Gate protection of MIS devices

Abstract

Gate shorts caused by electrical breakdown of the gate dielectric are a major yield and reliability problem for MOS transistors and integrated circuits. Diodes or diffused resistors with breakdown voltages of about 40 V can be used to protect the gate from high voltage transients or static discharges. This paper provides a uniform approach to gate protection. It is shown theoretically that in order to obtain effective gate protection: the protecting device should have a low dynamic resistance in breakdown; the breakdown voltage of the protecting device should be above, but close to, the maximum gate operating voltage; and protection by a diffused resistor in series with the gate is much more effective than by a diode in parallel with the gate. It is shown experimentally that, compared to the widely used fieldplate-induced breakdown, breakdown due to reach-through to a highly doped substrate provides: a dynamic resistance that is almost two orders of magnitude lower; reasonable control of the breakdown voltage; much better protection against simulated static discharges. Since under pilot line conditions no adverse effects on performance or yield have been observed, reach-through breakdown devices seem to improve gate protection decisively without any coincident disadvantages.