2.2.4 GaN Based HEMTs

For high-power applications such as amplifiers, GaN based transistors constantly set new output power records, especially for frequencies at X-band to K-band [320,321]. A combination of mediocre, but RF suitable transport properties together with excellent breakdown properties [258], and extremely good charge control results in outstanding high-power performance. The current record output power amounts to a saturated output power of ${\it P}_{\mathrm{sat}}$= 51 W in pulsed operation mode in the C-band (4.6 GHz-7.0 GHz) at 6 GHz [320]. For a comparison of power densities and overall output power, see Fig. 2.8 and Fig. 2.9. The highest available power density at 20 GHz has been reported by [181] as 4 W/mm for continuous wave (CW) operation on SiC substrate. The theoretical limit is taken from [145] and shows the optimization potential remaining. High frequency operation has been reported up to ${\it f}_\mathrm{T}$= 74 GHz for ${\it l}_{\mathrm{g}}$= 150 nm [81] and the highest ${\it f}_\mathrm{T}$$\times$ ${\it l}_{\mathrm{g}}$ product has been reported with 101 GHz ${\it f}_\mathrm{T}$ at ${\it l}_{\mathrm{g}}$= 120 nm with an astonishingly low transconductance [166]. The highest overall value ${\it f}_\mathrm{T}$= 110 GHz was reported for ${\it l}_{\mathrm{g}}$= 50 nm by Micovic et al. in [182]. These values suggest possible operation at least up to the Ka-band in this stage of process development. So far InGaN/AlGaN based HEMTs have faced harsh material problems due to the unsolved problem of high unintentionally doping concentrations. They are, however, under development [246] for high-speed operation due to the suggested lower effective carrier mass.

Figure 2.8: Power density versus frequency for GaN HEMTs circuits for different substrates and modes of operation.

Figure 2.9: Absolute output power as a function of frequency for GaAs HBTs and GaN HEMTs.