Rear entry photodiode with three contacts

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring initially to FIG.
1, one form of a semiconductor photodiode of the present invention is generally designated as 10.
Photodiode 10 includes a substrate 12 of a semi-insulating semiconductor material having a pair of opposed major surfaces 14 and 15.
On the surface 14 of the substrate 12 is a first layer 16 of the same semiconductor material as the substrate 12.
The first layer 16 is of one conductivity type and is highly doped so as to be conductive.
On the surface 18 of the first layer 16 is a second layer 20 of a semiconductor material which will absorb the light as the wavelength being detected.
The second layer 20 is the active layer of the photodiode 10.
The layer 20 is of the conductivity type opposite that of the first layer 16 and is lightly doped.
On the second layer 20 is a third layer 22 of the same semiconductor material as either the second layer 20 or the first layer 16.
The third layer 22 is of the same conductivity type as that of the second layer but is highly doped so as to be conductive.
The second and third layers 20 and 22 are smaller in area than the first layer 16 so as to form a mesa and to expose a portion of the surface 18 of the first layer 16.
During the formation of the photodiode 10, conductivity modifiers from the first layer 16 diffuse a distance into the second layer 20, forming a PN junction 34 therein.
An insulating layer 24, such as of a silicon oxide, is on the third layer 22 and has an opening 26 therethrough which exposes a portion of the surface of the third layer 22.
A conductive metal contact 28 is on the exposed portion of the surface of the third layer 22 within the opening 26 and makes ohmic contact with the third layer 22.
A second conductive metal contact 30 is on the exposed portion of the surface 18 of the first layer 16 and makes ohmic contact with the first layer 16.
The contacts 28 and 30 are of an area large enough to serve as a terminal pad for attaching a terminal thereto.
The surface 15 of the substrate 12 is polished and an anti-reflection coating 31, such as of silicon oxide is on the surface 15