Charge Control Model And Novel Structures For Gallium Nitride Based Heterojunction Transistors

Wide band-gap semiconductor material—Gallium Nitride (GaN) is a typicalrepresentative of the third generation semiconductor. The AlGaN/GaN HeterostructureField-Effect Transistor (HFET) has rapidly become a branch of the power electronicsindustry. Compared with conventional Silicon power devices, the AlGaN/GaN HFETcan normally operate in the harsh conditions, such as, high-power, high temperature andirradiation and the lower conduction loss can be obtained. Hence, in power electronicconversion system, the AlGaN/GaN HFET is chosen as one of ideal alternative devicefor conventional Silicon power devices. However, the conventional AlGaN/GaN HFETis a depletion-mode device featuring normally-on channel. Due to this normally-onproperty, its application is dramatically limited. Therefore, the normally-offAlGaN/GaN HFET with high reliability as the main issue must be solved. A vaiety oftechniques are achieved to realize the normally-off channel, such as, recessed gate,p-type GaN cap, fluorine ion implantation, selective epitaxial layer, etc. But the modeland mechanism about these techniques for the controlled channel lacks systematicresearch. So, the research to solve these problems is very important.In this desertation, based on the mechanism of the normally-off channel, theCharge Control Model (CCM) is established to propose the novel devices withnormally-off or enchancement-mode channel. So as to achieve the enhancement-modechannel and the increase of threshold voltage, the charge distribution in the barrier canbe varied to modulate the built-in potential and the2DEG concentration. Thedesertation is mainly about the CCM of HFET. The major innovations are as follows:First, based on the strucutre with single-heterojunction barrier layer, the CCM andnovel structures are proposed. The CCM is on the basis of polarization mechanism inheterojunction. According to the CCM, the expression of the rotation and2DEGconcentration are given in this dessertation. And then, the critical thickness andrelaxation factor are introduced in this CCM. The relationship between the criticalthickness, relaxation factor and threshold voltage is studied. At last, according to CCM,a Hybride-Anode Field Effect Rectifier (HA-FER) and Fin-HFET with normally-off channel are proposed and investigated.Second, based on the strucutre with double-heterojunctions barrier layer, thedouble-heterojunctions CCM is established and novel structures are proposed. ThisCCM takes the stress/strain relaxation, doping concentration, traps concentration andferroelectric polarization into account for calculating the threshold voltage. Thecharacteristics of HFETs with no doped barrier, doped barrier and ferroelectric barrierare investigated. Furthermore, based on the double-heterojunctions CCM, two kinds ofnovel enhancement-mode HFET are proposed, which are MIS-HFET with fluorionetreated barrier and HFET with ferroelectric dielectric barrier. The results show that thethreshold voltage of the Al2O3/AlGaN/GaN MIS-HFET can be enhanced from－4.8to+0.2V. The basis mechanism of CCM is verified by these results. The threshold voltageof LiNdO3/AlGaN/GaN HFET can be enhanced to be－3.9V,－2.1V and－1.1V atvaried thickness of the LiNdO3barrier. The theoretical values of threshold voltage agreewith the experimental data.Third, for extending the double-heterojunctions CCM, the multi-heterojunctionsCCM is established based on the structure with multi-heterojunctions barrier. Thedoping concentration, traps concenstration, polarization charge, etc. in the barrier layerare taken into account in this model. The general form of the threshold voltage is givenfor designing the enhancement-mode HFET.Fourth, based on CCM, the AlGaN/GaN HFET with bi-blocking capability isproposed. It features lower forward voltage drop and lower power consumption. Itsshort gate effect is also investigated. Finally, the integration solution for thenormally-off device is proposed, viz., the depeletion-mode AlGaN/GaN HFET can becontrolled by the Si-based NMOS as a switch.