Modeling the Impact of Incomplete Dopant Ionization on Built in Potential and C–V Characteristics of GaN p–n Junctions: A SCAPS-1D Study

Incomplete dopant ionization in wide-bandgap semiconductors plays a critical role in determining carrier concentration, electrostatic properties, and overall device performance; however, its impact on GaN p–n junctions for optical photovoltaic converters (OPCs) remains insufficiently understood. In this work, SCAPS-1D simulations are employed to systematically investigate GaN p–n junctions incorporating three p-type acceptors (Mg, Zn, Be) and three n-type donors (Si, O, S) over doping concentrations of 10¹⁵–10¹⁸ cm⁻³ and temperatures ranging from 77 K to 400 K. The temperature dependence of the bandgap is described by the Varshni relation (R² = 0.9721), while dopant ionization is modeled as a function of both temperature and doping level to capture its effects on carrier distribution, the built-in potential, and capacitance–voltage (C–V) characteristics. The results reveal a pronounced reduction in junction capacitance at lower temperatures due to incomplete acceptor ionization. For a representative doping level of 5×10¹⁷ cm⁻³, the capacitance decreases from approximately 3.2 pF at 400 K to 1.5 pF at 77 K (≈53% reduction), primarily due to partial ionization of Mg acceptors, while donor species remain nearly fully ionized. These findings demonstrate that conventional models that neglect incomplete ionization significantly overestimate junction capacitance at low temperatures. Although the analysis is based on a one-dimensional framework, it provides physically consistent insight into the role of deep-level dopants and establishes a basis for future multidimensional TCAD investigations. This study highlights the necessity of incorporating incomplete-ionization effects into the design and optimization of high-efficiency, radiation-resilient GaN-based OPCs operating in extreme environments.

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