7.6.2 Technology F: Metamorphic Depletion Type InAlAs/InGaAs HEMT on GaAs

A metamorphic technology for analog applications was investigated. Fig. 7.41 shows the simulated and measured transfer characteristic of a metamorphic In$_{0.52}$Al$_{0.48}$As/In$_{0.53}$Ga$_{0.47}$As/GaAs HEMT for two different temperatures ${\it T}_{\mathrm{sub}}$. A decrease of the current ${\it I}_{\mathrm{D}}$ with rising ${\it T}_\mathrm{L}$ for low ${\it V}_{\mathrm{DS}}$= 1 V can be observed.

Figure 7.41: Transfer characteristics of a metamorphic depletion-type HEMT with $l_g$= 150 nm.

Figure 7.42: Transconductance $g_m$ versus $V_{GS}$ of a $l_g$= 400 nm metamorphic HEMT.

Fig. 7.42 shows the transfer curves of a 2$\times$60 $\mu$m metamorphic depletion type HEMT and comparison with measurements for ${\it l}_{\mathrm{g}}$= 400 nm. The large gain for this relatively large gate length ${\it l}_{\mathrm{g}}$ makes this materials system well suitable for high gain applications.

Figure 7.43: $f_T$ as a function of material composition.

It is suggested by Bollaert et al. in [47] that the maximum ${\it f}_\mathrm{T}$ value versus channel In content $x$ for a device structure is found for $x$= 0.43 for the metamorphic HEMTs. Using the given parameter set from Chapter 3 this idea is not confirmed, as shown in Fig. 7.43. ${\it f}_\mathrm{T}$ of three devices available for ${\it l}_{\mathrm{g}}$= 150 nm and the simulation results are compared in Fig. 7.43. For the $x\leq$ 0.25 composition range the values are taken from typical pseudomorphic AlGaAs/In$_{0.25}$Ga$_{0.75}$As/GaAs HEMTs. The values suggest a linear dependence of ${\it f}_\mathrm{T}$. Between $x$= 0.25 and x= 0.53, no devices are experimentally available for this work. The simulation results obtained for $x$= 0.43 and $x$= 0.3 support the measured values and the monotonous decreasing behavior for decreasing $x$. Devices with $x$= 0.3 were demonstrated by Bollaert et al. in [47] for power applications, where the further reduction of the In content beyond $x$= 0.43 to $x$= 0.3 leads to a further decrease of ${\it f}_\mathrm{T}$. For power applications Zaknoune et al. in [325] reported a value ${\it f}_\mathrm{T}$= 125 GHz for ${\it l}_{\mathrm{g}}$= 150 nm using an extremely high $\delta$-doping of 1$\times$10$^{13}$ cm$^{-2}$.

Thus, the In content in metamorphic devices serves mainly to adjust the device in the trade-off between speed and breakdown considerations. The InAlAs/InGaAs HEMTs are limited by the on-state breakdown, as argued in Chapter 6. In [235] no significant improvement of the device burnout was found using a reduced In content to $x$= 0.41 for the device in the channel in comparison with the $x$= 0.53. This suggests when using the In$_y$Al$_{1-y}$As/In$_x$Ga$_{1-x}$As in high power applications in order to make use of the higher gain performance, that the In content $x$ needs to be further reduced than $x$= 0.41. However, such a reduction also decreases the possible improved speed performance in comparison with the PHEMT on GaAs. Hence, in order to make use of the higher gain in the InAlAs/InGaAs materials system to compensate the gain shortage of the pseudomorphic HEMTs on GaAs, the In content remaining to be investigated for reliable high-power application is 0.25 $\leq$ $x$$\leq$ 0.41. Comparing AlGaAs/InGaAs HEMT and AlGaAs/InGaAs HEMT lattice matched to InP del Alamo and Sommerville $\cite{Alamo1}$ determined the frequency ${\it f}$= 94 GHz at which better high-power performance was achieved for InAlAs/InGaAs devices for ${\it l}_{\mathrm{g}}$= 100 nm devices while PAE was not considered an issue. An In content $x$$\leq$ 0.4 is one choice to lower this frequency. Finding the break-even point in terms of the In composition $x$ for the change of sign of the temperature coefficient of impact ionization is necessary. Once this is achieved the concepts of electric field relaxation found useful for the PHEMT on GaAs can be used, as introduced in the last sections.