Channel length modulation: Difference between revisions

Revision as of 00:20, 14 November 2014

Revision as of 00:20, 14 November 2014

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One of several [[short-channel effect]]s in [[MOSFET#MOSFET scaling|MOSFET]] scaling, '''channel length modulation''' ('''CLM''') is 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. Channel length modulation occurs in all [[field effect transistors]], not just MOSFETs.

To understand the effect, first the notion of '''pinch -off a loaf''' of the channel is introduced. The channel is formed by attraction of carriers to the gate, and the current drawn through the channel is nearly a constant independent of drain voltage in saturation mode. However, near the drain, the gate ''and drain'' '''jointly''' determine the electric field pattern. Instead of flowing in a channel, beyond the pinch-off point the carriers flow in a subsurface pattern made possible because the drain and the gate both control the current. In the figure at the right, the channel is indicated by a dashed line and becomes weaker as the drain is approached, leaving a gap of uninverted silicon between the end of the formed inversion layer and the drain (the ''pinch-off'' region).

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''. Because resistance is proportional to length, shortening the channel decreases its resistance, causing an increase in current with increase in drain bias for a [[MOSFET]] operating in saturation. The effect is more pronounced the shorter the source-to-drain separation, the deeper the drain junction, and the thicker the oxide insulator.