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We report the first lattice matched InP HEWS fabricated using a T-gate process based on a bilayer of Shipley Uvm D W resist and PMMA. A DC gate resistance of 220 Q/mm was achieved, leading to fT of 193 GHz and Maximum Available Gain (MAG) values of 13 dB at 94 GHz for 100pm wide devices. I IntroductionGates with a T-shaped cross section are commonly used to obtain low gate resistance while retaining the very short footprint required for high gain, low noise operation at millimetre-wave frequencies [ 1 , 21. Electron beam exposure of bilayers of polymethylmethacrylate (Ph4M.A) and the copolymer (PMMA-MAA) are widely used for this process, but the small difference in sensitivity between these resists restricts process latitude [3]. To overcome this we developed a new process which uses PMMA and UVIII, a Shipley DUV photoresist. U W I resist is approximately five times more sensitive to electron beam exposure than PMMA giving rise to greater process latitude [4, 51. The process enables the fabrication of T-gates with a headwidth to footprint ratio in excess of 40:l combined with gate head thicknesses greater than 500nm. This is beneficial in reducing gate resistance. The uVIWPMh4.A bilayer gives improved control of the footprint size and increases pattem writing speeds by approximately 20% once all exposure overheads have been taken into account. In this letter, we report the first device results for the U'VIWPMMA gate stack which demonstrates the viability of incorporating the process in a complete HEMT process flow producing devices with good millimetrewave performance. I1 Device Fabrication:The devices were fabricated on a Si &doped In,,,Al,,,,As/ In,~,,Ga,,,,As lattice matched heterostructure grown by MBE on an InP substrate as schematically shown in figure 1. The as-grown room-temperature sheet concentration nrh and Hall mobility pH of the structure were measured as 2 . 1~1 0 '~c m -~ and 7500 cm2Ns respectively. Au/Ge/Ni based ohmic contacts with a contact resistance of 0.4

This paper reports on the performance and model extraction of indium phosphide based W-band (67-110 GHz) coplanar waveguide ratrace couplers, branchline couplers and Wilkinson power dividers. The extracted models consist of lumped and distributed circuit elements available in most microwave CAD packages. The accuracy of these models is verified by comparison with measured S-parameter data from 67 to 110 GHz.

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