The influence of epilayer structure on the properties of quantum well HEMT devices

Quantum well (QW) HEMT devices differ from conventional single heterojunction (SHJ) HEMT devices through the inclusion of an additional confining barrier in the conduction band, located below the conducting channel, the presence of which is of interest for a number of reasons. It offers the possibility of incorporating dopant below the well in addition to above the well thereby increasing the sheet carrier density. Transport measurements on the double quantum well (DQW) structures reveal strong trends at 4.2 K which are, however, largely masked at room temperature by phonon scattering. In order to investigate whether these low temperature trends are significant for device operation at room temperature, 0.5 μm HEMT devices were fabricated on the three DQW structures and the 450 Å SQW structure. The devices were designed for operation in the millimetre waveband, with an active width of 150 μm, made up of 6×25 μm unit gates. The devices were assessed at DC, using a computer controlled autoprober, and at frequencies from 1 to 26 GHz using a HP8510 network analyser, by mounting individual chipped devices in a calibrated test fixture