Selectively dopedn-Al_xGa_1−xAs/GaAs heterostructures with high-mobility two-dimensional electron gas for field effect transistors

Abstract

In selectively dopedn-Al_xGa_1−xAs/GaAs heterostructures with high-mobility two-dimensional electron gas (2 DEG) at the heterointerface a second conductive channel exists, if the Al_xGa_1−xAs layer is not totally depleted from free carries. The occurrence of parallel conductance has a deleterious effect on the performance of high-electron mobility transistors (HEMTs) fabricated from this material. Although in principle computable, parallel conductance depends on a large number of design parameters to be chosen for the heterostructure, which are additionally affected by the presence of deep electron traps inn-Al_xGa_1−xAs of composition 0.25<x<0.35. Capacitance-voltage, Hall effect, and transverse magnetoresistance measurements in the temperature range 4–300 K were used to detect the undesired parallel conductance and to demonstrate its effect on the result of these evaluation techniques. In addition, the significant influence of parallel conductance on the dc properties of HEMTs fabricated from selectively dopedn-Al_xGa_1−xAs/GaAs heterostructures is shown.