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Chapter 6
Bipolar Junction
Transistor




Current Controlled
Low input impedance
High gain
Temp, static sensitive
Field Effect Transistor





Voltage Controlled
High input impedance
Lower gain
More stable
Smaller footprint (ICs)
Both are used in amplifiers, switching circuits, and impedance
matching circuits
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JFET: Junction FET
MOSFET: Metal–Oxide–Semiconductor FET
D-MOSFET: Depletion MOSFET
E-MOSFET: Enhancement MOSFET
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Two main types
◦ N-channel (more common)
◦ P-channel

Three terminals
◦ Drain
◦ Source
◦ Gate
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The source is the accumulation of
electrons at the negative pole of the
drain-source voltage.
The drain is the electron deficiency
(or holes) at the positive pole of the
applied voltage.
The gate controls the width of the nchannel and, therefore, the flow of
charges from source to drain.
LABELS BASED ON ELECTRON FLOW
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There are three basic operating conditions for a JFET:
• VGS = 0 V, VDS increasing to some positive value
• VGS < 0 V, VDS at some positive value
• Voltage-controlled resistor
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Three things happen when VGS = 0 V and VDS increases
from 0 V to a more positive voltage:
• The size of the depletion region between
p- type gate and n-channel increases.
• Increasing the size of the depletion
region decreases the width of the nchannel, which increases its resistance.
• Even though the n-channel resistance is
increasing, the current from source to
drain (ID) through the n-channel is
increasing because VDS is increasing.
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Depletion ranges grow
till Vp
◦ ID reaches saturation (IDSS)
◦ No increase in ID with VDS
increases
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

Depletion region grows
as VGS becomes more
negative
Reaches IDSS at a lower
level
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As VGS becomes more negative:
• The JFET experiences
pinch-off at a lower voltage
(VP).
• ID decreases (ID < IDSS)
even when VDS increases
• ID eventually drops to 0 A.
The value of VGS that causes
this to occur is designated
VGS(off).
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The region to the left of the
pinch-off point is called the
ohmic region.
The JFET can be used as a
variable resistor, where VGS
controls the drain-source
resistance (rd).
As VGS becomes more negative, the resistance (rd)
increases.
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P and N channel JFETs
operate the same
◦ Voltage polarities and
current directions are
reversed.
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As VGS becomes more positive:
• The JFET experiences pinch-off
at a lower voltage (VP).
• The depletion region increases,
and ID decreases (ID < IDSS)
• ID eventually drops to 0 A
(when VGS = VGSoff)
Also note that at high levels of VDS the JFET reaches a breakdown
situation: ID increases uncontrollably if VDS > VDSmax.
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P-channel symbol
has the arrows
reversed
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JFET input-to-output transfer characteristics are not
as straightforward as they are for a BJT.
• BJT:  indicates the relationship between IB (input) and IC
(output).
• JFET: The relationship of VGS (input) and ID (output) :
Shockley’s equation
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Final version with limits shown on Pg. 393
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Manually

Solve for VGS = 0 V:
◦ ID = IDSS

Graphing Calculator

Solving for VGS =
VGS(off):
Solving for VGS = Vp/2:
◦

=
1−
◦ IDSS is in mA (enter
without e-3)
◦ Set view window for
proper view
◦ ID = 0 A

Plot 1 =

Trace graph
◦ X=VGS
◦ Y=ID
Draw curve
P-channel is drawn the same, except VP and VGS
are positive, and the curve is mirrored over the
y-axis
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Given:
IDSS = 8 mA
VP = -4 V


Draw transfer curve
Find VGS when ID = 2.5
mA
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Electrical
Characteristics
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Maximum Ratings
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
=

ID=IS
IG≅ 0 A

FET
1−



21
IC=βIB
IC ≅ IE
VBE = 0.7
BJT
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Metal Oxide Semiconductor Field Effect
Transistor
◦ Metal – D, G, and S connections
◦ Oxide – SiO2 insulation layer
◦ May also be called IGFET (insulated gate FET)

Two main types
◦ Depletion
◦ Enhancement
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The Drain (D) and Source (S)
connect to the to n-type regions.
These n-typed regions are
connected via an n-channel. This
n-channel is connected to the
Gate (G) via a thin insulating
layer of silicon dioxide (SiO2).
The n-type material lies on a ptype substrate that may have an
additional terminal connection
called the Substrate (SS).
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The characteristics are
similar to a JFET.
When VGS = 0 V, ID = IDSS
When VGS < 0 V, ID < IDSS
The formula used to plot the
transfer curve for a JFET applies to
a D-MOSFET as well:
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VGS > 0 V, ID increases above IDSS (ID > IDSS)
◦ Free electrons are pulled from the P-channel
◦ Very easy to exceed IDMAX
◦ Still plotted using Shockley’s equation
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
Given:
◦ IDSS = 10 mA
◦ VP = -4



Draw transfer curve
Find ID when VGS = -2 V
Find ID when VGS = 1 V
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Maximum Ratings
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Electrical
Characteristics
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The Drain (D) and Source (S) connect to the to n-type regions.
These n-type regions are connected via an n-channel
The Gate (G) connects to the p-type
substrate via a thin insulating layer of
silicon dioxide (SiO2)
There is no channel
The n-type material lies on a p-type
substrate that may have an additional
terminal connection called the
Substrate (SS)
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
Operates only in enhancement mode
◦ When
= 0
≥ 0,
◦ When 0 <
<
, = 0
◦ When
, > 0
≥
 Increases in VDS cause ID to increase till IDSAT
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To find current

k- use given conditions
and VT of MOSFET
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
Operating voltages
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The p-channel enhancement-type MOSFET is similar
to its n-channel counterpart, except that the voltage
polarities and current directions are reversed.
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Maximum Ratings
more…
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MOSFETs are very sensitive to static electricity.
Because of the very thin SiO2 layer between the external terminals
and the layers of the device, any small electrical discharge can
create an unwanted conduction.
Protection
• Always transport in a static sensitive bag
• Always wear a static strap when handling MOSFETS
• Apply voltage limiting devices between the gate and source,
such as back-to-back Zeners to limit any transient voltage.
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VMOS (vertical MOSFET) is a component structure that
provides greater
surface area.
Advantages
VMOS devices handle
higher currents by
providing more surface
area to dissipate the heat.
VMOS devices also have
faster switching times.
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CMOS (complementary MOSFET) uses a p-channel and
n-channel MOSFET; often on the same substrate as
shown here.
Advantages
• Useful in logic circuit designs
• Higher input impedance
• Faster switching speeds
• Lower operating power levels
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
Q. 20
◦ Region of operation should have at least four points
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