7.4 Technology C: Pseudomorphic HEMT on GaAs for Ka-Band to W-Band

For a completely different technology Fig. 7.31 shows the output characteristics of a pseudomorphic Al$_{0.2}$Ga$_{0.8}$As/InGa$_{0.25}$As/GaAs HEMT at ${\it T}_\mathrm{L}$= 300 K with ${\it l}_{\mathrm{g}}$= 150 nm. The device is a high current ( ${\it I}_{\mathrm{D}}$$\geq$ 900 mA/mm) and high gain device and therefore serves from the Ka-band (26.5-40 GHz) to the W-band (75-110 GHz). The maximum DC- transconductance amounts to ${\mit g}_{\mathrm{m}}$$\geq$ 800 mS/mm for a gate length ${\it l}_{\mathrm{g}}$= 150 nm.

Figure 7.31: Output characteristics of a 2$\times$60 $\mu$m pseudomorphic AlGaAs/InGaAs/GaAs HEMT.

Such devices are very suitable for high-power amplifiers in the Q-band, as shown by Bessemoulin et al. in [37], and in the V-band, as shown in [169]. For gate lengths of about ${\it l}_{\mathrm{g}}$= 150 nm-200 nm the output power of a pseudomorphic device is limited by the gain for these frequencies, as shown in the previous section. Applications beyond f= 42 GHz require more gain than typically available in double recess devices. Thus, the frequency of about 42 GHz represents the critical point of the usefulness of the double recess pseudomorphic AlGaAs/InGaAs/GaAs. The output power limitation switches from a breakdown voltage limitation to a gain limitation.

For the comparison of a pseudomorphic AlGaAs/InGaAs/GaAs HEMT to a GaAs MESFET of similar gate length ${\it l}_{\mathrm{g}}$, Fig. 7.32 shows the result of a simulation experiment to obtain the maximum transconductance. Using the model reported and information from [185,198] a simulation study is performed to obtain maximum transconductance for a MESFET with ${\it l}_{\mathrm{g}}$= 100 nm. The measured data shown in Fig. 7.32 are taken from [185].

Figure 7.32: Transconductance $g_m$ versus $V_{GS}$ for a $l_g$= 100 nm GaAs MESFET [185].

It is found that using the same parameter set and the information on the doping profile from [185,198] that a transconductance ${\mit g}_{\mathrm{m}}$$\approx$ 500 mS/mm can be obtained. Comparing the advantages and disadvantages of a MESFET at the same gate length ${\it l}_{\mathrm{g}}$ to a pseudomorphic HEMT, they are compiled as:

• Reduced ${\mit g}_{\mathrm{m}}$, less gain, less output power (Compare Fig. 7.4, Fig. 7.31, and Fig. 7.32).
• Reduced charge control, since doping profiles and implant pockets cannot be as sharp as MBE grown layers.

These findings agree very well with the published general experience, e.g. from Fig. 2.1-Fig. 2.4 in Chapter 2 and [196] who further state:

• Relatively higher ${\it C}_{\mathrm{gd}}$ for the MESFET.
• Reduced power added efficiency (PAE) for the MESFET.