High Gm MBE-grown InP based HEMTs with a very low contact resistance triple capping layer

A highly-doped triple capping layer consisting of n⁺-In _0.53Ga_0.47As, n⁺-In_0.52Al _0.48As, and n⁺-In_0.53Ga_0.47As was investigated with the goal of reducing parasitic source resistance in InAlAs/InGaAs HEMTs. Analysis of the source resistance revealed that the contribution of the resistance element at n⁺-In_0.53Ga_0.47As/un-In_0.52 Al_0.48As/un-In_0.53Ga_0.47As channel hetero-interfaces is as large as 70% of the total source resistance when non-alloyed ohmic electrodes are used. The highly-doped triple capping layer reduces the resistance element of vertical conduction between the capping layer and the 2DEG channel. Thus, source resistance was reduced to 0.57 Ω mm and contact resistance to 3×10^-5 Ω cm² in the HEMTs with a highly-doped triple capping layer. This source resistance is 60% that of HEMTs with a conventional single capping layer, and the contact resistance is one order of magnitude smaller. The low source resistance results in peak extrinsic transconductance as high as 1 S/mm for a device with a 0.4-μm-long gate, which is 42% higher than that of previously reported HEMTs with the same gate length