Charge carrier transport in field-effect transistors with two-dimensional electron gas channels studied using geometrical magnetoresistance effect

Sammanfattning: During the last decades, significant efforts have been made to exploit the excellent and promising electronic properties exhibited by field-effect transistors (FETs) with two-dimensional electron gas (2DEG) channels. The most prominent representatives of this class of devices are high-electron-mobility transistors (HEMTs) and graphene field-effect transistors (GFETs). Despite the relative maturity of the HEMTs and considerable efforts recently applied to develop the GFETs, a better understanding of the charge carrier transport mechanisms is required for their further development. This thesis work is focused on studying the charge carrier transport in the InGaAs/InP HEMTs and GFETs using the geometrical magnetoresistance (gMR) effect. The angular dependencies of output characteristics of the InGaAs/InP HEMTs oriented in a magnetic field (B) up to 14 T at 2 K were investigated. A strong angular dependence as a function of the B-field was identified. It was shown that the gMR effect governs the observed performance of the HEMTs, and the measured dependencies were accurately described by gMR theory. Additionally, the carrier velocity in InGaAs/InP HEMTs was studied using the gMR effect in the wide range of the drain fields at a cryogenic temperature of 2 K. The velocity peak was observed experimentally for the first time, and it was found that the peak velocity and corresponding field decreased significantly with the transverse field. The relevant scattering mechanisms were analyzed, and it was further demonstrated, that the low-field mobility and peak velocity reveal opposite dependencies on the transverse electric field, indicating the difference in carrier transport mechanisms dominating at low and high electric fields. It was demonstrated, that the mobility in the GFETs can be directly characterized and analyzed using the gMR and that the method is free from the limitations of other commonly used approaches requiring an assumption of constant mobility and knowledge of the gate capacitance. This allowed for interpretation of the measured dependencies of mobility on the gate voltage, i.e., carrier concentration, and identifying the corresponding scattering mechanisms. The charge carrier transport in the GFETs, characterized using the gMR method in combination with the drain-source resistance model, was also studied by applying a model of the quasi-ballistic charge carrier transport and transfer formalism. The charge carrier mean free path was found to be in the range of 374-390 nm. GFETs with a gate length of 2 μm were shown to have ≈ 20 % of the charge carriers moving ballistically, while at the gate length of 0.2 μm this number increases to above 60 %.