base material [80]. These results show ${\it f}_\mathrm{T}$ values as high as 305 GHz with promising breakdown properties. Due to several unresolved technological barriers, GaN based HBTs have only reached a status of principal demonstration so far [175]. An overview on remaining issues to produce RF devices is given by McCarthy et. al. in [176].