Mar 23 (Wed) @ 1:00pm: ”Evaluating the MIS- and Schottky- gate structures for N-polar GaN-HEMTs,” Wenjian Liu, ECE PhD Defense

Date and Time
Zoom Meeting –


GaN-based high electron mobility transistors (HEMTs) and monolithic microwave integrated circuits (MMICs) have emerged as a leading technology for power amplification at high frequency. N-polar GaN based HEMTs are very promising to achieve high output power and high efficiency for solid-state millimeter wave power amplifiers. The advantages come from both the GaN material, and the polarization engineering enabled by the N-polar orientation. Harnessing these advantages, Romanczyk et al. reported a record-high 8.84 W/mm power density at 94 GHz from a N-polar GaN deep-recessed HEMT with a SiN gate dielectric.

To further boost the high-frequency and high-power performance in N-polar GaN HEMTs, the improvement of the gate structure is critical. This dissertation is devoted to the evaluation of gate structures on N-polar GaN including MIS- and Schottky structures. The first part of the evaluation is conducted through the MIS-capacitor and Schottky diode structures to understand the interface properties. The final part of the evaluation is the application of these structures in N-polar GaN deep recess HEMTs to improve performance at 94 GHz.

A series of MIS- capacitors consisting of SiN and AlSiO with different thicknesses on N-polar GaN have been fabricated to investigate their interface states. An improved model combining the effects from interface states and bulk hole traps is proposed to extract the interface state density accurately. Based on the model, the Dit can be obtained by extrapolating the trap density to a zero-thickness dielectric. The average Dit value and the bulk trap parameters is thereby quantified. The results, model and analysis presented provide new insights into studying Dit of various dielectrics on GaN and other wide-bandgap semiconductors.

Schottky barrier diode using ruthenium (Ru) deposited by atomic layer deposition (ALD) on N-polar GaN was investigated. The Schottky diodes showed near-ideal thermionic current behavior under forward bias and reverse bias at various temperatures. The barrier height was extracted to be 0.77 eV at room temperature. The combination of the 0.77 eV barrier and thermionic current characteristic results in < 2 µA/cm2 reverse current at -5 V, which is a record-low value for N-polar GaN Schottky diodes.

ALD Ru gate was adopted into the N-polar GaN deep recess MIS- and Schottky HEMTs. The ALD Ru effectively fills the narrow T-gate stems aiding the realization of shorter gate lengths with lower gate resistance than in prior work. For the ALD Ru MIS-HEMTs, the gate length is scaled down to 48 nm resulting in the demonstration of a record-high 8.1 dB linear transducer gain measured at 94 GHz by load-pull. This increased gain enabled a record 33.8% power-added efficiency (PAE) with an associated output power density (PO) of 6.2 W/mm at 94 GHz. The Ru/N-polar GaN Schottky-HEMT operating at W-band has also shown good large signal performance. The fabricated Schottky HEMT with LG of 60 nm has a high peak extrinsic transconductance of 798 mS/mm with a gate leakage in pinch-off below 3 x 10-4 A/mm at VDS = 5V. The Schottky-HEMT showed a high PAE of 27.1% and PO of 4.87 W/mm at 94 GHz and VDS,Q = 16V. The peak PAE is 31.8% with associated PO of 3.31 W/mm at VDS,Q = 12V.


Wenjian Liu is a Ph.D. candidate from the ECE Department at the University of California, Santa Barbara. His research focuses on N-polar GaN-based high-frequency and high-power electronics, under the supervision of Prof. Umesh Mishra. He obtained his Bachelor’s degree and Master’s degree from Tsinghua University, China in 2015 and 2017, respectively. His research attempts to understand the physics, materials, devices and circuits in a systematic way.  

Hosted by: Prof. Umesh Mishra

Submitted by: Wenjian Liu <>