Channel Length Modulation in MOSFET

Channel Length Modulation

• Fig. 5.14.1 shows induced channel at different levels of V_DS. In the figure, the thickness of the induced channel layer qualitatively indicates the relative charge density.

• In Fig. 5.14.1 (a), applied V_DS is small and for this case the relative charge density is constant along the entire channel length.

• Fig. 5.14.1 (b) shows the situation when V_DS increases. As the drain voltage increases, the voltage drop across the oxide near the drain terminal decreases, which means that the induced inversion charge density near the drain also decreases. The incremental conductance of the channel at the drain then decreases, which causes the slope of the I_D versus V_DS curve to decrease. This effect is shown in the I_D versus curve in the figure.

• As V_DS increases to the point where the potential difference across the oxide at the drain terminal is equal to V_T, the induced inversion charge density at the drain terminal is zero. This is illustrated in Fig. 5.14.1 (c).

• When V_DS becomes larger than V_DS(sat), the point in the channel at which the inversion charge is just zero moves towards the source terminal. In this case, electrons enter the channel at the source, travel through the channel towards the drain, and then at the point where the charge goes to zero, are injected into the depletion region, where they are swept by the E‒field to the drain contact.

• It is observed that as V_DS increases beyond V_DS(sat) effective channel length decreases, producing the phenomenon called channel length modulation.

Important Concepts

• The MOSFET will have a smaller threshold voltage V_TH if the channel length is reduced.

• Reducing the channel length increases the drain current and hence the transconductance of the MOSFET.

Review Questions

1. What is channel length modulation.

2. State true or false: The transconductance increases if channel length is reduced.

3. Discuss the effect of channel length modulation.