High Mobility SiGe/Si n-MODFET Structures and Devices on Sapphire Substrates

SiGe/Si n-type modulation doped field effect structures and transistors (n-MODFETs) have been fabricated on r-plane sapphire substrates. Mobilities as high as 1380 cm 2 /Vs were measured at room temperature. Excellent carrier confinement was shown by Shubnikov-de Haas measurements. Atomic force microscopy indicated smooth surfaces, with rms roughness less than 4 nm, similar to the quality of SiGe/Si n-MODFET structures made on Si substrates. Transistors with 2 µm gate lengths and 200 µm gate widths were fabricated and tested. An I ds of 9 mA was obtained by operating the transistor in an enhancement mode (positive V GS ) and the maximum transconductance (g m ) was 37 mS/mm at a V ds of 2.5 V. The transducer gain (G t ) measured with a load-pull system was 6.4 dB at 1 GHz for a V ds of 2.5 V and V gs =-0.4 V.