Undergraduate thesis topic: A study on GaN based Double Heterojunction Field-Effect Transistors

The increase of requirements for millimeter-wave amplifiers in wireless communication systems and radars has induced innovative developments in solid-state device structures. One emerging area had been gallium nitride (GaN)-based field-effect transistors.  Not only because of their wide bandgap and large polarization charge enabling high carrier densities but also excellent thermal dissipation, AlGaN/GaN high-electron mobility transistors (HEMTs) have been established as the best candidate for microwave power applications. While AlGaN/GaN HEMTs are successfully introduced on the market for RF amplification, slightly different device characteristics are required for power conversion. The breakdown voltage must typically be larger in the off-state than the one required for RF power amplification. The other device specifications are defined by the main drivers for replacing Si MOSFETs or IGBTs: typically, reduced specific on-resistance, higher switching frequency, smaller gate capacitance and lower leakage currents at high reverse voltages.

In order to allow for high voltage operation, instead of using AlGaN/GaN single heterostructures (SH), recently Double Heterojunction Field-Effect (DHFET) Structures have emerged. Power switching devices is benefitted from ultrathin Al-rich barrier material. Indeed, the main device requirements are high breakdown together with low ON-resistance and no current collapse at high bias conditions. Furthermore, these devices should be cost effective and operate in a normally off configuration for circuit simplicity and safety reasons.

We have seen the possibility to achieve all the mentioned features by using ultrathin-barrier-layer AlN/GaN/AlGaN double heterojunction field-effect transistors (DHFETs). We have developed an analytical expression for the variation of the sheet carrier concentration with the applied gate voltage. It is found that our expression is more accurate than others proposed previously for analytical modeling of DHFETs. Our analytical charge control model will be very helpful in modeling other characteristics of DHFETs for their higher degree of accuracy.

We have also presented GaN based DHFET device simulation results, whose aim is to define a design guideline for the layout of AlN/GaN/AlGaN and AlGaN/GaN/AlGaN DHFETs. We demonstrated how the scaling of the S–G distance is a key factor for enhancing device performance, with positive consequences on both the output current and transconductance. We have also observed the effect of x-composition of Al_xGa_1-xN buffer on transfer characteristics. Increase in x-composition reduces device transconductance and in case of AlGaN/GaN/AlGaN DHFET device, gate leakage current increases as well.