Charge-sensitive infrared photo-transistors (CSIP) are quantum detectors of mid-infrared radiation (λ=4 µm−14 µm) which have been reported to have outstanding figures of merit and sensitivities that allow single photon detection. The typical absorbing region of a CSIP consists of an AlxGa1-xAs quantum heterostructure, where a GaAs quantum well, where the absorption takes place, is followed by a triangular barrier with a graded x(Al) composition that connects the quantum well to a source-drain channel. Here, we report a CSIP designed to work for a 9.3 µm wavelength where the Al composition is kept constant and the triangular barrier is replaced by tunnel-coupled quantum wells. This design is thus conceptually closer to quantum cascade detectors (QCDs) which are an established technology for detection in the mid-infrared range. While previously reported structures use metal gratings in order to couple infrared radiation in the absorbing quantum well, here, we employ a 45° wedge facet coupling geometry that allows a simplified and reliable estimation of the incident photon flux Φ in the device. Remarkably, these detectors have an “auto-calibrated” nature, which enables the precise assessment of the photon flux Φ solely by measuring the electrical characteristics and from knowledge of the device geometry. We identify an operation regime where CSIP detectors can be directly compared to other unipolar quantum detectors such as quantum well infrared photodetectors (QWIPs) and QCDs and we estimate the corresponding detector figure of merit under cryogenic conditions. The maximum responsivity R = 720 A/W and a photoconductive gain G~2.7 × 104 were measured, and were an order of magnitude larger than those for QCDs and quantum well infrared photodetectors (QWIPs). We also comment on the benefit of nano-antenna concepts to increase the efficiency of CSIP in the photon-counting regime.