Analog electronics Experiment 10 aim To study and plot the characteristics of MOSFET in its various configurations.

 Note remember page numbers

Aim: To study and plot the characteristics of MOSFET in its various configurations.

Apparatus:

Bread board, variable and fixed D.C. Sources, MOSFET, resistances, multimeters, connecting

wires.

OR

FET Characteristics trainer kit with connecting leads and power supply.

Theory: A metal–oxide–semiconductor field-effect transistor (MOSFET) is based on the

modulation of charge concentration by a MOS capacitance between a body electrode and

a gate electrode located above the body and insulated from all other device regions by a gate

dielectric layer which in the case of a MOSFET is an oxide, such as silicon dioxide. If dielectrics

other than an oxide such as silicon dioxide (often referred to as oxide) are employed the device

may be referred to as a metal–insulator–semiconductor FET (MISFET). Compared to

 the MOS

capacitor, the MOSFET includes two additional terminals (source and drain), each connected to

individual highly doped regions that are separated by the body region. These regions can be

either p or n type, but they must both be of the same type, and of opposite type to the body

region. The source and drain (unlike the body) are highly doped as signified by a "+" sign after

the type of doping.

If the MOSFET is an n-channel or N-MOS FET, then the source and drain are "n+" regions and

the body is a "p" region. If the MOSFET is a p-channel or P-MOS FET, then the source and

drain are "p+" regions and the body is a "n" region. The source is so named because it is the

source of the charge carriers (electrons for n-channel, holes for p-channel) that flow through the

channel; similarly, the drain is where the charge carriers leave the channel.

The occupancy of the energy bands in a semiconductor is set by the position of the Fermi

level relative to the semiconductor energy-band edges. As described above, and shown in the

figure, with sufficient gate voltage, the valence band edge is driven far from the Fermi level, and

holes from the body are driven away from the gate. At larger gate bias still, near the

semiconductor surface the conduction band edge is brought close to the Fermi level, populating

the surface with electrons in an inversion layer or n-channel at the interface between the p region

and the oxide. This conducting channel extends between the source and the drain, and current is

conducted through it when a voltage is applied between the two electrodes. Increasing the

voltage on the gate leads to a higher electron density in the inversion layer and therefore

increases the current flow between the source and drain.

For gate voltages below the threshold value, the channel is lightly populated, and only a very

small sub-threshold leakage current can flow between the source and the drain.

When a negative gate-source voltage (positive source-gate) is applied, it creates a p-channel at

the surface of the n region, analogous to the n-channel case, but with opposite polarities of 

charges and voltages. When a voltage less negative than the threshold value (a negative voltage 

for p-channel) is applied between gate and source, the channel disappears and only a very small 

sub-threshold current can flow between the source and the drain.

Three modes of operation of a typical n-channel enhancement type MOSFET are:

Cutoff mode, linear mode and active (saturation mode)

 Procedure:

(a)Connect the circuit as per the circuit diagram.

(b)Keep gate-source voltage constant.

(c)Vary drain-source voltage in steps and note down drain current.

(d) Readings are tabulated and graph is drawn.

(e) Other characteristics may also be observed, if required.

Conclusion: Transfer characteristics of MOSFET have been observed.

 Precautions:

(a) Always connect the voltmeter in parallel & ammeter in series as shown in fig.

(b) Connection should be proper & tight.

(c) Switch ‘ON’ the supply after completing the ckt.

(d) DC supply should be increased slowly in steps

(e) Reading of voltmeter & Ammeter should be accurate.