The indium content in metamorphic InxAl1-xAs/InxGa1-xAs HEMTs on GaAs substrate:: a new structure parameter

State-of-the art metamorphic InxAl1-xAs/InxGa1-xAs HEMTs (MM-HEMTs) on a GaAs substrate with different indium compositions x = 0.33, 0.4 and 0.5 have been realized and characterized. The gate lengths L-g are 0.1 and 0.25 mu m. These devices have been compared with lattice matched HEMTs on an InP substrate. DC-characteristics of 0.1 mu m gate length MM-HEMTs show drain-to-source current I-ds of the order of 550-650 mA/mm, and extrinsic transconductance of about 800 mS/mm. Schottky characteristics exhibit a gate reverse breakdown voltage varying from -14 to -7 V for x = 0.33-0.5, with an intermediate value of -10.5 V for x = 0.4. A small signal equivalent circuit of our 0.1 mu m MM-HEMTs give intrinsic transconductance higher than 1100 mS/mm, with similar values of 1350 and 1450 mS/mm for x = 0.5 and the lattice matched HEMT, respectively. The MM-HEMTs with a gate length of 0.25 mu m present a cutoff frequency f(T) close to 100 GHz. To achieve the same result with pseudomorphic HEMTs on GaAs, a smaller gate length has to be realized, which requires the use of an electron beam lithography and therefore increases the device costs. For L-g = 0.1 mu m, f(T) reaches 160, 195 and 180 GHz for x = 0.33, 0.4 and 0.5, respectively. These values are close to f(T) = 210 GHz obtained for a lattice matched HEMTs on InP realized with the same technological process. The MM-HEMTs are therefore good alternatives to PM-HEMTs on GaAs and LM-HEMTs on InP in the V bands and W bands while maintaining a GaAs substrate. Moreover. metamorphic In0.4Al0.6As/In(0.4)Gao(0.6)As HEMTs exhibit a comparable microwave performance with large voltage operation than the MM-HEMT with a 0.5 indium content and the lattice matched HEMTs. These results indicate that a device with indium content x = 0.4 is particularly attractive for the realization of low-noise and power circuits on the same wafer. (C) 2000 Elsevier Science Ltd. All rights reserved.