MOSFET Drain Current and Source

Triode mode or linear region (also known as the ohmic mode)

When V_GS > V_th and V_DS < ( V_GS - V_th )

 

The transistor is turned on, and a channel has been created which allows current to flow between the drain and the source. The MOSFET operates like a resistor, controlled by the gate voltage relative to both the source and drain voltages. The current from drain to source is modeled as:

 

   

 

where μ_n is the charge-carrier effective mobility, W is the gate width, L is the gate length and C_ox is the gate oxide capacitance per unit area. The transition from the exponential subthreshold region to the triode region is not as sharp as the equations suggest.

Saturation or active mode

When V_GS > V_th and V_DS > ( V_GS - V_th )

 

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 gate 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. The drain current is now weakly dependent upon drain voltage and controlled primarily by the gate-source voltage, and modeled very approximately as:

   

The additional factor involving λ, the channel-length modulation parameter, models current dependence on drain voltage due to the Early effect, or channel length modulation. According to this equation, a key design parameter, the MOSFET transconductance is:

,

where the combination V_ov = V_GS - V_th is called the overdrive voltage. Another key design parameter is the MOSFET output resistance r_O given by:

.

If λ is taken as zero, an infinite output resistance of the device results that leads to unrealistic circuit predictions, particularly in analog circuits.

As the channel length becomes very short, these equations become quite inaccurate. New physical effects arise. For example, carrier transport in the active mode may become limited by velocity saturation. When velocity saturation dominates, the saturation drain current is more nearly linear than quadratic in V_GS. At even shorter lengths, carriers transport with near zero scattering, known as quasi-ballistic transport. In addition, the output current is affected by drain-induced barrier lowering of the threshold voltage.

Nombre: Orozco Quiroz Deivid G.

Asignatura: E.E.S.

Fuente:en.wikipedia.org

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