In this paper a resonant tunnelling quantum well infrared photodetector (RT-QWIP) is discussed. Each period of this photodetector structure comprises of a resonant tunnelling structure (AlAs/AlGaAs/AlAs) nearby a quantum well (AlGaAs/GaAs). In this photodetector, photocurrent is produced when an electron makes a transition from the ground state of the well to an excited state which is coupled to the resonant state of the resonant tunnelling structure. The effect of structural parameters of the photodetector on the electronic states and oscillator strength of transitions between energy subbands of the structure is investigated. Then using the obtained results, an appropriate structure for the RT-QWIP is determined which exhibits a photoresponse peak at 7.3 μm at 77 K.