Current Research

Vertical GaN Diodes and MOSFETs

Quasi- and Fully-Vertical GaN p-i-n Diodes on Si

Fully- and quasi-vertical GaN-on-Si p-i-n diodes were grown and fabricated. A record high Baliga figure of merit of 304 and 152 MW/cm2 is reported for fully-and quasi-vertical GaN-on-Si p-i-n diodes, respectively. A comprehensive comparison has been made between the two kinds of diodes in regard ON-resistance, breakdown voltage, and switching performance.

Fig. 1. Schematic of (a) epitaxial structure of p-i-n diodes grown on (111) Si, (b) quasi-vertical p-i-n diodes on original substrate (with no vertical overlap of two electrodes), and (c) fully-vertical p-i-n diodes with GaN thin film transferred onto a (100)Si receiver (with vertical overlap of two electrodes).

Fig. 2. (a)–(e) Forward bias I–V characteristics of fully- and quasi-vertical diodes with diode mesa diameter ranging from 60 to 300 μm (black symbols); the corresponding specific ON-resistance for all the current level is also plotted (blue symbols). Very low specific RON less than 1 mΩ•cm2 could be readily observed for fully-vertical diodes at high current injection. (f) Ideality factor for two kinds of diodes with a diameter of 150 μm.

Related publications:

  1. Xinbo Zou, Xu Zhang, Xing Lu, Chak Wah Tang, and Kei May Lau, " Fully Vertical GaN p-i-n Diodes Using GaN-on-Si Epilayers ", IEEE ELECTRON DEVICE LETTERS, Vol. 37, No. 5, May 2016

  2. Xu Zhang, Xinbo Zou, Xing Lu, Chak Wah Tang, and Kei May Lau, " Fully- and Quasi-Vertical GaN-on-Si p-i-n Diodes: High Performance and Comprehensive Comparison ", IEEE TRANSACTIONS ON ELECTRON DEVICES,  Vol. 64, No. 3, 2017

Vertical GaN trench MOSFETs

The influence of p-GaN body doping concentration on the ON-state performance of vertical GaN trench MOSFETs is investigated. Decreasing the p-GaN body doping concentration leads to an enhanced maximum drain current (ID,max), reduced specific on-resistance (RON,sp), but also a decreased threshold voltage (Vth), suggesting that the p-GaN doping plays an important role in balancing the Vth, RON,sp and ID,max in vertical GaN trench MOSFETs. Resulting from the tuning of Mg concentration in the p-GaN, high ON-performance including a high ID,max of 2.8 kA/cm2, a low RON,sp of 0.87 mΩ·cm2, a large Vth of 4.8 V in a quasi-vertical GaN trench MOSFET on sapphire with a 2.5-μm-thick drift layer is demonstrated, while maintaining a breakdown voltage of 273 V.

Fig.3. (a) 3-D cross-sectional schematic of fabricated quasi-vertical GaN trench MOSFETs. (b) Secondary ion mass spectrometry (SIMS) depth-profiling results of Mg concentration in the p-GaN layer of different samples. (c) Cross-sectional SEM images of the gate trench region of a fabricated device.

Fig. 4. (a) Transfer characteristics (b) output characteristics of five samples with different Mg concentrations of p-GaN.

The ON-state device performance was effectively improved by reducing MOS channel interface charges with a piranha cleaning process prior to the gate dielectric deposition. For the OFF-state, the breakdown voltage of the device was greatly enhanced via suppressing the electric field in the gate dielectric near the bottom of the gate trench with a thick bottom dielectric process.

Fig. 5. Cross-sectional schematics of fabricated quasi-vertical GaN trench MOSFETs (a) without a thick bottom dielectric (TBD) process and (b) with a TBD process. ”S”, ”D”, ”G”, and ”B” refers to “Source”, ”Drain”, “Gate”, and “Body”, respectively. Cross-sectional SEM images of the gate trench region of devices (c) without TBD and (d) with TBD.

Step-graded channel doping can achieve an improved trade-off between ION, RON, and Vth than uniform channel doping for GaN trench MOSFETs.

Fig. 6. Experimental results of (a) RON,sp vs Vth and (b) ID,max vs Vth of all fabricated seven samples. (c) Simulation structures of device with two-step graded-doped channel. The simulation results are also included in (a) and (b)

GaN quasi-vertical trench MOSFETs grown on Si substrate with ON-current exceeding 1 A

Fig. 7. (a) Top-view layout of the large-area trench MOSFET with multiple-finger design. (b) transfer, (c) output, and (d) OFF-state I-V curves of the large-area vertical MOSFET on sample C. (e) RON,sp versus VBR benchmarking of the GaN-on-Si trench MOSFETs in this work with other reported GaN vertical transistors.

Related publications:

  1. Renqiang Zhu, Huaxing Jiang, Chak Wah Tang, and Kei May Lau, “Effects of p-GaN Body Doping Concentration on the ON-State Performance of Vertical GaN Trench MOSFETs”, IEEE Electron Device Letters, Vol. 42, No. 7, pp.970-973, 2021

  2. Renqiang Zhu, Huaxing Jiang, Chak Wah Tang, and Kei May Lau “Enhancing ON- and OFF-State Performance of Quasi-Vertical GaN Trench MOSFETs on Sapphire with Reduced Interface Charges and a Thick Bottom Dielectric”, in IEEE Electron Device Letters, vol. 43, no. 3, pp. 346-349, March 2022.

  3. Renqiang Zhu, Huaxing Jiang, Chak Wah Tang, and Kei May Lau “Vertical GaN Trench MOSFETs with Step-graded Channel Doping”, in Applied Physics Letters, vol. 120, issue 24, 242104, 2022.

  4. Renqiang Zhu, Huaxing Jiang, Chak Wah Tang, and Kei May Lau “GaN quasi-vertical trench MOSFETs grown on Si substrate with ON-current exceeding 1 A”, accepted by Applied Physics Express,2022.