Do GITs Have An Edge Over HEMTs?
Conventional GaN-based High Electron Mobility Transistors (HEMTs) are “normally-on” devices. This means that they have a negative threshold voltage. However, for intrinsic safety in electronic circuits, the use of normally-off devices is preferable. The concept of a "normally-off" transistor has been recently proposed by researchers from Panasonic Corporation. This company is working in close cooperation with University of Padova (M. Meneghini, G. Meneghesso and E. Zanoni) to investigate the incredible performance of this new type of gallium nitride (GaN) Gate-Injection Transistor (GIT). Since GITs are GaN-based power transistors with low on-state resistances and high breakdown voltages, they are good candidates for the replacement of currently used Si-based power transistors. GITs are expected to remarkably reduce the energy consumption in power conversion systems, since they have inherently low on-state resistance and low switching losses. This kind of device has a p-type AlGaN gate. The reason for this, explains Matteo Meneghini, from the University of Padova, is that this ensures a significant decrease in the electron concentration in the 2-Dimensional Electron Gas (2DEG) under the gate. This is the key to making normally-off operation possible. Despite the incredible potential of GaN-based GITs (Uemoto et al, IEEE TED December 2007), a detailed description of the electrical and optical properties of the devices was necessary to completely understand the different operating regimes of these devices. The initial idea of University of Padova was to use spectrally and spatially resolved electroluminescence measurements to demonstrate the role of holes in determining the electrical properties of GITs. A recent work, described in Applied Physics Letters, demonstrates that an interesting feature of GITs is the possibility of modulating the conductivity of the channel through the injection of holes from the p-type gate to the channel. Injected holes can recall an equal number of electrons in the region immediately under the gate; these electrons cannot move towards the gate, due to the presence of the AlGaN/GaN heterointerface, and are therefore subject to the electric field present into the channel that attracts them towards the drain. As a result of the injection of holes, the drain current can be significantly increased. This can result in an increase in transconductance for gate voltage levels greater than the turn-on voltage of the p-AlGaN/i-AlGaN junction. This paper, however, suggests another understanding of the electrical and optical operation modes of GITs is now possible. It allows one to create models for the simulation of these devices, to improve the design of device structure in order to achieve the best trade-off between threshold voltage and device transconductance, and to design applications capable of exploiting the possibility of having devices with two transconductance peaks. Electroluminescence is a powerful technique for the investigation of the characteristics of GaN-based transistors. In normal HEMT transistors, EL is emitted in the region between gate and drain, due to the presence of hot-electrons that are strongly accelerated by the gate-drain electric field. In this paper, the authors have described the presence of Gate-Source luminescence in GaN-based transistors. The emitted light had a peak at 364 nm corresponding to the optical bandgap of GaN. This, says Meneghini is one of the very first reports of electron-hole recombination in GaN-based transistors. The group says a surprise was the observation of hole-related luminescence ; this was the starting point for developing a theory on the electrical operation principles of GITs. Panasonic is now working towards the demonstration of different applications of GITs in power systems. Lately, Panasonic demonstrated the world first monolithic GaN inverter IC for a motor drive with high efficiency.
Electroluminescence micrographs measured under different bias conditions on one of the analyzed GITs. At low gate voltage levels, EL signal is emitted in the region between gate and drain, due to the presence of hot electrons, accelerated by the high gate-drain field. On the other hand, for high gate voltages, the EL signal is emitted between gate and source. This is one of the very first reports of gate-source emission. Emission is due to the recombination of the holes injected from the gate with the electrons present in the channel
