Channel length modulation

One of several short-channel effects in 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 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.

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 bipolar devices a similar increase in current is seen with increased collector voltage due to base-narrowing, known as the Early effect. The similarity in effect upon the current has led to use of the term "Early effect" for MOSFETs as well, as an alternative name for "channel-length modulation".

In textbooks, channel length modulation in active mode usually is described using the Shichman–Hodges model, accurate only for old technology: where ${\displaystyle I_{D}}$ = drain current, ${\displaystyle K'_{n}}$ = technology parameter sometimes called the transconductance coefficient, W ,L = MOSFET width and length, ${\displaystyle V_{GS}}$ = gate-to-source voltage, ${\displaystyle V_{th}}$ =threshold voltage, ${\displaystyle V_{DS}}$ = drain-to-source voltage, ${\displaystyle V_{DS,sat}=V_{GS}-V_{th}}$, and λ = channel-length modulation parameter. In the classic Shichman–Hodges model, ${\displaystyle V_{th}}$ is a device constant, which reflects the reality of transistors with long channels.

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