# Channel Length Modulation

## What is channel length modulation?

Pinch-off

It was previously said that a MOS transistor pinches off its channel when the drain-source voltage reaches the override voltage ($V_{DS} = V_{GS} - V_{TH}$). But what happens as we keep increasing the drain voltage? As we know, when the drain reaches pinch-off, we have $V_{GD} = V_{TH}$. More generally, the pinch-off occurs in the channel when the difference in potential between gate and that point in the channel is $V_{TH}$. If $V_{DS}$ keeps increasing, the pinch-off must occur at a position $x$ within the channel, somewhere between the drain and source. $$V_{Gx} = V_{TH}$$ $$V_G - V_x = V_{TH}$$ $$V_x = V_G - V_{TH}$$ $$V_{xS} = V_{GS} - V_{TH}$$ and $$V_{DS} > V_{GS} - V_{TH}.$$ There is a voltage drop between the pinch-off point $x$ and the drain. Whatever distance $\Delta L$ remains between these two is a depletion region. Since the channel is being modulated by $V_{DS}$, we call this effect channel-length modulation.

Channel Length modulation

## What effect has channel length modulation?

To see the effect of channel length modulation on transistor operation, we replace the length of the transistor by the effective length $L - \Delta L$ in the $I_{DS}$ equation in saturation: $$I_{DS} = \frac{1}{2}\mu C_{ox}\frac{W}{L - \Delta L}(V_{GS} - V_{TH})^2$$ $$I_{DS} = \frac{1}{2}\mu C_{ox}\frac{W}{L} \frac{1}{1 - \Delta L/L}(V_{GS} - V_{TH})^2.$$ Assuming that $\Delta L \ll L$, we can linearize $\frac{1}{1 - \Delta L/L}$ around $\Delta L/L = 0$ to get $\frac{1}{1 - \Delta L/L} \approx 1 + \frac{\Delta L}{L}$. Then: $$$$I_{DS} = \frac{1}{2}\mu C_{ox}\frac{W}{L} (1 + \frac{\Delta L}{L})(V_{GS} - V_{TH})^2.\label{eq:a}$$$$ Next we state that $\Delta L$ is influenced linearly by $V_{DS}$ by a factor of $\lambda'$ (this is more or less true), which is a process parameter: $$\Delta L = \lambda' V_{DS}.$$ But usually, a parameter $\lambda = \lambda'/L$ is used instead, so: $$$$\frac{\Delta L}{L} = \frac{\lambda' V_{DS}}{L} = \lambda V_{DS}\label{eq:b}$$.$$ We can replace $\eqref{eq:b}$ in $\eqref{eq:a}$ to get $$I_{DS} = \frac{1}{2}\mu C_{ox}\frac{W}{L}(V_{GS} - V_{TH})^2(1 + \lambda V_{DS}).$$ By the above equation it is easy to see that increasing $V_{DS}$ increases $I_{DS}$. The following plots show the $I_{DS}-V_{DS}$ curve with channel-length modulation.

### The $I_{DS}-V_{DS}$ curve

Now the plateaus are not flat, but have some slope. The meaning and value of the slope is discussed in the small-signal model.

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