Channel length modulation

Channel length modulation (CLM) is an effect in field effect transistors, a shortening of the length of the inverted channel region with increase in drain bias for large drain biases.

The result of CLM is an increase in current with drain bias and a reduction of output resistance.

As the drain voltage increases, its control over the current extends further toward the source, so the uninverted region expands toward the source, shortening the length of the channel region, the effect called channel-length modulation.

The effect is more pronounced the shorter the source-to-drain separation, the deeper the drain junction, and the thicker the oxide insulator.

In the weak inversion region, the influence of the drain analogous to channel-length modulation leads to poorer device turn off behavior known as drain-induced barrier lowering, a drain induced lowering of threshold voltage.

In textbooks, channel length modulation in active mode usually is described using the Shichman–Hodges model, accurate only for old technology:[2] where

= technology parameter sometimes called the transconductance coefficient, W, L = MOSFET width and length,

The channel-length modulation parameter usually is taken to be inversely proportional to MOSFET channel length L, as shown in the last form above for rO:[3] where VE is a fitting parameter, although it is similar in concept to the Early Voltage for BJTs.

However, no simple formula used for λ to date provides accurate length or voltage dependence of rO for modern devices, forcing use of computer models, as discussed briefly next.

The effect of channel-length modulation upon the MOSFET output resistance varies both with the device, particularly its channel length, and with the applied bias.

The main factor affecting the output resistance in longer MOSFETs is channel length modulation as just described.