Via‑Hole‑Length Modulation Unlocks Normally Off GaN HEMTs with 300 nm Channels

Abstract

We report a breakthrough in normally‑off AlGaN/GaN high‑electron‑mobility transistors (HEMTs) by controlling the via‑hole‑length in a multi‑mesa‑channel (MMC) architecture. Enhancement‑mode (E‑mode) devices with channel widths up to 300 nm were fabricated, demonstrating positive threshold voltages of +0.79 V (100 nm / 2 µm) and +0.46 V (300 nm / 6 µm). These devices exhibit on‑resistances lower than conventional tri‑gate nanoribbon HEMTs while maintaining high drain current and improved thermal stability. The via‑hole‑length governs surface‑pinning and lateral depletion, enabling scalable normally‑off GaN power electronics.

Background

III‑V wide‑bandgap nitrides offer unparalleled performance for high‑frequency and high‑voltage applications due to their large bandgaps, high critical fields, and high electron saturation velocities. AlGaN/GaN heterostructures form 2‑DEG channels that underpin high‑performance transistors, yet their intrinsic depletion‑mode (D‑mode) behavior necessitates complex circuitry for digital, RF, and power applications. Conventional strategies to induce normally‑off operation—recessed gates, p‑type capping, tunnel junctions, ion implantation, or rapid‑thermal annealing—often degrade device yield or introduce reliability issues.

Recent fin‑nanostructures have shown that reducing the channel width below ~100 nm shifts the threshold voltage (Vth) positively, owing to side‑wall surface‑pinning and strain relaxation. However, achieving low on‑resistance (Ron) with sub‑100 nm channels remains challenging, as gate‑width scaling adversely impacts leakage and transconductance. Our work addresses this by integrating a tri‑gate MMC structure with adjustable via‑hole‑lengths to simultaneously achieve E‑mode operation, low Ron, and robust thermal performance.

Methods

The AlGaN/GaN epilayer was grown on c‑plane sapphire by MOCVD (Nippon Sanso SR‑2000). A 600 °C GaN nucleation layer was followed by a 2 µm unintentionally doped GaN buffer, a 21.8 nm unintentionally doped AlGaN barrier (23 % Al), and a 2 nm GaN cap, all deposited at 1180 °C. Mesa isolation and periodic trench etching were performed by ICP‑RIE (BCl3/Cl2). Post‑etch crystallographic restoration used molten KOH for side‑wall smoothing, and piranha solution removed organics. Photolithography defined source, drain, gate, and contact pads. Ohmic contacts (Ti/Al/Ni/Au 30/120/20/80 nm) were annealed at 850 °C for 30 s under vacuum; gate electrodes were Ni/Au 20/80 nm. Device dimensions: gate length 2 µm, MMC width 100–500 nm, via‑hole‑length 1–6 µm, height 130 nm. No passivation was applied to the via‑hole‑length to preserve surface‑pinning effects.

Figure 1. (a) Cross‑section of the HEMT structure; (b) top‑view; (c) 3‑D representation.

Figure 2. (a) SEM of source/drain metallization; (b) OM of full device; (c) SEM of channel dimensions.

Results and Discussion

Most GaN HEMTs remain D‑mode due to the inherent 2‑DEG. Achieving normally‑off operation demands precise control of surface depletion and gate control. The side‑wall dangling bonds (~10^15 cm⁻²) produce a surface‑pinning field that depletes the channel laterally. In our tri‑gate MMC, the via‑hole‑length dictates the extent of this depletion.

Figure 3a shows the lateral depletion around the side‑walls. Transfer characteristics (Figure 3b) for LMMC = 2 µm and WMMC = 100, 300, 500 nm reveal Vth of +0.79, –1.32, and –2.18 V at VDS = 8 V, confirming the positive shift with reduced width. The effect arises from combined lateral depletion and via‑hole‑length surface bending.

Figure 3. (a) Schematic of wide vs. narrow MMC channels; (b) ID‑VG transfer curves for LMMC = 2 µm, varying WMMC.

By varying LMMC at fixed WMMC = 300 nm (Figure 4), we obtained Vth of –2.12, –1.07, and +0.46 V for LMMC = 1, 2, and 6 µm, respectively. The 6 µm LMMC device achieves E‑mode operation. Table 1 summarizes the dependence of Vth, IDSsat, and gm on both WMMC and LMMC. Notably, increasing LMMC from 0.8 to 6 µm shifts Vth from negative to positive while reducing IDSsat and gm—an expected trade‑off that is acceptable for power applications where low Ron is critical.

Figure 4. ID‑VG transfer curves for fixed WMMC = 300 nm and varying LMMC.

Compared to prior art, our E‑mode devices exhibit markedly lower Ron while maintaining acceptable drain current and transconductance, thanks to the engineered surface‑pinning and tri‑gate geometry. Thermal measurements confirm reduced Joule heating due to distributed current flow.

Conclusions

We have demonstrated that via‑hole‑length modulation in a multi‑mesa‑channel GaN HEMT allows normally‑off operation with channel widths up to 300 nm. Devices with WMMC = 300 nm and LMMC = 6 µm deliver Vth = +0.46 V, low Ron, and improved power handling, paving the way for next‑generation GaN power electronics.