6.4.1 Technology A and B: Pseudomorphic HEMT on GaAs at 30 GHz and 38 GHz

A PHEMT technology is very suitable for the Ka-band since it can cope with both possible restrictions, firstly to supply the necessary gain and secondly to provide the breakdown hardness. In Fig. 6.18 the power handling capabilities in class A operation at ${\it I}_{\mathrm{D}}$= 40% of ${\it I}_{\mathrm{Dmax}}$ can be observed for a standard HEMT used e.g. for medium power amplifier design. For the frequency range between 30 GHz and 40 GHz large-signal gain, $P_{-1dB}$, and ${\it P}_{\mathrm{sat}}$ are obtained for a ${\it W}_{\mathrm{g}}$= 4$\times$60 $\mu$m pseudomorphic AlGaAs/InGaAs/GaAs HEMT.

Fig. 6.19-6.21 show measurements of several HEMTs with ${\it W}_{\mathrm{g}}$= 6$\times$60 $\mu$m gate width. For high-power applications Fig. 6.19 and Fig. 6.20 show a comparison of two different variations of Technology B at f= 30 GHz. The variations differ by slight changes in the epitaxial structure. The most significant change is a higher $\delta$-doping concentration in variation B. All other device and process parameters, such a gate length ${\it l}_{\mathrm{g}}$, were kept constant. A better output power performance of Variation B can be observed already at 30 GHz with higher power added efficiency of 38%. For Variation A, the gain is higher for low input power levels, but the onset of gain compression occurs earlier, so that $P_{-1dB}$ is significantly higher for Variation B.

Figure 6.18: Load-pull measurement of P$_{sat}$, P$_{-1dB}$, and gain for 4$\times$60 $\mu$m HEMT versus frequency of Technology A.

Figure 6.19: Output power, gain, and PAE for f= 30 GHz, Technology B, Variation A.

For applications at 38 GHz and 40 GHz a pseudomporphic double recess HEMT is limited even more by gain than by breakdown voltage. Fig. 6.21 shows the critical impact of the lattice temperature increase for another 6$\times$60 $\mu$m device of Technology B at f= 40 GHz. The relative reduction of the saturated output power as a function of temperature goes along with the gain reduction.

Figure 6.20: Output power, gain, and PAE for f= 30 GHz, Technology B, Variation B.

Figure 6.21: Output power, gain, and PAE for f= 40 GHz for variation B for $T_L$= 298 K and $T_L$= 343 K

For the development of high-power amplifiers on coplanar wave-guide technology [37], power measurements were performed varying the substrate temperature ${\it T}_{\mathrm{sub}}$. Fig. 6.22 shows the dependence of the saturated output power and the gain on temperature at f= 35 GHz for a two stage high-power amplifier. Over the wide range of the substrate temperature ${\it T}_{\mathrm{sub}}$ between 278 K and 423 K we observe a linear dependence of both gain and output power, represented by P$_{-1dB}$, P$_{-3dB}$, and ${\it P}_{\mathrm{sat}}$.