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CHAPTER 3:
TRANSISTOR MOSFET
DR. PHAM NGUYEN THANH LOAN

Hà Nội, 9/24/2012
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Chapter 3: MOSFET

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Introduction
Classifications

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JFET
 D-FET (Depletion MOS)
 MOSFET (Enhancement E-FET)

DC biasing
Small signal analysis
Equivalent small signal circuit

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FET Introduction

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High input impedance, nMΩ-n100MΩ
Controlled by voltage (≠ BJT)
Low power consumption
Low noise, suitable for small signal
Low impact of temperature
Using as switch for low power application
Small size and adapt for integrated circuit

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Classification
JFET-Junction Field Effect Transistor

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MOSFET-Metal Oxide Semiconductor FET
and P channels

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 Enhancement

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 Depletion

and P channels

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and P channels

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Classification (cont’d)
JFET

D-FET

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E-FET (MOSFET)

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JFET
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Structure and Operation
Characteristic Curve
Compare with BJT
Examples, datasheets

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JFET – Structure

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JFET – Operation

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

VGS = 0, VDS>0 increase gradually, ID increases and then saturates
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JFET – Operation

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VGS = 0, VDS = VP, ID = IDSS
VP : pinch off voltage (pinch-off)
ID = IDSS(1 - VGS/VP)2
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JFET – Operation

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VGS < 0, VDS > 0, Saturation current reduces when VGS  Vpinch-off
VGS = VP, ID = 0
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JFET – Characteristic Curves

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ID = f(VGS) Shockley equation: ID = IDSS(1 - VGS/VP)2

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IG ≈ 0A

(gate current)

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ID = IS

(ID drain current, IS source current)
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JFET – Characteristic Curves

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N-channel, IDSS = 8mA, VP = - 4V

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P-channel, IDSS = 6mA, VP = 6V

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JFET – Symbol

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JFET
2N5457

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Datasheet-2N5457
Symbol

Value

Unit

Drain-Source voltage

VDS

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Vdc

Drain-Gate voltage

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Vdc

VGSR

-25

Vdc

IG

10

nAdc

Device dissipation 250C
Derate above 250C

PD

310
2.82

mW
mW/0C

Junction temp range

TJ

125

0C

Storage channel temp range

Tstg

-60 to
+150

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Rating

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Gate current

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Reverse G-S voltage

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Datasheet-2N5457-characteristics

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Symbol

Min

VG-S breakdown

V(BR)GSS

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Igate reverse(Vgs=-15, Vds=0)

IGSS

VG-S cutoff

VGS(off)

Typ

Max

Unit
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Characteristic

nAdc

-1.0

Vdc

-2.5

-6.0

Vdc

3.0

5.0

mAdc

Ciss

4.5

7.0

pF

Crss

1.5

3.0

pF

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ID-zero gate volage

-0.5

VGS

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-1.0

Creverse transfer
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IDSS

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MOSFET
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Structures
Operation
Characteristic Curves

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MOSFET – Structure

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N-channel Depletion DMOS

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N-channel Enhancement EMOS

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MOSFET – Operation

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N-channel DMOS
VGS = 0, VDS > 0
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N-channel EMOS
VGS > VTH, VDS > 0
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DMOS – Transfer characteristic curves

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

Similar to JFET, transfer characteristic curve ID = f(VGS)
follows Shockley equation: ID = IDSS(1 - VGS/VP)2

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Can work at: VGS > 0, ID > 0
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EMOS – Transfer characteristic curve

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Transfer characteristic curve:
ID = k(VGS – VT)2 with VT > 0 (for NMOS) and Vt< 0 for PMOS)
When VGS < VT, ID = 0
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MOSFET – Transfer characteristic curve

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P-channel depletion

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MOSFET – Transfer characteristic curve

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P-channel enhancement

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MOSFET – Symbol

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DMOS
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EMOS
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EMOS
2N4351

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