btech vi sem 1

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Two Terminal MOS Structure

 MOSFET

• Now add source and drain to MOS capacitor. The new structure is called MOS transistor.

• Two types of MOS transistors : NMOS and PMOS

• The distance between S and D is called channel length L and lateral extent of channel is width W. Both W and L are very important parameters. The channel is formed through applied gate voltage between S and D.

 MOSFET Types

Figure: Circuit symbol for an enhancement-mode n-channel MOSFET.

Figure: n-Channel Enhancement MOSFET showing channel length L and channel width W.

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nMOS Transistor - Structure

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MOS Structure – p substrate

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MOS Structure – n substrate

Lecture-3@IIIT-Allahabad

N-Channel MOSFET Operation (acc, dep, inv)

Lecture-3@IIIT-Allahabad

N-Channel MOSFET Operation : I-V Curve (1)

NMOS in Linear Region :• if small drain voltage is applied, drain current flows through the conducting

channel.• As drain voltage increases, drain current also increases linearly with voltage. The

channel region acts as voltage controlled resistor. • This operation mode is called linear mode.

Lecture-3@IIIT-Allahabad

N-Channel MOSFET Operation : I-V Curve (2)NMOS in saturation Region :• As drain voltage increases again, the gate voltage is not sufficient to maintain

channel below the gate.• The charge and channel depth start to decrease at drain end. At Vds=Vdsat the

charge and depth at drain end become zero. • This point is called pinch off point. Beyond pinch-off depletion region forms

adjacent to drain and grows to source with increasing Vds.• This operation mode is called saturation mode. Near pinch-off point, high field

region forms between channel-end and drain. Electrons arriving at this end injected to drain region and accelerated to drain.

Lecture-3@IIIT-Allahabad

Lecture-3@IIIT-Allahabad

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MOS Device Design Equations

The drain to source current of an nMOS device is given by

Ids = 0 ; Cut-off region; Vgs<Vtn

Ids = n/2 [2(Vgs – Vtn)Vds - Vds 2 ] ;

Linear Region; Vds < (Vgs – Vtn) Ids = n/2 (Vgs – Vtn)2 ;

Saturation Region; Vds (Vgs – Vtn);

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MOS Device Design Equations

n = (n ox/tox) (W/L) A/V2

MOS Transistor Gain Factor n = Mobility of electrons, cm2/V-Sec ox = Oxide Permittivity, F/cm, tox = Oxide thickness, cm W = Width of the transistor, microns L = Length of the transistor, microns

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V-I Characteristics

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V-I Characteristics

P channel• The physical structure of a PMOS is

identical to a NMOS except that the semiconductor types are interchanged, i.e.,

• body and gate are made of n-type material and source and drain are made of p-type material and a p-type channel is formed.

• As the sign of the charge carriers are reversed, all voltages and currents in a PMOS are reversed.

• By convention, the drain current is owing out of the drain as is shown. With this, all of the NMOS discussion above applies to PMOS as long as we multiply all voltages by a minus sign:

Next class

• Consider NMOS circuit below with K = 0:25 mA/V2 and Vt = 2 V. Find vo

• when vi = 0, 6, and 12 V for RD = 1 K and VDD = 12 V.

Threshold voltage

• Dependence• Numerical • Second order effects• Strong and Weak one’s and zero’s………………………………………………………………………….• DC characteristics of inverter• Realization of circuits using CMOS

[Gates, Mux, Flip flops (sequential)]