2.2.1 Pseudomorphic AlGaAs/InGaAs/GaAs HEMTs

As was shown in [170,172], even for moderate RF frequencies, GaAs HEMTs, and pseudomorphic AlGaAs/InGaAs HEMTs (PHEMTs) on GaAs substrates offer the highest PAE, output power and gain on the FET side in this frequency range. Extremely high-power amplifiers at 2 GHz using three terminal devices with saturated output power ${\it P}_{\mathrm{sat}}$= 200 W [136] have been realized. By introducing a fourth terminal output powers up to ${\it P}_{\mathrm{sat}}$= 230 W [172] have been achieved. In the X-band (8-12 GHz) amplifiers up to ${\it P}_{\mathrm{sat}}$= 39 dBm have been demonstrated, e.g. by TNO in [74]. Combining these circuits in modules up to 70 W of output power have been realized [283].

In the K-band (18-26.5 GHz) PAE is a major concern especially for satellite communication. A PAE of 68% was demonstrated at 18 GHz using PHEMTs [245], where also a compilation of data on K-band power amplifiers is given.

High-power amplifiers have been developed for radar applications at 35 GHz to replace traveling wave tube (TWT) amplifiers. Recently, however, the Ka-band (26.5-40 GHz) has been faced with growing attention due to the interest in high bandwidth, high-speed digital data transmission applications such as Local Multipoint Distribution Services (LMDS) or Multichannel-Multipoint Distribution Services (MMDS). The highest overall output power for a Ka-band high-power amplifier on a single chip was realized by both [150] and [261] with ${\it P}_{\mathrm{sat}}$= 5 W at 27.5 GHz to 29 GHz and ${\it P}_{\mathrm{sat}}$= 4 W for ${\it f}$= 29 GHz and 31 GHz, respectively. Very recently, ${\it P}_{\mathrm{sat}}$= 6 W at 30 GHz on a single chip have been published in [86]. LMDS chip solutions for mixers have been proposed, see e.g. [27,301]. An overview for LMDS modules is given in [11]. Module solutions up to ${\it P}_{\mathrm{sat}}$= 6 W have been demonstrated by TRW in [133]. A space qualified process for this frequency has been reported by UMS in [125], or in a more complete description in [227]. Low noise amplifiers at 38 GHz were demonstrated by Ali et al. in [157].

Figure 2.5: Noise figure versus frequency for low noise amplifiers.

As HEMTs are typical high gain and thus low noise devices reported noise figures are shown in Fig. 2.5 versus frequency for several low noise amplifiers (LNA) and compared to values reported for Si [240,253]. The straight line given in Fig. 2.5 indicates a lower bound, which has not been beaten so far. The slope amounts to 3 dB/100 GHz. The measurement technique for the values reported near that line [155] has not yet been completely published. Various chip solutions for the 60 GHz range have been proposed using PHEMTs, e.g. by [169], who demonstrated ${\it P}_{\mathrm{sat}}$= 200 mW for 60 GHz for satellite data communication using a 150 nm pseudomorphic AlGaAs/InGaAs HEMT process. Using a 100 nm pseudomorphic process for 60 GHz a ${\it P}_{\mathrm{sat}}$= 560 mW was demonstrated [289]. A 1.5 W module for the 60 GHz range consisting of two circuits with ${\it P}_{\mathrm{sat}}$= 800 mW each was demonstrated in [168]. For automotive collision avoidance radar the 77 GHz frequency range is used. Chip sets can be found from industrial vendors (see [57,189]) based on ${\it l}_{\mathrm{g}}$$\approx$ 150 nm pseudomorphic HEMTs. Using a similar pseudomorphic process on GaAs substrate for low noise applications, TRW [305] demonstrated 3.5 dB noise figure at ${\it f}$= 94 GHz for remote sensing applications.