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  Metal Oxide Field Effect Transistor, Physics tutorial

MOS-FET operation:

Both JFETs and MOSFETs are conductivity modulated devices, utilizing only one type of charge carrier that signifies they are unipolar devices as distinct from Bipolar Transistors that use both electrons and holes.

In the MOS-FET, P-N junction is replaced with the metal-oxide layer that is much simpler to mass produce; mainly in microchips involving thousands integrated MOSFET devices.

688_MOSFET structure.jpg

Unlike the JFET in which the conducting channel is formed by doping and its geometry modulated by applied voltages, MOSFETs change the carrier concentration in their channel that in turn changes conductivity of channel.

In illustration of the p - type MOSFET above, source and drain are n+ regions in the p-substrate while gate is capacitive coupled to channel region through insulating layer; this insulating layer is generally the thin layer of Silicon dioxide (SiO2).

When the positive voltage is applied to gate, electron concentration at silicon surface beneath gate increases and just as in JFET the combination of gate and drain voltages control conductivity of channel.

In absence of any special surface preparation surface of silicon is n-type that is p-type silicon inverts at surface. The n-channel MOSFET uses the n-channel in the p-substrate, so application of the positive potential to gate forms inversion layer required for channel.

As in JFET, combination of current flow in channel and applied potentials forms the depletion region which is greatest near drain. At a sufficiently large drain potential channel pinches off.

Illustrations below show relationship between Drain voltage and saturation in MOSFET. In illustration drain voltage is less than saturation voltage which can be defined as VD < Vsat. In this region MOSFET presents resistive channel. When VD = Vsat as illustrated in (b) saturation starts to set in and when VD > Vsat as illustration (c) depicts, output current of the MOFET is saturated.

744_Relationship between Drain voltage and saturation in MOSFET.jpg

MOSFET output curves:

Output curves of the MOSFET are like Junction Field Effect Transistor and similarly, drain voltage needed to get saturation increases with operating current.

When MOSFET is in saturation region

ID = (W/L)(μCi/2)(VG - VT)2

Where Ci is gate capacitance per unit area eox /dox and VT is gate threshold voltage that corresponds to commencement of strong inversion.

From above, transconductance is:

gm = (W/L)Ciμ(VG - VT) = (W/L)(εox/dox)μ(VG - VT) = √(W/L)(εox/dox)μ.ID

And with the given width W and drain current ID transconductance is increased by decreasing channel length L and thickness of gate oxide dox

2334_MOSFET Characteristic Curves.jpg

MOSFET device types:

MOSFETs can be executed with either n or p-channels depending on substrate doping while the thin surface layer can be implanted to adjust threshold voltage. This determines whether device is usually on at zero gate voltage (depletion mode device) or usually off at zero gate voltage whereby gate needs increased voltage to form the inversion layer (enhancement mode device)

P-Channel Enhancement Mode: P-Channel Enhancement mode MOSFET is usually off and applied gate voltage decreases channel resistance.

2215_P-Channel Depletion mode MOSFET.jpg

N-Channel Enhancement Mode: N-Channel Enhancement mode MOSFET is usually off and applied gate voltage decreases channel resistance.

P-Channel Depletion Mode: P-Channel Depletion mode MOSFET is usually off and applied gate voltage decreases channel resistance.

N-Channel Depletion Mode: N-Channel Depletion mode MOSFET is usually off and applied gate voltage decreases channel resistance.

1234_N-Channel Depletion mode MOSFET.png

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