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Question: Write short note on MOS capacitance
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Subject: Electronic Devices and Circuits II

Topic: Introduction to MOSFET

Difficulty: Medium

edc2(34) • 372 views
 modified 5 months ago  • written 8 months ago by
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The MOS capacitor consists of a Metal-Oxide-Semiconductor structure as illustrated by Figure:

the semiconductor substrate with a thin oxide layer and a top metal contact, referred to as the gate. A second metal layer forms an Ohmic contact to the back of the semiconductor and is called the bulk contact. T

The metal may be aluminum or some other type of metal, this metal is deposited on oxide layer. the thickness of this layer is defined by tox and permittivity of the oxide is given by Ԑox .

The metal plate ,oxide layer and semiconductor layer structure shown in the above figure is equivalent to parallel plate capacitor. The metal plate and semiconductor substrate will act as two parallel plates of this capacitor and oxide layer is act as an insulator.

The working of mos capacitor is same as parallel plate capacitor.If a voltage source is connected across a capacitor where the +Ve (positive terminal) is connected to the positive terminal of a capacitor and negative terminal is connected to –Ve (negative terminal) of the capacitor. Then, the energy which is stored in the capacitor is directly proportional to the applied voltage.

Where ’C’ is a proportionality constant, which is familiar as the capacitance of the capacitor. The unit capacitance of the capacitor is the Farad. According to the equation Q=CV, 1 F = coulomb/ volt. From the above equation, we can conclude that capacitance depends on voltage and charge, but this is not true.

The capacitance of the capacitor mainly depends on the sizes of the plates and dielectric among two plates. C= ε A/ d

The capacitance of the capacitor mainly depends upon the surface area of each plate, the distance between two plates and the permitivity of the material between the two plates.

Normally, the capacitance value of a capacitor doesn't change with values of voltage applied across its terminals. However, this is not the case with MOS capacitor. It is found that the capacitance of MOS capacitor changes its value with the variation in Gate voltage. This is because application of gate voltage results in band bending in silicon substrate and hence variation in charge concentration at Si-SiO2 interface.

Depending upon the value of gate voltage applied, the MOS capacitor works in three modes:

1.Accumulation

1. Depletion

3.Strong Inversion

The threshold voltage is given by: