We perform a detailed investigation of the light and x-ray response of a newly developed planar avalanche photodiode (APD) of the deep diffused type. This type of photodetector design has the low noise characteristics obtained from the deep diffusion process, but it is built using only standard planar technology. We measure an excess noise factor of 1.8 at a gain of 6, which is similar to that of other commercial APDs. With this type of structure, the expected gain is ~500 at 1600 V. We do not achieve this gain because of an early breakdown at the surface. Future designs will incorporate improvements in the structure to avoid surface breakdown. In spite of the low gain, the level of noise that we find indicates that this structure, reaching full breakdown, has potential for very good performance. At this gain, it is found that the dominant contribution to the broadening of the energy resolution comes from preamplifier voltage noise. The measured capacitance is 20 pF, and the rise time is 9 ns at 1300 V. The dependence of the gain on the density of primary carriers generated is also investigated. When the gain is measured with α particles (these particles generate a large number of primary electrons in an area of a few micrometers), it is lower than the gain measured with light (primary electrons generated by light are spread over a large area). The decrease starts to be seen at a gain of 2. When a large amount of charge (90×106 e) is injected via a light pulse into a larger area (0.3 mm diameter), no effect is seen.