The indium content in metamorphic [formula omitted]As/ [formula omitted]As HEMTs on GaAs substrate: a new structure parameter

Abstract

State-of-the art metamorphic In x Al 1−x As/ In x Ga 1−x As 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 μm. These devices have been compared with lattice matched HEMTs on an InP substrate. DC-characteristics of 0.1 μm 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 μm 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 μm 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 μm, 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 In 0.4Al 0.6As/In 0.4Ga 0.6As 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.