Gate-voltage-dependent transport measurements on heterostructure field-effect transistors

Heterostructure field-effect transistors (HFET's) with a quasi-two-dimensional electron gas as a channel need further optimization with respect to transport properties such as mobility, velocity, and carrier concentration and confinement. For the control of optimization procedures, analytical test methods are required. One suitable method is described here. It is an extension of the magnetotran-sconductance mobility measurement method [9] and allows a detailed analysis of transconductance, mobility, and sheet carrier concentration in the quasi-two-dimensional channel as a function of gate bias. The theory is described and experimental results on double (quantum well) heterojunction FET's are given. The assumption that parallel Conduction in HFET's at 77 K and at room temperature has different reasons [1], [2] will be proved experimentally. The great advantage of this method is its applicability to fully processed short-channel devices; its limitation is the restriction to the low-field constant-mobility regime.