Microwave performance of ZnO/ZnMgO heterostructure field effect transistors

We report the first microwave performance of single crystalline ZnO/ZnMgO heterostructure field-effect transistors (HFETs). The structure consisted of a 15-nm-thick ZnO channel layer was grown by molecular beam epitaxy (MBE) on an a-sapphire substrate. Two-finger type HFETs with 1 or 2-mu m-long gate were fabricated and measured for microwave performance. The transconductance of the HFETs are 28 and 23 mS/mm for 1 and 2-mu m-gate devices, respectively. The microwave measurement of ZnO-based TFTs revealed that the current gain cutoff frequency f(T) of 1.75 GHz and that of unilateral power gain f(max) of 2.45 GHz for 1-mu m-gate HFET. Electron velocity obtained by two-terminal measurements implies that the structural design is crucial for further improvement of the high-frequency performance of ZnO/ZnMgO HFETs. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim