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The variable depletion layer model In this section, we present three different models for the MOSFET, the linear model, the quadratic model and the variable depletion layer model. The quadratic model includes the voltage variation along the channel between source and drain. This model is most commonly used despite the fact that the variation of the depletion layer charge is ignored.
The variable depletion layer model is more complex as it includes the variation of the depletion layer along the channel. More specifically, it can be modeled as a linear resistor whose resistance is modulated by the gate-to-source voltage. The general expression for the drain current equals the total charge in the inversion layer divided by the time the carriers need to flow from the source to the drain: If the velocity of the carriers is constant between source and drain, the transit time equals: This leads to the following expression for the drain current: We also assume that the basic assumption described in section 6.
Replacing the inversion layer charge density in the expression for the drain current yields the linear model: Also note that the drain current is zero if the gate-to-source voltage is less than the threshold voltage.
The linear model is only valid if the drain-to-source voltage is much smaller than the gate-to-source voltage minus the threshold voltage. This insures that the velocity, the electric field and the inversion layer charge density is indeed constant between the source and the drain.
The figure illustrates the behavior of the device in the linear regime: While there is no drain current if the gate voltage is less than the threshold voltage, the current increases with gate voltage once it is larger than the threshold voltage.
The slope of the curves equals the conductance of the device, which increases linearly with the applied gate voltage. The quadratic model The quadratic model uses the same assumptions as the linear model. However, this model allows the inversion layer charge to vary between the source and the drain.
The derivation is based on the fact that the current is continuous throughout the channel. The linear model as described by equation 7. Both sides of the equation can be integrated from the source to the drain, so that y varies from 0 to the gate length, L, and the channel voltage VC varies from 0 to the drain-source voltage, VDS.
According to the above equation the current would even decrease and eventually become negative. The charge density at the drain end of the channel is zero at that maximum and changes sign as the drain current decreases.
As explained in section 6. However, these holes cannot contribute to the drain current since the reversed-biased p-n diode between the drain and the substrate blocks any flow of holes into the drain. Instead the current reaches its maximum value and maintains that value for higher drain-to-source voltages.
A depletion layer located at the drain end of the gate accommodates the additional drain-to-source voltage.
This behavior is referred to as drain current saturation. Drain current saturation therefore occurs when the drain-to-source voltage equals the gate-to-source voltage minus the threshold voltage.FET Questions and Answers pdf free download also objective type multiple choice interview 2 mark important interview questions lab viva manual book.
UNIT I. COMPLEX NUMBERS AND INFINITE SERIES: De Moivre’s theorem and roots of complex benjaminpohle.com’s theorem, Logarithmic Functions, Circular, Hyperbolic Functions and their Inverses. Convergence and Divergence of Infinite series, Comparison test d’Alembert’s ratio test.
As channel lengths are reduced without proportional reduction in drain voltage, raising the electric field in the channel, the result is velocity saturation of the carriers, limiting the current and the transconductance.
at −30 V. DC source–drain current–voltage characteristics notice an increase of the saturation current with decreasing undoped layer width for three device structures except device (a).
This seemingly contradictory behaviour can transconductance, the rate of transconductance change. This paper describes the conception of the current follower transconductance amplifier (CFTA) with electronically and linearly current tunable.
the voltage-current characteristic of this device can be expressed as where its circuit symbol is the DC and AC current characteristics of the output currents from and terminals against the. The current-voltage (I-V) characteristics were studied within voltage sweep 0 - V, and carried out under normal conditions at room temperature using the four point probe technique, a Jandel four point probe tool (model TYMP), and a Keithly A dual channel source/measurement equipment unit all sealed in a dark room.