Chapter 5: Field Effect Transistor
5.1 NMOS Transistors
Major difference with BJT – there is no dc gate current.
Thus power dissipation is much less.
Figure 5.2 Circuit symbol for an
enhancement mode n-channel
MOSFET
The gate is insulated from the substrate or body using an oxide layer device characteristics depend on L, W, the doping
level, and the thickness of the oxide layer.
Gate used to be made of Al but now mostly it is made of highly doped polysilicon. Very recently, we have transitioned
back to metal gates to lower the gate resistance.
Operation in the Cutoff Region
For VGS = 0 there are two back to back p-n
junctions. One is at the drain body and the other
is at the source body. The reverse biased p-n
junction results in zero drain current.
This is called the cutoff region of operation.
This situation persists until VGS reaches a
threshold voltage, Vt0.
Notice also that the body is shorted to the source. Thus the device is a three terminal device.
Note that the BJT and MOSFET are both back-to-back p-n diodes, so that no current flows when there is no gate bias.
However, the switching on part is different for each of them – one uses a contact to the gate to allow electrons to go to
the base (BJT), while the other uses field effect to flip the carrier type in the base to produce conduction (MOSFET).
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Operation in the Triode Region
For VGS > Vt0, the electric field resulting from the gate voltage repels holes from the region under the gate, but attracts
electrons, which can now flow easily between two n-type contacts (drain and source).
This phenomenon manifests itself as an n-type channel between the drain and the source (thus the name n-MOS).
Thus when positive VDS is applied current flows into the drain, through the channel, and out of the source.
For small values of VDS the drain current, iD is proportional to VDS. iD is also proportional to VGS – Vt0. This called the
Triode Region.
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Operation in the Triode Region contd.
In the triode region, the NMOS device behaves as a resistor connected between the source and the drain. The
resistance decreases as VGS increases. Thus, FETs are sometimes used as voltage controlled resistors.
For instance, for an amplifier, in which the gain depends on a certain resistance value we can control the gain by using
a FET. The FET will provide variable resistance and hence can be used for automatic gain control (AGC).
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Operation in the Saturation Region
As VDS continues to increase the uniform channel starts to taper off in the direction of the drain. This is due to increase in
electric field in that region. The consequence is higher channel resistance and an almost constant drain current after the VDS
becomes larger than VGS - Vt0 . This region is called the saturation region of the NMOS.
In summary:
For VDS < VGS-Vt0
For VDS ≥ VGS-Vt0
Triode region
Saturation region
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Expression of drain current in the Triode Region
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Operation in the Saturation Region
Fig. 5.6 Characteristic curves
for an NMOS transistor
Note that the different
characteristics are a function of
voltage, not current, unlike BJTs
Since at the boundary VDS = VGS–Vt0, we have the drain current given as
i D  K V DS
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
(5 .7 )
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Example 5.1
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