Influence of substrate current on hold-time characteristics of dynamic MOS IC´s

The charge hold-time characteristics of three-transistor-type MOS memory cells were investigated. It was observed that, at first, the holding-node voltage V/SUB G/ decreased slowly, due to junction leakage current. However, when V/SUB G/ reached a value determined by the sense-inverter dc supply voltage and external load resistance, V/SUB G/ fell rapidly. The onset value of the rapid drop coincided well with the value of V/SUB G/ that caused substrate current to flow in the sensing inverter. It is suggested that an extremely small amount of substrate current I/SUB sub/ arrives at the holding node and that positive feedback between V/SUB G/ and I/SUB sub/ causes the rapid drop of V/SUB G/. A simple analysis was pursued to estimate the transport probability α of the substrate current I/SUB sub/ arriving at the holding node. It was also observed that the hold-time characteristics of neighboring physically independent cells were degraded by substrate current generated by one cell. The transport probability between these holding nodes and the driver transistor generating the I/SUB sub/ was estimated as a function of the separating distance.