CHARGE STORAGE DIODE WITH GRADED DEFECT DENSITY PHOTOCAPACITIVE LAYER

摘要

In a light actuated device such as an alternating current driven light valve or other display device requiring the photocapacitance of a light responsive layer in a photodiode to be modulated in response to changes in incident or writing light, sensitivity is an important factor, especially when a cathode ray tube phosphor image is the source of such light. This sensitivity can be improved by more than an order of magnitude by using a graded defect center (as defined hereinbelow) concentration, graded band gap layer in said diode which can produce a graded optical absorption coefficient between two regions of the layer so that most of the incident light is absorbed in the region near the semiconductor rectifying junction of the diode to store charge near this junction by this or any similar action. In one particular embodiment disclosed a cadmium sulfide photoconductor is used and the defect center density and hence the optical absorption coefficient for the incident light beam in the light sensitive region is graded from a low value on the light input side to a higher value near the junction forming interface with a cadmium telluride light blocking layer by varying the defect concentration in the cadmium sulfide during the film preparation by progressively lowering the temperature during sputter deposition. In a second embodiment this grading is accomplished by alloying cadmium sulfide with cadmium selenide near the cadmium telluride region thus introducing defect centers and lowering the absorption band edge. In both embodiments charge carriers are generated which are stored in the region adjacent to the junction interface and thereby change the photocapacitive depletion width adjacent to the junction to enhance the sensitivity of the diode.