Outline

 General overview on FET

 Comparison of BJT and FET
 The classifications of FET
 The physical structure of the JFET & MOSFET and how it works
 Equations that describe VI-characteristics
FIELD-EFFECT TRANSISTORS (FETS)
 How these class of transistor can be used to make an amplifier

By: Behailu T.

Field-Effect Transistors - FETs FET verses BJT

 FET is a three-terminal device used for a variety of applications that Field Effect Transistor (FET) Bipolar JunctionTransistor (BJT)

match, to a large extent, those of the BJT transistor. Unipolar Bipolar

1 Current depends only on one type of Current depends on both electrons and
charge carriers holes
 Both BJT and FET can be used for:
2 Voltage controlled device Current controlled device
 Amplifiers 3 Very high input impedance Low input impedance

 Switching devices 4
Simpler in fabrication and occupies
occupies large space in integrated form
less space in integrated form
 Impedance matching circuits
Low voltage gain High voltage gain
5
High current gain Low current gain

6 Easily damaged Robust

 FET: Classification JFET : Structure
Terminals: Gate - G | Drain - D | Source - S
MOSFET : Metal Oxide Semiconductor FET FET
JFET : Junction FET

MOSFET JFET

p n p
n p n

Enhancement Depletion p-channel n-channel

MOSFET MOSFET JFET JFET

n-channel JFET p-channel JFET

n-Channel p-Channel n-Channel p-Channel
EMOSFET EMOSFET DMOSFET DMOSFET
 The n-channel is more widely used

JFET: Operation JFET: Operation

 The JFET is always operated with the Gate-

Source pn-junction reverse-biased.
 Reverse biasing of the GS-junction produces a
depletion region along the pn junction, which
extends into the n channel and thus increases
its resistance by restricting the channel width.
 The channel width and thus the channel
resistance can be controlled by varying the gate
voltage, thereby controlling the amount of Physical operation of the n -channel JFET:
drain current, ID. a) For small the channel is uniform and the device functions as a resistance whose value is controlled
by (more in negative  larger depletion region  narrower channel  lower current )
Sets the reverse-bias voltage b) Increasing causes the channel to acquire a tapered shape and eventually pinch-off occurs, so the
current reaches the max value . If = 0  the max value is
 JFET: Characteristics & Parameters JFET: controls
JFET must be operated between
 Pinch-Off Voltage ( ): For = 0 V, the value = 0 V and ( )
of at which becomes essentially constant
(point B on the curve).
 For a given JFET, has a fixed value Active (Pinch-off) region

 Drain current is (Drain to Source current

with gate Shorted) and is always specified on JFET
datasheets.
 is the maximum drain current that a specific
JFET can produce regardless of the external
circuit.

Cutoff Voltage

JFET versus BJT

JFET: Transfer Characteristic
JFET transfer characteristic curve
Saturation region
is expressed approximately as :
≥−
Linear region

Active (Pinch-off) region

Saturation region
Linear region

Channel Off

Channel Off
≤− ≤−
JFET: Small-Signal Model JFET: Small-Signal Model
 Transconductance:  Input Impedance:

 The relationship of a change in ID to Z i  

the corresponding change in VGS is  Output Impedance:

called transconductance 1
Z o  rd 
y os
 Transconductance is denoted gm and
where,
given by:
VDS
ΔI D rd  VGS  constant
gm  I D
ΔV GS
yos= admittance parameter listed on FET
specification sheets. Fig. n-channel JFET and its small signal equivalent

JFET: Common-Source(CS) Fixed-Bias Circuit JFET: Common-Source(CS) Fixed-Bias Circuit

DS

• The resistor RG is
present to ensure that
Vi appears at the input
to the FET amplifier for
the ac analysis.
• The zero-volt drop Fig. AC equivalent Circuit
across RG permits
Fig. CS Fixed-bias Circuit Fig. CS Fixed-bias Circuit
Fig. DC equivalent Circuit replacing RG by a short-
circuit equivalent.
JFET: Common-Source(CS) Fixed-Bias Circuit JFET: Common-Source(CS) Self-Bias Circuit

Input impedance:
Zi  RG
Output impedance:
Z o  R D || rd
Zo  R D
rd  10R D

Voltage gain: Fig. AC equivalent Circuit

Vo
Av    g m (rd || R D )
Vi
V
A v  o  g m R D Fig. CS Self-bias Circuit
Vi rd  10R D
Fig. CS Self-bias DC equivalent Circuit

JFET: Self-Bias Configuration – Graphical Soln. JFET: Common-Source(CS) Self-Bias Circuit

