AlGaN/GaN High Electron Mobility Transistors with a p-GaN Backgate Structure
Authors: Lin Wei-Tse , Lin Wei-Chun , Zhong Yi Nan , Hsin Yue-Ming 
Published: February 2020

GaN based materials are widely used for power devices. AlGaN/GaN heterostructure is able to form high mobility 2DEG channel by its spontaneous and piezoelectric polarization at the AlGaN/GaN interface. Therefore, AlGaN/GaN HEMT is a promising candidate for next generation power device because of its high breakdown voltage, high current density and low on-state resistance (Ron). For efficient power switching applications, the E-mode operation with positive threshold voltage and low leakage current are required. Many efforts have been devoted to make AlGaN/GaN HEMT to achieve E-mode operation, such as gate recess, F-ion implements and p-GaN gate structures.

In this study, a p-GaN backgate layer is applied to AlGaN/GaN HEMTs. Additional control ability under the channel layer is observed by applying bias at the p-GaN layer. The device can be turned off easily due to the p-GaN layer under channel layer, which depletes carriers in channel under suitable bias. Simulation and measurement results show the shift of threshold voltage toward positive for possible E-mode operation when backgate bias is negative.

The simulation of AlGaN/GaN HEMT with a p-GaN backgate layer was performed with Silvaco TCAD tools including device structures, band diagrams, electron concentrations and ID-VG characteristic. A 30 nm p-GaN backgate layer was implemented under the 30-nm GaN channel with p-type doping of 2 ´ 1017 cm-3. The AlGaN/GaN HEMT epitaxial wafers on low-resistivity Si (111) substrates were grown by MOCVD. Device fabrication started with p-GaN activation in nitrogen atmosphere at 700°C for 15 minutes. Mesa isolation was etched by ICP down to the buffer. The ohmic contact was made by the deposition of Ti/Al/Ni/Au (25/125/45/55 nm) and annealing at 875°C in a nitrogen atmosphere for 40 s. The contact to p-GaN layer was etched by ICP. Backgate metal was made by deposition of Ni/Au (20/20 nm) and annealing at 500°C in an oxygen atmosphere for 5 minutes. Ni/Ti/Al/Ti/Au (30/25/250/25/200 nm) metals were used to form front gate (FG). All the devices under test are with a gate-source distance of 4 μm, a gate length of 2 μm, and a gate-drain distance of 10 μm.

The p-GaN backagte layer with bias is able to adjust the fermi levels in the quantum well thus the 2DEG concentration. Therefore, the threshold voltage shifts to positive values while applying negative backgate bias. Electrons are localized in the GaN channel layer when negative backgate bias was applied. This result is to justify the reduction of leakage current because leakage path through buffer layer is confined.

From the simulated transfer characteristics of AlGaN/GaN HEMT with a p-GaN backgate under different backgate bias (VBG
). It is clearly to observe that the positive shift of threshold voltage (VTH) is due to the negative VBG
. The simulated results showed the positive shift of VTH which was 5 V from VBG
of 0 V to -14 V. Therefore, it possible to operate HEMT in either D-mode or E-mode operation by adjusting VBG
. The measurement results showed the positive shift of VTH which was 0.55 V from VBG
of 0 V to -14 V, with associated off-state leakage current reduction of 73.1%. However the shift of VTH is not as significant as in the simulation, the major reason is due to the bad p-GaN contact.

The impact of p-GaN backgate structure in this study on the DC characteristics of AlGaN/GaN HEMTs was investigated. AlGaN/GaN HEMTs with a p-GaN backgate shows an additional control capability in 2DEG channel. Therefore, threshold voltage is adjustable and leakage current is improved by backgate bias. This extra gate control under channel layer is useful for circuit flexibility to achieve both D-mode and E-mode operations in the power device circuit design.