MOSFET PRIMER

Sameer Sonkusalehttp://nanolab.ece.tufts.edu

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MOSFET

Metal Oxide Semiconductor Field Effect Transistor

Gate electrode is used to control the electric field in the channel region – which in turn controls the flow of charges between source and drain

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Quick Look At MOS Vs BJT Structure

In BJT, current conduction is due to both holes and electrons – hence the nameBipolar Transistor

In MOSFET, current conduction is due to one carrier – either holes or electrons

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Dissecting the MOSFET Structure

• Source-Drain are either p-type or n-type

• Gate is metal or polysilicon

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Views and Types of MOSFET

N-channel MOSFETNMOS

P-channel MOSFETPMOS

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NMOS as a switch

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PMOS as a switch

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N-channel MOSFET and P-channel MOSFET on same Substrate

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Array of FETs in an Integrated Circuit Environment

FOX – Field oxideN-well process example is shown above

Next problem : how do you connect these transistors ?- metal layers

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Metal layers and connections

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Side view showing metal interconnect layers in an IC environment

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Studying NMOS Regions of Operation

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Control the channel in NMOS

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Linear Region (Vgs > Vt and Vds small)

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MOSFET in linear region resembles a resistor (small Vds)

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As Vds is increased, channel tapers

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Pinch-Off

When pinch-off is reached, the amount of charge in the channel remains fixed* and therefore the current saturates.

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I-V characteristics of the MOSFET

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ID vs VGS

Square Law

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The iD–vDS characteristics for a device with k’n (W/L) = 1.0 mA/V2.

I-V characteristics of the MOSFET

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Figure 4.15 Increasing vDS beyond vDSsat causes the channel pinch-off point to move slightly away from the drain, thus reducing the effective channel length (by DL).

Actually Pinch-off point moves as Vds increases

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I-V characteristics showing finite output impedance

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Equations

Cut-off: VGS < VT ID = 0

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Key Observations

Control the ratio of Width and Length of the transistor to control transistor properties

It is a square law device. BJT was an exponential device

Terminologies for Triode and Saturation for MOSFETs are different than in BJT

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Symbol for NMOS

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Large Signal Model For MOSFET

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PMOS

OFF

ON

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Table 4.1

Large Signal Models Revisited