New low contact resistance triple capping layer enabling very high G(m) InAlAs/InGaAs HEMTs

It has been demonstrated that a highly doped (Si:3 x 10(19) cm(-3)) triple capping layer consisting of n(+)-In0.53Ga0.47As, n(+)-In0.52Al0.48As, and n(+)-In0.53Ga0.47As can remarkably reduce the parasitic source resistance in InP-based high electron mobility transistors (HEMTs). The analysis of the source resistance revealed that the resistance element at the n(+)-In0.53Ga0.47As/un-In0.52Al0.48As/un-In-0.53 Ga0.47As channel heterointerfaces was as large as 70% of the source resistance when nonalloyed ohmic electrodes were used. The highly doped triple capping layer reduces the resistance contribution of vertical conduction between the capping layer and 2DEG channel. A low source resistance of 0.57 Omega mm and a low contact resistivity of 3 x 10(-5) Omega cm(2) were obtained for the HEMTs with the highly doped triple capping layer, which were 60% lower and one order of magnitude smaller than those for the HEMTs with a conventional single capping layer doped 5 x 10(18) cm(-3), respectively. These values were also 70 and 30% lower than those for the HEMTs with a highly doped (3 x 10(19) cm(-3)) single capping layer, respectively. The low source resistance brings high peak extrinsic transconductance of 1 S/mm for a device with 0.4 mu m long gate, which was 42% higher than the previously reported HEMTs with the same gate length.