Scanning Capacitance Microscopy (SCM) was applied to obtain the 2-dimensional carrier distribution on the cross-section of planar type InGaAs/InAlAs pixels. The profile of pn junction in the device structure was able to be depicted with high space resolution. Besides, for InGaAs/InP detector, the SCM study helps to disclose the distinct diffusion behavior of p-type impurities in different functional layers. The lateral diffusion speed of zinc in InGaAs absorption layer was decided as 3.3 times than that in the depth direction, which is significantly higher than the lateral to depth ratio of 0.67 in the n-InP cap layer, this could affect both the capacitance and dark current properties of the diode pixels.