Fig. CS Self-bias DC
Fig. CS Self-bias Circuit Fig. CS Self-bias AC equivalent Circuit
equivalent Circuit
JFET: Common-Source(CS) Self-Bias Circuit JFET: Common-Source(CS) Voltage-divider-Bias Circuit

 Input impedance:
Zi  RG

 Output impedance:
Z o  rd || R D

Zo  R D
rd  10R D

 Voltage gain:
A v   g m (rd || R D )
Fig. CS Voltage-divider-bias Circuit Fig. CS Voltage-divider-bias
A v  g m R D Fig. CS Self-bias AC equivalent Circuit
rd  10R D DC equivalent Circuit

JFET: Common-Source(CS) Voltage-divider-Bias Circuit JFET: Common-Source(CS) Voltage-divider-Bias Circuit

Input impedance:
Z i  R 1 || R 2
Output impedance:
Z o  rd || R D
Zo  R D
rd  10R D
Voltage gain:
A v   g m (rd || R D )
Fig. CS Voltage-divider-bias AC equivalent Circuit Fig. CS Voltage-divider-bias AC equivalent Circuit
Fig. CS Voltage-divider-bias Circuit A v  g m R D
rd  10R D
JFET: Reading Assignment MOSFET- Metal Oxide Semiconductor Field-Effect Transistor

 Has no pn junction structure; instead,

 Common-Gate Configuration the Gate of the MOSFET is insulated
from the channel by a silicon dioxide
 Common-Drain(Source-Follower) Configuration
(SiO2) layer.
 The two basic types of MOSFETs
are:
 Enhancement(E) MOSFET &
 Depletion(D) MOSFET

 The enhancement MOSFET is more Symbols for: (a) n-channel symbols for: (a) n-channel
enhancement-type depletion-type MOSFETs
widely used. and (b) p-channel depletion-
MOSFETs and (b) p-channel
enhancement type type MOSFETs.
MOSFETs.

Depletion-Type MOSFET Construction D-MOSFET: Operation and Characteristics

 The Drain (D) and Source (S) connect to
the n-doped regions.  = 0 and is applied across the drain-
 These n-doped regions are connected via an source terminals
n-channel.  This results to attraction of free electrons of
 This n-channel is connected to the Gate(G)
the n-channel to the drain, and hence
via a thin insulating layer of SiO2.
current flows.
 The n-doped material lies on a p-doped
substrate that may have an additional
terminal connection called Substrate(SS).

Fig. n-Channel depletion-type MOSFET

with VGS = 0 V and applied voltage VDD.
D-MOSFET: Operation and Characteristics D-MOSFET: Operation and Characteristics

 For negative value of , the negative potential at  For positive values of , the positive gate will  A depletion-type MOSFET can
operate in two modes:
the gate pressures electrons toward the p-type draw additional electrons (free carriers) from the
p-type substrate and hence increases. Depletion Mode
substrate and attract holes from p-type substrate.
The characteristics are similar to a
 This will reduce the number of free electrons in JFET.
 When =0 , =
the n-channel available for conduction.  When <0 , <
 The more negative the , the resulting level of Enhancement Mode
drain current is reduced.  >0
 increases above
 When is reduced to (Pinchoff voltage), then
= 0mA. The formula used to plot the transfer
curve still applies:
Fig. Reduction in free carriers in a channel
due to a negative potential at the gate
= −
terminal.

p-channel D-Type MOSFET Enhancement-Type MOSFET Construction

 The Drain(D) and Source(S) connect
to the n-doped regions.
 The Gate(G) connects to the p-doped
substrate via a thin insulating layer of
SiO2.
 There is no physical channel.
 The n-doped material lies in the p-
doped substrate that may have an
additional terminal connection called
the Substrate (SS).
Fig. n-channel enhancement-type MOSFET.
E-MOSFET : Operation and Characteristics E-MOSFET : Operation and Characteristics
 For = 0 , = 0 (no channel).  The enhancement-type MOSFET operates
 For some positive voltage, and = 0, two only in the enhancement mode.
reverse biased pn-junctions and no significant
 is always positive.
flow between Drain and Source.
 For > 0 and > 0 , the positive voltage
 As increases, increases.
at Gate pressure holes to enter deeper regions  As is kept constant and is increased,
of the p-substrate, and the electrons in the p- then saturates and the saturation level,
substrate will be attracted to the positive Gate. is reached.
 The level of that results in the significant = -
increase in the Drain current is called threshold
voltage ( ).
 For < , = 0mA.
Fig. For the case of > 0 and >0

E-MOSFET : Operation and Characteristics

MOSFET: Reading Assignment

 MOSFET Small Signal Model Analysis