MOSFET - Metal Oxide Semiconductor FETs

The gate input is a capacitance; therefore the gate requires zero current. The only dependence upon terminal voltages on the current flowing from source to drain.

Two back to back P-N junctions from source-to-drain; therefore, minimal channel current flows unless an applied gate voltage decrease this barrier

We have two types of MOSFETs, nFET and pFET, depending upon their respective substrate doping, p-type or n-type, respectively.

We have only one current to consider in a MOSFET, that is the current flowing from source-to-drain.

A MOSFET is a symmetric device; that is physically swapping the drain and source terminals does not change the circuit properties

MOS Capacitors are the fundamental element of MOSFET devices. k is due to the capacitive voltage divider between gate voltage and the surface potential.

A MOSFET may be considered in sub-threshold or in above-threshold operation.

Subthreshold MOSFET:

Many textbooks assume that the current in this region is zero; we know otherwise

Low current levels ® The charge density in the channel is small ® Channel potential (Y) is constant ® Current is due to diffusion

A MOSFET in subthreshold is nearly an ideal barrier modulated by its gate and substrate.

A subthreshold MOSFET modulates its surface potential by capacitive coupling; therefore no gate current. k (referred to as "kappa") is due to the capacitive coupling.

We can model a subthreshold nFET as I = I_o exp(k V_g / U_T ) ( exp( -V_s/ U_T ) - exp( -V_d / U_T ) )

Above-threshold MOSFET:

The charge due to the high current results in a decreasing potential from drain to source

High current levels ® The charge density in the channel is significant ® Channel potential (negative voltage) decreases between the source and drain regions ® Current is due to drift

A MOSFET may be considered to be operated in the ohmic or saturated regions

Ohmic Region - Strong dependence on drain voltage

Subthreshold: I = I_o exp( ( k V_g ) / U_T ) (exp( -V_s / U_T ) - exp( -V_d / U_T ) )

Above-threshold: I = ( k K / 2 ) (( V_g - V_T0 - V_s / k )² - ( V_g - V_T0 - V_d / k )²)

Saturation Region - Weak dependence on drain voltage. If Early effect is negligible, then current is independent upon drain voltage.

Subthreshold: I = I_o exp( ( k V_g - V_s) / U_T )

Above-threshold: I = ( k K / 2 ) ( V_g - V_T0 - V_s / k )²

Saturation voltage, V_dssat , is the voltage between the ohmic and saturation regions.

Subthreshold: V_dssat ~ 100mV ((V_ds > V_s + 4 U_T )in subthreshold, and is independant of gate voltage.

Above-threshold: V_dsat = k (V_g - V_T0 ) in above threshold. (Mobile charge at the drain is zero)

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