Impact of Surface Donor Traps and Aluminum Composition on 2DEG Density and Surface Potential in AlGaN/GaN HEMTs

Abstract

Surface traps profoundly influence the performance of AlGaN/GaN HEMTs. By simulating 2DEG electron concentration across a wide range of aluminum fractions (5–50 %) and surface donor trap parameters (concentration 10¹¹–10¹⁶ cm⁻², energy 0.2–1.4 eV), we demonstrate that 2DEG density saturates at roughly 8 % when donor traps shift from deep to shallow. Concurrently, the depth of the quantum well below the Fermi level reaches a plateau of about 2 %. These results, obtained through comprehensive TCAD modeling, provide a quantitative framework for designing HEMTs with optimized electron transport and surface potential control.

Introduction

AlGaN/GaN heterostructures are the cornerstone of high‑frequency and high‑power devices due to their intrinsic two‑dimensional electron gas (2DEG) formed at the AlGaN–GaN interface. This 2DEG arises from spontaneous and piezoelectric polarization charges in the AlGaN barrier, but surface donor states—ionized at ~1.42 eV below the conduction band—contribute additional positive sheet charge that is essential for stabilizing the triangular quantum well. Prior studies have examined surface traps in isolation, yet the combined influence of trap concentration, energy, and aluminum mole fraction on 2DEG formation remains underexplored. This work addresses that gap by systematically varying these parameters and analyzing their effects on surface potential, electric field distribution, and 2DEG characteristics.

Methodology and Simulation Setup

All simulations were performed with Synopsys Sentaurus TCAD L‑2016.12, calibrated against experimental data for a 30 nm Al₀.₂₈Ga₀.₇₂N barrier on a 2 µm GaN buffer (Fig. 1). The device features a 1 µm Schottky gate, 2.5 µm ungated spacing, and 150 µm width. Polarization charge was calculated using the established Al‑fraction dependent expression |σ(x)| = |2(a(0)−a(x))/a(x){e₃₁(x)−e₃₃C₁₃(x)/C₃₃(x)}+P_SP(x)−P_SP(0)| (Eq. 1). Quantum confinement was captured via the density‑gradient model, introducing the Λ correction term (Eq. 2‑3). Surface donor states were modeled as a positive sheet charge σ_D at the AlGaN surface. All simulations ran at 300 K with a 2‑dimensional grid to capture the full device physics.

Results and Discussion

2DEG Density versus Aluminum Fraction and Surface Traps

For each Al fraction, 2DEG density remains flat at low donor concentrations (Region 1). As the trap concentration rises into Region 2, the density increases linearly until a threshold where further trap activation no longer alters the 2DEG—this reflects Fermi‑level pinning by fully ionized donors. Higher aluminum content amplifies the internal polarization field, shifting the threshold to higher trap concentrations and extending the linear regime. Across all Al fractions, the transition from deep (1.4 eV) to shallow (0.2 eV) donor energy yields a pronounced increase in 2DEG density, but this effect saturates beyond ~30 % Al (Fig. 4). At 20 % Al and above, the 2DEG sheet density approaches 1–3 × 10¹³ cm⁻².

Surface Potential Modulation

Surface potential decreases linearly as donor energy deepens, thereby raising the 2DEG density. For donor concentrations > 10¹³ cm⁻², the potential varies proportionally with donor energy (Fig. 7a). Increasing Al fraction from 5 % to 50 % raises the 2DEG density from 7.8 × 10¹¹ cm⁻² to 2.75 × 10¹³ cm⁻², while the surface potential shifts by ~0.1 eV (Fig. 7b). These trends confirm that both trap characteristics and Al content jointly dictate surface potential.

Conduction Band Profile and Quantum Well Depth

Aluminum enrichment increases the conduction band offset and polarization charge, steepening the band slope in the AlGaN layer. For Al ≥ 20 %, the conduction band bends sufficiently below the Fermi level to form a robust triangular well even for deep donor traps. The depth of the well, quantified by E_F − E, saturates after 20 % Al, aligning with the observed plateau in 2DEG density (Fig. 9‑11). Drain‑source current simulations corroborate this, showing negligible current for Al ≤ 10 % and significant conduction above 20 % (Fig. 12‑13).

Conclusion

Our comprehensive TCAD study reveals that the interplay between surface donor traps and aluminum composition critically shapes 2DEG density, surface potential, and quantum well depth in AlGaN/GaN HEMTs. The 2DEG density stabilizes at ~8 % and the well depth at ~2 % beyond 20 % Al, while 5 % Al fails to support a 2DEG for deep traps. These insights provide a solid foundation for engineering HEMTs with tailored electron transport properties.

Data Availability

All data underlying these results are available upon request.

Abbreviations

• GaN – Gallium nitride
• HEMT – High‑electron‑mobility transistor
• 2DEG – Two‑dimensional electron gas
• DD – Drift and diffusion transport model
• SRH – Shockley–Read–Hall recombination model