GaN High Electron Mobility Transistor with Floating Gate for Accurate Threshold Voltage Control
Authors: Kosaka Hayato , Iwata Hiroki , Watariguchi Yudai , Shirota Riichiro , Amemiya Yoshiteru , Takatani Shinichiro , Suwa Tomoyuki , Teramoto Akinobu 
Published: September 2023

Gallium nitride (GaN) high electron mobility transistors (HEMTs) have high electron mobility, high breakdown voltage, and low on-resistance, and are expected to operate as power devices. When GaN HEMTs are normally-on devices, drain current flow even for 0-gate-voltage. Thus, the normally-off operation is essentially required to the power devices from a fail-safe standpoint. Figure A shows a schematic of the structure, which is constructed with GaN HEMT including floating gate and the injection gate. The block oxide and tunnel oxide were formed between control gate and floating gate, and between injection gate and floating gate, respectively. It is impossible to inject electrons from the channel to the floating gate by the applying voltage to the substrate because AlGaN layer is located on the GaN layer. We introduced the injection gate for injecting electrons into the floating gate, and then the normally-off operation is obtained by the injecting electrons into the floating gate. The injecting electrons decrease the floating gate potential and form the depletion region under the gate oxide. Because the channel is the same as that of a conventional GaN HEMTs during on-operation, the high electron mobility of GaN HEMTs can be maintained, and then the on-operation behaviors are the same as conventional GaN HEMTs except threshold voltage. In this study, we demonstrate the normally-off operation of the fabricated GaN HEMTs with floating gate.
Figure B shows the microscopy image of the fabricated device where the gate length, gate width and thickness of gate oxide are 2 μm, 110 μm and 20 nm, respectively. In floating gate device, the coupling ratio between the induced voltage of floating gate and the applied voltage of control gate is important. To increase the coupling ratio, we extended the overlap areas of floating gate and control gate to the drain side. The area of the control gate and the thickness of block oxide are set to be 3600 μm2 and 50 nm, respectively. Then, the coupling ratio becomes 91%, and then the induced voltage of floating gate is almost the same as the applied voltage of control gate. High-quality oxides are required in our devices for block oxide, tunnel oxide and gate oxide, however high-quality oxide films cannot be deposited by the thermal oxidation processes for GaN and polycrystalline Si. To form high-quality dielectric films, we used the microwave exited Plasma Enhanced Chemical Vapor Deposition (PECVD) [1][2][3].
The Id-Vcg characteristics before and after the electron injection into the floating gate are shown in figure C. To change the threshold voltage in positive value, the voltages of control gate and injection gate are determined for the electron injection. The threshold voltage before the electron injection is -10.7 V and the electron injection increases the threshold voltage, resulting in a positive threshold voltage of 3.2 V. This voltage is sufficiently high voltage for normally-off operation. The Id-Vd characteristics after electron injection are shown in the figure D. It is found that the electron mobility is 2100 cm2/Vs, which is the similar value of 1500~2250 cm2/Vs in the reported conventional GaN HEMT [4][5][6] and this indicates that the device can be maintained high electron mobility as same as the conventional GaN HEMT.
We fabricated the GaN HEMTs with floating gate, which realize the normally-off and high electron mobility. The threshold voltage can be controlled to be 3.2 V and the electron mobility of 2100 cm2/Vs is achieved. It is indicated that normally-off GaN HEMT device with high electron mobility can be demonstrated.
[1] T. Ohmi, et al., J. Phys. D: Appl. Phys., 39, p.R,2006.
[2] H. Tanaka, et al., Jpn. J. Appl. Phys. 42, p. 1911, 2003.
[3] H. Kambayashi, et al., Solid-State Electronics, 54, p. 660
[4] J.-T. Chen, et al., Appl. Phys. Lett. 106, 251601, 2015.
[5] K. Shinohara et al., IEEE Trans. Electron Devices, 60, p. 2982, 2013.
[6] X. Ding et al., CES Transaction on Electrical Machines and Systems, 3, p54.

Figure 1