Short wavelength infrared (SWIR) focal plane arrays (FPAs) working at room temperature have many important military and cosmonautic applications. The dimensions of detectors in FPAs have gradually been reduced. Then the problem of the increase of effective optically sensitive area in photovoltaic detectors by conventional technology has been more thought of. The response of n-on-p HgCdTe photodiodes at 300K was measured by laser beam induced current(LBIC). From the results, we may see that the increase of effective optically sensitive area does exit, and the lateral diffusion of carriers to the junction periphery is the main reason.