Design of insulator/AlGaN structures in MIS AlGaN/GaN HFETsfor higher device performance

摘要

To improve the device performance of GaN-based Metal-Insulator-Semiconductor (MIS) heterostructure field-effecttransistors (HFETs), the layer design of MIS AlGaN/GaN HFETs has been investigated by considering theinsulator/AlGaN structures as the total barrier layers of MIS HFETs. Since deposition of insulators on AlGaN hasproven to change electrical properties of the heterostructures as the increase in AlGaN thickness does, considering theinsulator/AlGaN structures as the total barrier layers is effective and indispensable in designing the MIS AlGaN/GaNHFETs. With an increased insulator thickness and decreased AlGaN thickness, where the gate capacitance is keptconstant, a high transconductance was successfully obtained that was equivalent to that of HFET, in addition to thereduced gate leakage current in MIS HFETs. Moreover, Al_2O_3-based MIS HFETs were revealed to exhibit superior DCcharacteristics over conventional Si3N4 MIS HFETs, which can be ascribed to larger bandgap energy of Al203 thanSi3N4. Channel doping design has also been proposed to incorporate into MIS HFETs to modulate the threshold voltageaiming at the same application of depression mode as conventional HFETs. In a MIS HFET with channel doping design,a significantly reduced gate leakage current was obtained with a transconductance and threshold voltage that are keptclose to typical values of HFETs, indicating that the device has a potential to replace conventional HFETs. Thus, designof insulator/AlGaN structures and incorporation of channel doping design are the key, with which MIS HFETsexhibiting a low gate leakage current and high transconductance with a desired threshold voltage can be realized.