Q7VLSI Design

Channel length modulation refers to the effective shortening of a MOSFET's conducting channel length as the drain-to-source voltage VDS increases beyond the saturation (pinch-off) point - as VDS rises further past pinch-off, the pinch-off point itself moves slightly toward the source, effectively reducing the conducting channel's length below the transistor's nominal (mask) length L. Since drain current in the ideal square-law model is inversely proportional to channel length, this effective shortening causes the saturation-region drain current to increase slightly with VDS (rather than remaining perfectly constant/flat as the simple ideal model predicts), modeled by multiplying the ideal saturation current by a factor (1+lambda*VDS), where lambda is the channel length modulation parameter, giving the MOSFET's output characteristic curves a small positive (non-zero) slope in saturation rather than being perfectly horizontal.