Goa University > Electronics Engineering > Sem 6 > VLSI

Marks: 6M

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The MOS transistor works in three modes:

1.Cut off:

A MOS device is said to be operating when the gate-to-source voltage is less than threshold voltage. Cut-off region is also known as sub-threshold region. In this region, the dependence of current on gate voltage is exponential. The magnitude of current flowing through MOS in cut-off region is negligible as the channel is not present. The conduction happening in this region is known as sub-threshold conduction.

(Drain Current ) $ID=0$

2.Saturation (active) region:

The switch is turned on, and a channel has been created, which allows current to flow between the drain and source. Since the drain voltage is higher than the source voltage, the electrons spread out, and conduction is not through a narrow channel but through a broader, two- or three-dimensional current distribution extending away from the interface and deeper in the substrate. The onset of this region is also known as pinch-off to indicate the lack of channel region near the drain. Although the channel does not extend the full length of the device, the electric field between the drain and the channel is very high, and conduction continues. The drain current is now weakly dependent upon drain voltage and controlled primarily by the gate–source voltage, and modeled approximately as:

$Id$(saturation) = $µ C_o \times \frac {W}L (V_{gs} – V_{th} – V_{ds/2})^2$

Linear Region::

For an NMOS, as gate voltage increases beyond threshold voltage, channel is formed between source and drain terminals. Now, if there is voltage difference between source and drain, current will flow. The magnitude of current increases linearly with increasing drain voltage till a particular drain voltage.

The current is, then, represented as a linear function of gate-to-source and drain-to-source voltages. That is why, MOS is said to be operating in linear region. The linear region voltage-current relation is given as follows:

$Id$ (Linear) = $ µ C_o \times \frac {W}L (V_{gs} – V_{th} – V_{ds/2}) V_{ds}$

The figure below shows the figure of MOS structure: