New Insights in Optimizing CMOS Inverter Circuits with Respect to Hot-Carrier Degradation

Peter M. LEE  

IEICE TRANSACTIONS on Electronics   Vol.E77-C   No.2   pp.194-199
Publication Date: 1994/02/25
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Issue on 1993 VLSI Process and Device Modeling Workshop (VPAD 93))
Category: Coupled Device & Circuit Modeling
integrated electronics,  electronic circuits,  semiconductor materials and devices,  

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New insights pertaining to hot-carrier degradation of CMOS inverters have been obtained using an in-house reliability simulator named HIRES (Hitachi Reliability Simulator). The simulation of three out of four different inverter configurations which utilize series-connected NMOSFET devices between the output node and ground results in higher levels if degradation than that induced by intuition. For two of the configurations--the cascode inverter (where the gate of all NMOSFET's are connected to the input) and the two-input NAND gate--degradation levels are comparable to that of a simple two-transistor CMOS inverter. This high level of degradation is found to be caused by the fact that most of the output voltage is dropped across one of the series-connected NMOSFET transistors rather than being equally divided between the two. From degradation simulation results, a design methodology is developed to optimize the inverter circuits to minimize hot-carrier degradation by balancing the degradation suffered between the two series-connected NMOSFET's. Using this approach, up to a factor of 109 improvement in device lifetime is achieved.