MOSFETs

Metal-Oxide-Semiconductor

Field Effect Transistors

Classes of Field Effect Transistors

• Metal-Oxide-Semiconductor Field Effect Transistor– Which will be the type that we will study in this course.

• Metal-Semiconductor Field Effect Transistor– MESFET

• Junction Field Effect Transistor– JFET

• High Electron Mobility Transistor or Modulation Doped Field Effect Transistor– HEMT or MODFET

• Fast Reverse/Fast Recovery Epitaxial Diode– FREDFET

• DNA Field Effect Transistor– The conduction path is through a strand of DNA

Field Effect Transistors

• The conductivity (or resistivity) of the path between two contacts, the source and the drain, is altered by the voltage applied to the gate.– Device is also known as a voltage controlled

resistor.

Types of MOSFETS

n-channel

Enhancement Mode

(nMOSFET)

p-channel

Enhancement Mode

(pMOSFET)

n-channel

Depletion Mode

(nMOSFET)

p-channel

Depletion Mode

(pMOSFET)

Cross-Sectional View of n channel planar Enhancement Mode Transistor

p channel Enhancement Mode Transistor

n channel Depletion Mode Transistor

p channel Depletion Mode Transistor

Symbols for n channel Enhancement Mode MOSFET

VGS ≥ 0V, VDS ≥ 0V

VTN is positive

Symbols for p channel Enhancement Mode MOSFET

VGS ≤ 0V, VDS ≤ 0V

VTP is negative

Symbols for n channel Depletion Mode MOSFET

Symbols for p channel Depletion Mode MOSFET

PSpice MOSFET Symbols

• The IRF150 is an nMOS and the IRF9140 is a pMOS. Both are enhancement mode transistors.– The body terminal is connected to the source terminal on

the FET.

– “M” is used to denote that the device is a MOSFET.

MOS Capacitor

MOS Capacitor Under Bias:Electric Field and Charge

Parallel plate capacitor

Accumulation

Positive gate bias

Electrons attracted to gate

Depletion Inversion

Negative gate bias: Holes attracted to gate

MOS Capacitor: p-type semiconductor

Accumulation Depletion Inversion

Threshold Voltage

The gate voltage that causes the concentration of electrons immediately under the gate oxide is equal to the concentration of holes is called the threshold voltage.

• Enhancement mode FETs• NMOS VG = VTN

• When enough electrons have been attracted to the oxide-semiconductor interface to create a path for current to flow between the source and drain.

• PMOS VG = VTP • When holes have been attracted to the oxide-semiconductor interface to create a path for

current to flow between the source and drain.

• Depletion mode FETs• NMOS VG = VTN

• When holes have been attracted to the oxide-semiconductor interface to stop current from flowing between the source and drain.

• PMOS VG = VTP • When electrons have been attracted to the oxide-semiconductor interface to stop current

from flowing between the source and drain.

Capacitance

OX

ox

OX

oxOX

OXOX

TTC

ACCC

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MOSFETs

Enhancement mode Depletion mode

• Also known as Normally Off transistors.– A voltage must be applied

to the gate of the transistor, at least equal to the threshold voltage, to create a conduction path between the source and the drain of the transistor before current can flow between the source and drain.

• Also known as Normally On transistors.– A voltage must be applied

to the gate of the transistor, at least equal to the threshold voltage, to destroy a conduction path between the source and the drain of the transistor to prevent current from flowing between the source and drain.

Before electron inversion layer is

formed

After electron inversion layer is

formed

Family of ID Versus VDS Curves:Enhancement-Mode nMOSFET

VDS > VGS – VTNVDS < VGS – VTN

Pinch-off/Saturation

Triode/Nonsaturation

Cut-off VGS < VTN

Family of iD Versus vDS Curves:Depletion-Mode nMOSFET

Assuming that VTN < -1V

For this discussion

• I am going to emphasize the operation and applications of n channel enhancement mode FETs

Piecewise ModelCut-off Region

VGS < VTN

VTN is positiveID = 0 mA

Piecewise ModelNonsaturation/Triode Region

VGS > VTN

VDS < VGS – VTN

ID ≤ 0 mAVTN is positive

Piecewise ModelSaturation/Pinch-off Region

VGS > VTN

VDS > VGS – VTN

ID ≤ 0 mAVTN is positive

Summary of I-V Relationships

Region NMOS

Nonsaturation/

Triode

VDS < VDS(sat)

Saturation/

Pinch-off

VDS > VDS(sat)

Transition between triode and pinch-off

VDS(sat) = VGS - VTN

Enhancement Mode VTN > 0 V, ID ≥ 0 mA, ID = IS, IG = 0 mA

D

DSDSon

DSDSTNGSnD

I

VR

VVVVL

WkI

2'

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D VVL

WkI

Questions• To increase the drain current ID at a particular VDS and VGS, should you

use a MOSFET with a larger or smaller W/L ratio?

• Compare the operation of two FETs, where MOS #1 has a smaller VTN than MOS #2. Sketch the differences on a graph of ID-VDS.

• The microelectronics industry is working to decrease the channel length L. If W is held constant, how will:

– the capacitance between the gate and the channel change?

– the time it takes for an electron to move from the source to the drain be altered?

– the value of VTN change?

– this modify RDSon for a particular set of VDS and VGS?

• The microelectronics industry is also working to decrease the thickness of the gate oxide TOX and is researching high and low dielectrics to replace silicon dioxide as the gate dielectric?

– If TOX decreases, how will the capacitance between the gate and channel change?

– Should a low or high dielectric be used to increase the capacitance?