TRANSPORT-PROPERTIES OF A 2-DIMENSIONAL ELECTRON-GAS CLOSELY SEPARATED FROM AN UNDERLYING N+ GAAS LAYER - THE FABRICATION OF INDEPENDENT OHMIC CONTACTS USING MOLECULAR-BEAM EPITAXIAL REGROWTH AND INSITU FOCUSED ION-BEAMS

Recently we have described a new method for the fabrication of independent ohmic contacts to a two-dimensional electron gas (2DEG) closely separated from an underlying n+ GaAs layer. This was achieved by using molecular beam epitaxial regrowth on an in situ focused ion implanted epilayer. In this article, we present further transport properties of backgated high electron mobility transistors which indicate the versatility of this fabrication technique. Results are presented for two wafers, both with a separation of almost-equal-to 1.38 mum between the backgate and 2DEG: one having a high carrier concentration, the other a low carrier concentration. For the high carrier concentration wafer, it is shown that up to - 7 V could be applied to the backgate at 1.2 K while maintaining leakage currents of < 1 nA. Further, in combination with a surface Schottky gate, the onset of second subband occupation was investigated as the 2DEG confining potential was altered. For the low carrier concentration wafer, it is shown that successful backgating action was maintained even when the temperature was increased to almost-equal-to 150 K.