Physics-Based Modeling and Charge Density Saturation in GaN/AlGaN MOS-HEMTs

Ashkhen Yesayan, Farzan Jazaeri, Jean-Michel Sallese Swiss Federal Institute of Technology in Lausanne (EPFL) Physics-Based Modeling and Charge Density Saturation in GaN/AlGaN MIS-HEMTs 12 December 2025 Electronic Device Modeling Group Head: Prof. Jean-Michel Sallese Project: 200021 213116 Outlines: ❑Introduction to GaN MIS-HEMTs ❑Increasing demand for advances in MIS-HEMTs ❑charge density saturation in undoped GaN MIS-HEMTs ❑Analytical model of charge saturation ❑Model validation with COMSOL simulations ❑Conclusion Metal-Insulator-Semiconductor –High Electron Mobility Transistor (MIS-HEMT): typical structure vertical cut of MIS_HEMT structure Energy band diagram Quantum well n2D is two-dimensional electron gas density and  pol is polarization charges density The polarization induced fixed charge induces a 2D electron gas (2DEG) in the quantum well (QW) created by the conduction band discontinuity at the AlGaN/GaN interface. AlxGa1-xN GaN Insulator Metallic Gate d tins ------------------  pol n2D +++++++++++++ ❑Increasing demand for advances in MIS-HEMTs ✓As a result, the wireless communication industry has gradually increased operating frequencies to meet this demand! What are driving forces today? •High data-rates •Low energy consumption ✓5G products are being rolled out in sub-6GHz frequency bands, and products targeting 28/39GHz are already showcased. The remaining challenges are the reduced output power and lower efficiency of most technologies at these frequencies. GaN semiconductors are the best candidates for these technologies due to their higher mobility, charge density and breakdown voltage. However, the gate leakage current remains an issue for HEMTs. In order to suppress the gate leakage current, the introduction of an additional dielectric layer in form of MIS-HEMT offers significant improvement in terms of gate leakage. The insulating layer beneath the gate enables operation at high gate voltages, effectively mitigating conventional gate leakage issues. One would expect that the 2DEG density under high gate voltages would continuously increase, however.. Operation beyond the charge neutrality point leads to the accumulation of mobile charges under the gate insulator. These charges screen the gate voltage, resulting in the saturation of the 2DEG density in the quantum well. The barrier thickness and aluminum content determine the maximum achievable, n2D_max for the given structure This effect was first observed and modelled by our modelling group [1]. 1. A. Yesayan, F. Jazaeri, B. Parvais and J. M. Sallese, "Intrinsic Limitation of 2DEG Modulation in GaNMISHEMT," in IEEE Trans. Electron Devices, doi: 10.1109/TED.2025.3610338. (2025). ❑Core analytical model: channel charge saturation Charge neutrality reaches when n2D=  pol AlxGa1-xN GaN Insulator Metallic Gate d tins ------------------  pol n2D +++++++++++++ ❑Core analytical model 𝑉𝐺∗=𝜀𝑏 𝐶𝑖𝑛𝑠 𝐹𝑏_𝑠 + 𝜑𝑠 𝜑𝑠= 𝑈𝑇𝑙𝑛 𝐹𝑏_𝑠2− 𝐶𝑜 α+ 𝑉 𝑜 n2D=f (VG, VD) 𝐶𝑜=𝑞𝑛2𝐷 − 𝜎𝑝𝑜𝑙 𝜀𝑏 2 − α exp − 𝑉 𝑜 𝑈𝑇 𝐹𝑏_𝑠 = 𝐶𝑜𝑐𝑜𝑡ℎ 𝑎𝑐𝑜𝑡ℎ 𝑞𝑛2𝐷−𝜎𝑝𝑜𝑙 𝜀𝑏𝐶𝑜−𝐶𝑜 2𝑈𝑇𝑑 𝑓𝑜𝑟 𝐶𝑜>0 𝐹𝑏_𝑠 = −𝐶𝑜𝑡𝑎𝑛 𝑧 −𝐶𝑜 2𝑈𝑇+ atan 𝑞𝑛2𝐷−𝜎𝑝𝑜𝑙 𝜀𝑏−𝐶𝑜𝑓𝑜𝑟 𝐶𝑜<0 𝑞𝑉 𝑜= Δ𝐸𝑐−𝐸𝐺𝑏 2+𝑘𝑇 𝑙𝑛 𝑁𝑉𝑏 𝑁𝐶𝑏 − 𝐸𝐹 𝐸𝐹=𝑘𝑇 𝑙𝑛 𝑒𝑥𝑝 𝑛2𝐷 𝐷𝑂𝑆 𝑘𝑇 − 1 + 𝛾 𝑛2𝐷 2 3 Poisson-Boltzmann equation in the barrier region has been analytically solved and following charge density –voltage dependencies have been derived: 𝐸𝐹(𝑛2𝐷) Fermi level 𝑉 𝑜(𝑛2𝐷) Potential at heterointerface 𝐶𝑜(𝑛2𝐷) 𝐹𝑏_𝑠(𝑛2𝐷) Surface electric field 𝜑𝑠(𝑛2𝐷) Surface potential VG(𝑛2𝐷) Gate potential Calculation method The outcomes of presented model We developed a physics-based analytical model that explains how electrons accumulating at the insulator/AlGaN interface screen the electrostatic potential and thereby modify the charge density in the AlGaN/GaN MIS-HEMT quantum well. The presented method enables quick and explicit estimation of the asymptotic (maximum) 2DEG density ( nmax ) at given system parameters. An accurate evaluation of charge saturation is critical for distinguishing the respective contributions of the ungated access regions and charge saturation to drain current saturation at high gate voltages. This model can be further expended for calculations of gate leakage current in HEMTs. AlxGa1-xN GaN Insulator Metallic Gate d tins Symbol Definition qAbsolute charge of electron 0Dielectric permittivity of vacuum kbBoltzmann's constant Symbol Definition Magnitude used in calculations xAluminum content 0.3, 07 pol Polarization charge density (varying with x) 0.016, 0.056 (C/m2) [4] TTemperature 300 (K) UTThermal voltage 0.026 (V) AlGaN Relative permittivity of AlGaN 3.410-16 (cm-3) tins Insulator thickness 5 (nm) ins Insulator relative permittivity 7 (for SiN4) Cins 𝐶𝑖𝑛𝑠 = 𝜀𝑖𝑛𝑠 𝜀0/𝑡𝑖𝑛𝑠 0.01 (F/m2) dThickness of AlGaN layer 320 (nm) DOS Two-dimensional density of states in GaN 91019 (eV-1cm-2) Parameter for ground subband 2.0910-31 (CVm4/3) ECConduction band offset (depends on x) 0.55, 1.38 (eV) 𝑛𝑖_𝐴𝑙𝐺𝑎𝑁 Intrinsic concentration of AlGaN ~310-25 (cm-3) EAlGaN Band gap of AlGaN (depends on x) 4.15, 5.26 (eV) 𝛼 = 2𝑞𝑈𝑇𝑛𝑖_𝐴𝑙𝐺𝑎𝑁/𝜀𝑖_𝐴𝑙𝐺𝑎𝑁 4.0610-20 (V2/m2) Variable parameters used in calculations The same set of parameters were used in COMSOL Multiphysics and MATHCAD calculations.