AlGaN/GaN HEMTs with 803 V Breakdown: Fluorine‑Doped SiNx Passivation Yields Low Dynamic ON‑Resistance

Abstract

We introduce an AlGaN/GaN high‑electron‑mobility transistor (HEMT) that achieves a 803 V breakdown voltage (BV) and a modest 23 % increase in dynamic ON‑resistance (R_ON, D) compared to a conventional device. The key innovation is fluorine ion implantation within the thick Si₃N₄ passivation layer located between the gate and drain. Unlike fluorine implantation in the thin AlGaN barrier, this configuration places the ion peak and vacancy distribution far from the two‑dimensional electron gas (2DEG), thereby suppressing both DC and pulsed‑dynamic degradation. Additionally, the implanted fluorine extends the depletion region and raises the average electric field (E‑field) across the gate‑drain channel, which underpins the enhanced BV.

Background

GaN‑based AlGaN/GaN HEMTs are the front‑line technology for high‑power, high‑frequency, and low‑loss applications, thanks to their high critical breakdown field and exceptional electron mobility. [^9][^10][^11][^12][^13][^14] Yet, the maximum reported breakdown voltages remain well below theoretical limits, making it imperative to push BV further without enlarging device dimensions. [^15][^16] Several termination strategies—such as field plates, fluorine ion implantation, and recessed gate‑edge termination—have been explored to raise BV. [^17][^18][^19][^20][^21][^22][^23][^24]

Fluorine implantation in the thin AlGaN barrier (FBL) offers a simple process and minimal parasitic capacitance, but the proximity of the fluorine peak and vacancy distribution to the 2DEG can degrade both static and dynamic device performance. [^22][^27][^28]

In this work, we experimentally demonstrate that fluorine implantation in the thick Si₃N₄ passivation layer (FPL) preserves the integrity of the 2DEG while simultaneously optimizing the surface E‑field distribution. The resulting HEMT exhibits superior static and dynamic characteristics.

Fabrication Methods

Figure 1 illustrates the three‑dimensional layout of the FPL, FBL, and conventional (Conv.) HEMTs. All devices share identical gate dimensions: gate length L_G=2.5 µm, gate‑source spacing L_GS=1.5 µm, and gate‑drain spacing L_GD=10 µm. The epitaxial stack, grown on a 6‑inch (111) Si substrate by metal‑organic chemical vapor deposition (MOCVD), consists of a 2‑nm GaN cap, 23‑nm Al_0.25Ga_0.75N barrier, 1‑nm AlN interlayer, 150‑nm GaN channel, and a 3.5‑µm GaN buffer. Hall measurements yielded a 2DEG density of 9.5×10¹² cm^–2 and mobility of 1500 cm²/V·s.

Device fabrication began with mesa isolation via a high‑power Cl₂/BCl₃ inductively coupled plasma (ICP) etch. A 40‑nm LPCVD Si₃N₄ layer was then deposited at 780 °C/300 mTorr (NH₃=280 sccm, SiH₂Cl₂=70 sccm), achieving 3.7 nm/min. Ellipsometry measured a refractive index of 1.978 and an N/Si ratio of 1.31; HR‑TEM confirmed the amorphous crystallinity (inset, Fig. 1a).

Source and drain windows were opened by SF₆ dry etch, followed by deposition of Ti/Al/Ni/Au (20/150/45/55 nm) and rapid annealing at 890 °C for 30 s in N₂. The resulting contact resistance was 1 Ω·mm and sheet resistance 400 Ω/□.

The gate electrode was defined by Ni/Au (50/150 nm) lift‑off. A 3‑µm fluorine‑implantation window was patterned with AZ5214 resist, and ions were implanted at 10 keV with a dose of 1×10¹² cm^–2 using an SEN NV‑GSD‑HE implanter. Final annealing at 400 °C for 15 min in N₂ completed the process.

AlGaN/GaN HEMTs with 803 V Breakdown: Fluorine‑Doped SiNx Passivation Yields Low Dynamic ON‑Resistance

Results and Discussion

Figure 2 presents the secondary ion mass spectrometry (SIMS) profile of fluorine and the simulated vacancy distribution from TRIM for both the FPL (a) and FBL (b) devices. While the fluorine peak concentration and depth are similar in both cases, the vacancy distribution in the FBL device extends into the 2DEG channel, whereas in the FPL device it remains confined within the Si₃N₄ layer, far from the channel. Because vacancies near the 2DEG can trap carriers, the FPL approach dramatically reduces static and dynamic degradation.

Figure 3 shows the transfer and output I‑V curves. Compared with the Conv. HEMT, both FPL and FBL devices exhibit a slight increase in static ON‑resistance due to 2DEG depletion and reduced mobility (post‑implantation mobilities: 228 cm²/V·s for FPL, 203 cm²/V·s for FBL). At low drain bias (V_DS < 3 V), the ON‑resistance of the FPL and FBL devices is nearly identical. However, for V_DS > 3 V, the FBL device suffers from saturation drain‑current collapse because its vacancy profile reaches the 2DEG channel, whereas the FPL device maintains stable drain current.

Figure 4 demonstrates the off‑state I‑V characteristics and simulated surface E‑field distributions. The measured BVs are 803 V (FPL), 746 V (FBL), and 680 V (Conv.). Fluorine implantation between gate and drain lowers the E‑field peak at the gate edge and introduces a new peak at the edge of the implantation region, yielding a more uniform field and higher BV. The FBL device benefits from stronger field modulation but suffers from additional barrier damage, leading to a slightly lower BV compared to FPL.

Pulsed I‑V measurements were conducted to assess dynamic ON‑resistance (R_ON, D) under slow switching (3 ms pulse width, 5 ms period). Figure 5 compares the pulsed output under zero bias and a 100 V drain stress. The Conv. HEMT shows only a 12.3 % rise in R_ON, D, while the FBL device’s R_ON, D nearly doubles (98 % increase). In contrast, the FPL device exhibits only a 23 % increase, demonstrating that the vacancy distribution confined to the passivation layer effectively suppresses charge trapping.

Conclusions

We have demonstrated a high‑performance AlGaN/GaN HEMT that achieves a 803 V breakdown voltage while maintaining a dynamic ON‑resistance only 1.23 times its static value after a 100 V drain‑stress pulse. Fluorine implantation in the thick Si₃N₄ passivation layer preserves the 2DEG integrity, suppresses charge trapping, and optimizes the surface electric field, enabling a substantial BV increase from 680 V (conventional) to 803 V.