Turn-off transients in circular geometry MOS pass transistors

Errors induced by turn-off transients in pass transistors can be significantly reduced by using asymmetric transistor geometries. A detailed analysis is presented of such transients in circular geometry pass transistors. Conventional single-lump models, which assume quasi-static operation, can introduce substantial errors in the analysis of pass-transistor behavior under high-speed transient conditions. Therefore both a distributed model and a simpler two-lump model have been devised to investigate turn-off transients in circular geometry pass transistors operating in the diffusion mode. A test chip including pairs of semicircular and rectangular pass transistors has been designed, fabricated, and tested to verify the analytical results. Both experimental and analytical results suggest that circular geometries offer a means of substantially reducing transient charge injection errors in MOS pass transistors.