The lattice-matched XBn structures of InAsSb, grown on GaSb substrates exhibit high crystal quality, and can achieve extremely low dark current at high operating temperatures (HOT). Its superior performance is attributed to the unipolar barrier, which blocks the majority carriers while allowing unhindered hole transport. To further analyze the energy band and carrier transport mechanisms of the XBn unipolar barrier structure, this paper systematically investigates the influence of doping on the dark current, photocurrent, and tunneling characteristics of InAsSb photodetectors in the PBn structure. Three high-quality InAsSb samples with unintentionally doped absorb layers (AL) were prepared, with varying p-type doping concentrations in the GaSb contact layer (CL) and the AlAsSb barrier layer (BL). As the p-type doping concentration in the CL increased, the device’s turn-on bias voltage also increased, and p-type doping in the BL led to tunneling occurring at lower bias voltages. Notably, the sample with an unintentionally doped BL exhibited an extremely low dark current 5×10^-6 A/cm^2. The photocurrent characteristics of the unintentionally doped BL sample were well-fitted using a back-to-back diode model, revealing the presence of two opposing space charge regions on either side of the BL.