This thesis reports a novel system capable of testing both the spectral responsivity and the spatial quantum efficiency uniformity of heterostructure photodiodes using optical fiber coupled radiation. Testing was performed to confirm device specifications. This study undertakes to quantify the spectral bandwidth of an AlGaAs I GaAs double heterostructure photodiode and two InGaAs I InP double heterostructure PIN photodiodes at D.C., through the use of spatial scanning. The spatial scanning was done using lasers at 670 nm, 780 nm, 848 nm, 1300 nm, and 1550 nm, coupled through singlemode optical fiber. The AlGaAs I GaAs material system covers the 600 - 870 nm wavelength region of research interest in the visible spectrum. The InGaAs I InP material system covers the 800 - 1650 nm region which contains the fiberoptic communications spectrum. The spatial measurement system incorporates a nearly diffraction limited spot of light that is scanned across the surf ace of nominally circular photodiodes using a piezoelectric driven stage. The devices tested range in size from 17 to 52 μin diameter. The smallest device scanned has a diameter approximately four times the diffraction limit of the radiation used for spatial scanning. This is the smallest diode yet reported as being spatially mapped. This is the first simultaneously reported spectral and spatial scans of the same heterostructure PIN photodiodes in the InGaAs I InP and AlGaAs I GaAs systems. The testing arrangement allows both spectral and spatial scans to be taken on the same stage. The diodes tested were taken from intermediate runs during their process development. All testing was performed at room temperature. This study describes the mechanical assembly, calibration and testing of a spatial quantum efficiency uniformity measurement system. The spectral quantum efficiency was measured with low power, incoherent broadband radiation coupled through multimode fiber from a tunable wavelength source to the device under test. The magnitude was corrected to the measured peak external quantum efficiency (Q.E.), determined during spatial scanning at a mid-spectral bandwidth wavelength using continuous wave (CW) higher power lasers. A procedure to improve the accuracy of the correction is recommended. This process has been automated through the use of National Instruments LabVIEW II software. The results from this procedure are plotted to show 2.5 D (pseudo 3D) and 2 D contour spatial quantum efficiency maps. These results give a quantified map of the relative homogeneity of the response. The non-homogeneity of the spatial scans on the smallest devices has not previously been reported. The Q.E. measurements made agree well with previously published results for similar device structures. The AlGaAs I GaAs device achieved a peak external Q.E. of 58.7% at 849 nm with -lOV bias. An InGaAs I InP device achieved 63.5% at 1300 nm with the same bias. The Q.E. results obtained are compared to theoretical calculations. The calculations were performed using the best optical constant data available in the literature at this time. The measured peak Q.E. was found to agree with the theoretical calculations to within 16% at longer wavelengths for both devices tested.