The spatial resolution obtainable in a photosensing array used for optical imaging may be limited by the diffusion of photogenerated carriers within a uniformly doped semiconductor even if other components of the optical system are optimized and scattered light is reduced. A technique has been developed to improve the spatial resolution for critical applications by incorporating subsurface electric fields that accelerate the photogenerated carriers toward or away from the surface so that the carriers are prevented from diffusing to distant photosensing elements. The subsurface fields are obtained by incorporating suitable dopant concentration gradients into the structure. In one structure fabricated the subsurface field was formed by using a heavily doped buried layer and a lightly doped epitaxial film over a lightly doped substrate, all of the same conductivity type. This structure is compatible with the incorporation of other semiconductor devices in the same monolithic substrate. The technique has been applied to an array of photodiodes in a silicon integrated circuit, but the principle is directly applicable to other types of photosensing arrays, such as charge-coupled devices (CCD's), and other semiconductor materials.
[1]
D. H. Seib,et al.
Carrier diffusion degradation of modulation transfer function in charge coupled imagers
,
1974
.
[2]
D.F. Barbe,et al.
Imaging devices using the charge-coupled concept
,
1975,
Proceedings of the IEEE.
[3]
R. Newman,et al.
Intrinsic Optical Absorption in Single-Crystal Germanium and Silicon at 77°K and 300°K
,
1955
.
[4]
R. H. Dyck,et al.
Integrated arrays of silicon photodetectors for image sensing
,
1968
.
[5]
J. McKelvey,et al.
Solid State and Semiconductor Physics
,
1966
.