We present a comprehensive experimental and TCAD Sentaurus simulation study of p-Si/n-CdS heterojunctions, focusing on the effects of temperature, doping, and incomplete ionization on the electro-optical performance from cryogenic to elevated temperatures (20–800 K). CdS thin films exhibit a sharp optical absorption edge near 500 nm, corresponding to a bandgap of 2.42 eV, while TCAD simulations reveal that Eg(Si) decreases from 1.1697 eV at 20 K to 0.9516 eV at 800 K, and Eg(CdS) decreases from 2.5586 eV to 2.1625 eV, narrowing the band offset (ΔEg) from 1.3889 eV to 1.2109 eV (~ 13%). Electrical characterization shows that increasing doping from \(N_{A} \, = \, N_{D} \, = \, 1 \cdot 10^{16} cm^{ - 3}\) to 1 × 1017 cm −3 1 × 1017 cm−3 raises the built-in potential from 0.55 V to 0.73 V (33% increase) and triples the maximum junction capacitance from ~430 pF/cm2 to ~1350 pF/cm2. Under incomplete ionization, low-temperature capacitance at 20 K drops to ~22 pF/cm2 (45–50% reduction compared to full ionization), whereas at 300 K the difference is < 5% (~ 48 pF/cm2). The reverse saturation current is dominated by p-Si, increasing over two orders of magnitude from ~1.2 × 10−13 A at 50 K to ~2.5 × 10−11 A at 350 K, while n-CdS contributes < 5%. CdS film surfaces exhibit RMS roughness of 50–80 nm and grain sizes of 120–320 nm, enhancing light absorption and carrier separation. The study demonstrates that neglecting temperature-dependent ionization and dopant freeze-out leads to significant errors in predicting depletion width, capacitance, and carrier transport. These results provide quantitative guidance for optimizing doping, built-in potential, and thermal operation, offering a predictive framework for designing high-efficiency p-Si/n-CdS photodetectors, solar cells, and optoelectronic devices across 20–800 K.
