This paper presents a quantitative study on the electrical properties of HgCdTe epitaxial materials with ultra-low background carrier concentrations, to support the development of fully depleted infrared structure. Conventional Hall measurements at 77 K reveals a distinct thickness-dependent carrier concentration in undoped mid-wavelength HgCdTe (Cd composition ≈ 0.29–0.32): the measured Hall concentration decreases from >1×10¹⁴ cm^–3 to ~5×10¹³ cm^–3 as the epilayer thickness increased from 5 μm to 13 μm. This phenomenon is attributed to surface states induced by oxidation and dangling bonds, which distort the standard single-layer Hall effect analysis and lead to inaccurate bulk parameter extraction.

To decouple surface and bulk contributions, a double-layer Hall model is developed, where the effective Hall concentration n_eff is the combined response of a uniform bulk layer and a near-surface mixed layer. Assuming equal carrier mobilities in both layers, the model simplifies to n_eff = n₁ + n_surface/d, predicting a linear correlation between n_eff and 1/d. Differential Hall measurements with ~1 μm stepwise etching precision are performed on four ultra-low-background HgCdTe samples, and the experimental results confirm this linear relationship, validating the model. The intrinsic bulk background concentrations extracted from fitted line intercepts ranges from 8×10¹² to 2×10¹³ cm^–3, comparable to international state-of-the-art values (e.g., Teledyne). Slope variations among samples reflect surface microstate differences, associated with chemical etching, dislocation density, and compositional uniformity.

The model is further verified by In-doped HgCdTe samples, with bulk concentrations derived from the model matching secondary ion mass spectrometry (SIMS) results within experimental error. Two-dimensional numerical simulations of mid-wavelength fully depleted HgCdTe devices show that a 5μm depletion width is achieved at reverse bias >0.1 V for 2×10¹³ cm^–3 doping, and >0.5 V for 5×10¹³ cm^–3. These results confirm that the HgCdTe materials, with reproducible ultra-low background concentrations, provide a material basis for fabricating HgCdTe fully depleted structures.