Fabrication of high performance GaN modulation doped field effect transistors

Fabrication of GaN devices presents a number of processing issues. Since th e AlGaN/GaN material and sapphire substrates are transparent in the ultravi olet, light scattering can degrade the quality of the lithography. GaN and AlGaN are chemically inert materials, highly resistant to wet etching. Scho ttky contacts are fabricated with noble metals (Pt, Au) which present adhes ion problems. The approaches to solving these difficulties and the routes t o fabrication of high performance GaN-based modulation doped field effect t ransistors are presented. The light scattering has been addressed by the de position of amorphous Si on the backside of the wafers, Mesa etching For de vice isolation has been performed with both chemically assisted ion beam et ching and a newly developed wet etching technique. Finally, metal adhesion has been greatly improved by the initial deposition of a thin sputtered Pt layer, followed by a thick layer of Pt/Au by e-beam evaporation. A 100 mu m wide device showed a maximum de current density of 946 mA/mm, a peak trans conductance of 160 mS/mm, a short circuit current gain cutoff frequency of 15.6 GHz, and a maximum oscillation frequency of 49.4 GHz. (C) 2000 America n Vacuum Society. [S0734-2101(00)06902-5].