The effective channel length of MOSFET in saturation decreases with increase in
Drain voltage
MOSFET Channel Length Modulation
The Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is a fundamental building block in modern electronics. Its operation relies on controlling the current flow between the source and drain terminals using a voltage applied to the gate terminal. When a MOSFET operates in the saturation region, its drain current becomes largely independent of the drain-source voltage (VDS) for ideal long-channel devices. However, in real MOSFETs, especially short-channel devices, the effective channel length changes with the drain voltage, a phenomenon known as channel length modulation.
Effective Channel Length in Saturation
In the saturation region, the channel in a MOSFET is said to be "pinched off" at the drain end. This means that the channel, which connects the source to the drain, narrows down significantly near the drain. The point where the channel pinches off marks the boundary between the active channel region and the depletion region extending from the drain. The effective channel length (\(L_{eff}\)) is the length of the channel that is still conducting current, from the source to this pinch-off point.
For a MOSFET to be in saturation, the drain-source voltage (VDS) must be greater than or equal to the overdrive voltage (VGS - VTH), where VGS is the gate-source voltage and VTH is the threshold voltage.
- When \(V_{DS} \ge V_{GS} - V_{TH}\), the MOSFET enters the saturation region.
- At the pinch-off point, the channel voltage relative to the source is approximately \(V_{GS} - V_{TH}\).
Drain Voltage and Channel Length Decrease
When the MOSFET is operating in the saturation region, increasing the drain voltage (VDS) further causes the depletion region at the drain end to expand more aggressively towards the source. This expansion effectively shortens the length of the conductive channel. The point where the channel pinches off shifts closer to the source terminal. This reduction in the effective channel length (\(L_{eff}\)) with an increase in drain voltage is called channel length modulation (CLM).
The relationship between the actual channel length (\(L\)) and the effective channel length (\(L_{eff}\)) in saturation can be understood as:
\[L_{eff} = L - \Delta L\]Where \(\Delta L\) is the reduction in channel length due to the expansion of the depletion region. As the drain voltage increases, \(\Delta L\) increases, leading to a decrease in \(L_{eff}\).
This decrease in effective channel length has a significant impact on the MOSFET's output characteristics. Since the drain current in saturation is inversely proportional to the channel length, a decrease in \(L_{eff}\) leads to a slight increase in the drain current with increasing drain voltage, even in the saturation region. This phenomenon gives the MOSFET output characteristics a non-zero slope in saturation, which is modeled by the channel length modulation parameter (\(\lambda\)).
Therefore, the effective channel length of a MOSFET in saturation decreases with an increase in the drain voltage.
Important Questions from MOSFET - Teaching
In enhancement type of MOSFET, at zero gate bias:
A. Channel conductance is very low
B. Channel conductance is very high
C. Channel resistance is very low
D. Channel resistance is very high
Choose the correct answer from the options given below:
The maximum operating frequency of a MOSFET is
A. \(\rm \frac{\omega_m}{2\pi}\)
B. \(\rm \frac{\mu_nV_D}{2\pi L^2}\)
C. \(\rm \frac{\mu_nL^2}{2\pi}\)
D. \(\rm \frac{\mu_nV_D}{2\pi L}\)
Choose the correct answer from the options given below:
In a MOSFET, the transconductance in linear region ean be expressed as:
For a thermally grown silicon dioxide layer, in a MOSFET, the leakage current between the gate and channel is:
If N is the doping level of substrate and V is the effective voltage across the junction in a MOSFET, then the depletion region width for the junction is given by:
