2.2.3 InAlAs/InGaAs HEMTs Grown Metamorphically on GaAs

A recent achievement is the broadening development of InAlAs/InGaAs HEMTs on GaAs substrates using a so-called metamorphic buffer. The obvious advantage lies on the more advanced development status of the GaAs substrates in comparison with InP substrates. GaAs substrates are available on 6 inch substrates with 8 inch under development. In comparison with InP substrates typically available on 4 inch, GaAs substrates are more cost effective for a diameter given. In [314] improved gain performance of metamorphic HEMTs was demonstrated in comparison with pseudomorphic HEMTs. It is discussed that in comparison with HEMTs lattice matched to InP breakdown problems can be reduced using the In contents between pseudomorphic GaAs (x$\leq$ 0.25) and lattice matched InP HEMTs (x= 0.53). A systematic investigation of the channel In composition variation is given by Bollaert et al. in [47]. For low noise applications noise figures of a metamorphic technology at 89 GHz comparable to InP based HEMTs were demonstrated in [128]. In [327], an integration concept of pin diodes and metamorphic HEMTs similar to InP based processes was demonstrated. Manufacturing issues of metamorphic HEMTs were discussed by Rohdin et al. in [235]. Preliminary results reveal no disadvantage in terms of long term reliability [33,235] in comparison with InP HEMTs. It is suggested in [33] that the disadvantage of the reduced thermal conductivity of the GaAs substrates is more than balanced by the possibility of producing high gain amplifiers at relaxed gate lengths ${\it l}_{\mathrm{g}}$.

Figure 2.6: Comparison of $f_{T}$$\cdot$$l_{g}$ for different FET types.

To evaluate device speed performance versus gate length, Fig. 2.6 shows the comparison of the product ${\it l}_{\mathrm{g}}$$\times$ ${\it f}_\mathrm{T}$ of different III-V technologies as a function of gate length ${\it l}_{\mathrm{g}}$. To balance the uncertainties of applied deembedding methods and observed gate-lengths, lines are introduced to give an estimate for ${\it l}_{\mathrm{g}}$= 150 nm. For comparison with other device types or materials, Fig. 2.7 shows a world map of analog applications in a diagram showing maximum cut-off frequency ${\it f}_\mathrm{T}$ versus breakdown voltage. Fig. 2.7 is far from being complete and the comparability of different breakdown voltages is under discussion, too. However, several aspects can be observed in detail. Looking at industrially available devices, the pseudomorphic AlGaAs/InGaAs HEMTs cover a relatively large frequency range up to 100 GHz at very good power capabilities represented by the breakdown voltage. A similar tendency can be observed for the InGaP HBT although the frequency range is smaller. Si bipolar technologies are restricted by the Johnson limit [138]. SiGe based HBTs achieve a higher Johnson limit due to the higher average carrier velocity of SiGe relative to Si. Comparing SiGe and GaAs based HBTs, the GaAs HBT is superior due to the relatively higher band gap of GaAs, which leads to improved breakdown hardness. For high power applications, very promising candidates are GaN based HEMTs with outstanding breakdown voltages and cut-off frequencies up to 74 GHz at ${\it l}_{\mathrm{g}}$= 150 nm and 111 GHz at ${\it l}_{\mathrm{g}}$= 50 nm. Although only available in small scale processes InP HBTs, in particular double heterostructure bipolar transistors (DHBTs), offer very promising power capabilities for very high frequencies. Si based technologies are generally discriminated in this kind of comparison, which does not consider their high density integration capabilities.

Figure 2.7: Comparison of the $BV_{i}$ versus $f_{T}$ for MESFET, HEMT, and HBT taken from different material systems.