MOSFET Ids-Vds characteristics    

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MOSFET Ids-Vds characteristics

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This application plots the Ids-Vds characteristics of a n-channel MOSFET according to the input data characterizing the transistor and its functional state.

How to use this application

On the right side of the screen the desired settings may be inputted. On changing a value, the plot on the left side automatically changes by recalculating the transistor equations described below. More than one curve parametrized by the gate-source voltage may be plotted at once. To access this help click the    -icon in the top right corner. It may be closed by clicking here or by using the close label in the top right corner of this overlay.

MOSFET drain current

n channel MOSFET symbol

The formula for the drain current Ids is derived with the gradual channel approximation. This model assumes a voltage drop across the channel caused by the outer drain-source voltage Vds which shrinks the conducting channel limiting the current. This approximation leads to the following formula:

linear region drain source current


k formula

containing the transistor's geometry and material properties. Vth is the threshold voltage of the transistor describing the minimum gate voltage for strong inversion and thus starting to conduct:

Vth formula

Note that in the formula above the intrinsic carrier concentration is temperature dependent as well. This application correctly consideres this fact, so no action has to be taken by the user when changing temperature.

The formula for Ids above describes an upside down parabola. Experiments show that the drain current does not decrease as the formula suggests when Vds gets bigger and bigger. If Vds gets bigger than the so called saturation voltage (which is the drain-source voltage at the maximum drain current), the transistor is said to be in saturation. Otherwise it's said to be in linear operation. In saturation, the current through the transistor can not be increased by an increase in drain source voltage. It basically acts as a voltage controlled current source. The current drive of the transistor in the saturation region can be calculated with the following equation:

saturation region drain source current

Note the extra term with λ inside. This quantity is called the channel length modulation coefficient. Experiments show that the drain current slightly increases when increasing the drain-source voltage in saturation. So a MOSFET is not an ideal current source, as the current is dependent on the voltage applied. To regard this fact in the formula, this coefficient was introduced. When being zero, the transistor acts like an ideal current source. This value may only be determined experimentially, typical values lie between 0.0001V-1 and 0.1V-1.