INTEGRATED OVERVOLTAGE PROTECTION CIRCUIT

The overvoltage protection circuit, when used with CMOS circuits, protects them from overvoltage conditions while minimizing latch-up conditions in the structure. It consists of a well region (16) of an opposite conductivity to that of the substrate (10, 12) defining a pocket region (18) having a conductivity type which is similar to that of the substrate (10, 12). A first PN junction diode (34) is formed in a portion of the well region (16) and a second PN junction diode (32) is formed in the pocket region (18). The two diodes have opposite polarity and they both are connected to a signal line in such a way that one of the two diodes will be forward biased if the voltage on the signal line exceeds the bounds of the power supply voltages. The pocket region (18) is connected to a Vss terminal which is normally grounded and the well region (16) is connected to a power supply VDD. The doping concentration in the well region (16) is predetermined to have a gradient so that minority carriers injected from one of the diodes in the well region will be repulsed and prevented from moving into the substrate region where they would be majority carriers and they could cause latch-up in the structure or at the very least adversely affect the voltage level of the substrate. Instead the injected carriers recombine in the well region (16) or are collected by the adjacent isolated pocket region (18). When the second diode (32) is forward biased, the minority carriers are injected into the isolated pocket region (18) and are prevented from reaching the substrate (10) by the underlying well region (14). This prevents these carriers from affecting the operation of adjacent circuits.