Effect of Intrinsic Layer Thickness on the Power Conversion Efficiency of GaN p-i-n Betavoltaic Cells

The effect of unintentionally doped GaN (UID-GaN) layer thickness on depletion characteristics and power conversion efficiency (PCE) in GaN p-i-n betavoltaic cells was investigated by fabricating devices with UID-GaN layer thicknesses of 0.3 µm and 2 µm. The penetration and energy-deposition profiles of 17 keV electrons in GaN, representing the mean energy of the Ni-63 beta spectrum, were analyzed by Monte Carlo simulation to determine an appropriate UID-GaN layer thickness range, and the devices were characterized under the same 17 keV electron-beam condition. The depletion ratios of the 0.3 µm and 2 µm devices were calculated to be ~82% and ~12%, respectively. Despite a smaller fraction of electron-hole pair (EHP) generation occurring within the UID-GaN layer, the 0.3 µm device achieved a higher fill factor (FF = 0.77 vs. 0.56) and PCE (4.11% vs. 3.23%) compared to the 2 µm device. TCAD simulations at two additional intermediate thicknesses (0.8 and 1.3 µm) further showed a monotonic decrease in normalized PCE at the sampled thicknesses. These results indicate that the depletion ratio is a key design parameter governing the trade-off between EHP generation and carrier collection in GaN p-i-n betavoltaic cells.