Abstract The effect of photon flux density on the noise properties of optically illuminated Double Drift Region (DDR) Mixed Tunneling Avalanche Transit Time (MITATT) device is investigated in this paper. An analytical model is proposed to obtain a relationship between the normalized difference of electron and hole current density of DDR device and the incident photon flux on it. Simulation is carried out to study the small signal noise spectral density and noise measure of the device under optical illumination. Two illumination configurations i.e., Flip Chip (FC) and Top Mounted (TM) structures are considered in the simulation. The device is designed to operate at mm-wave window frequency at W-band. The results show that avalanche noise measures of FC and TM structures of photo irradiated device are 37.1 dB and 40.2dB respectively for an incident photon flux density of 1024m-2sec-1 at 1000nm wavelength near band gap absorption of Silicon. The noise measure of the un-illuminated device is found to be 35dB. The results indicate that the increase of avalanche noise due to the incident photon flux can be reduced if FC structure is taken instead of TM structure.