Programmable bipolar structures

Abstract

A switchable bipolar structure, suitable for use in a programmable read only memory, is provided which includes a rectifying contact disposed on a N type semiconductor substrate with a P type diffusion region formed in the substrate spaced within a minority carrier diffusion length from the rectifying contact. A conductive filament is selectively formed between the rectifying contact and the P type diffusion region by applying a reverse bias voltage between the rectifying contact and the N type substrate having a magnitude sufficiently large so as to form a liquid alloy having a front moving in the direction of current flow. By maintaining the P type diffusion region at a positive voltage with respect to the voltage on the rectifying contact, the liquid alloy front moves from the rectifying contact to the P type diffusion region forming a conductive filament or segment therebetween. If the rectifying contact is aluminum and the semiconductor substrate is made of silicon, the filament becomes a silicide made of aluminum and silicon. By arranging the rectifying contact within a minority carrier diffusion length of, e.g., the base of a NPN transistor, a dense programmable read only memory system may be formed which is programmed by merely applying appropriate pulses of relatively low magnitude to the access lines of the memory system after the system has been fabricated. Alternatively, a dense Schottky barrier diode read only memory system may be provided by arranging a first rectifying contact within a minority carrier diffusion length of a P type diffusion region which is connected to a second rectifying contact of a Schottky barrier diode to selectively provide parallel current paths through the two rectifying contacts. In either embodiment, the presence or absence of the filament corresponds to the storing of a 0 or 1 binary digit of information in the memory systems.