Validated SILVACO physical models were exploited to optimise electrical and optical characteristics of 1.5 µm wavelength InAlAs–InGaAs Avalanche photodiodes and InGaAs PIN diodes. Optimised SILVACO models were created by selectively thinning down the absorption layers to further reduce the carrier transit time. Further optimisation was accomplished by scaling the light window aperture and mesa area sizes with the aim of reducing the device capacitances. The optimised PIN diode provides a maximum opto-electric bandwidth of 35 GHz with a current responsivity of 0.4 A/W under −5 V bias voltage and 10 µW incident optical power. At 1 µW incident optical power, the maximum opto-electric bandwidth and current responsivity of the optimised avalanche diode is 21 GHz and 1.4 A/W under −21.5 V bias voltage. The optimised APD and PIN photo-detectors are capable of working at a data rate of up to 25  and 40 Gb/s, respectively.