We present a characterisation of a GaInNAs/GaNAs quantum well-based photodetector with a bottom distributed Bragg reflector (quasi-cavity). The detector is designed to be used at the 1.3 μm optical fibre communication window. The quantum efficiency of the photodetector is measured as 24% at 1286 nm under −2 V applied reverse bias. As the reverse bias voltage is increased, a carrier multiplication-related increase and oscillations are observed in the voltagedependent responsivity curve. The observed carrier multiplication is explained by the high electrical field-induced impact ionisation mechanism in the pin junction region, while the observed voltage-dependent oscillations are explained by the Franz-Keldysh effect (FKE). At the wavelength of 1286 nm, which is close to the absorption wavelength of the active region of the photodetector, FKE-related oscillations (FKOs), start at very low reverse bias values and the responsivity of the photodetector is dominated by FKOs. On the other hand, FKOs quench at higher wavelengths and an impact ionisation-related increase at the voltage-dependent responsivity curve dominates. At λ=1310 nm, only impact ionisation mechanisms have an effect over the R (V) curve. The multiplication factor for 1310 nm is calculated as M=12 at room temperature. The applied electric field and excitation wavelength dependence of the absorption coefficient is calculated and a good match with the experimental results at the applied voltages is achieved.