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Nov 2, 2013 (3 years and 11 months ago)

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Transistor

Transistor
-

Definition


A
transistor

is a
semiconductor device

that uses a small amount of voltage or
electrical current to control a larger change
in voltage or current.

Transistor
-

Uses


Because of its fast response and
accuracy, the transistor may be used in a
wide variety of
digital

and
analog

functions, including
amplification
,
switching
,
voltage regulation
, signal
modulation
, and
oscillators
. Transistors
may be packaged individually or as part of
an
integrated circuit

chip, which may hold
thousands of transistors in a very small
area.


Transistor
-

Uses


Although millions of individual (known as
discrete
) transistors are still used, the vast
majority of transistors are fabricated into
integrated circuits

(
IC

, also called
microchips

or
simply
chips
) along with
diodes
,
resistors
,
capacitors

and other
electronic components

to
produce complete electronic circuits.


A
logic gate

consists of about twenty transistors
whereas an advanced microprocessor, as of
2006, can use as many as 1.7 billion transistors
(
MOSFETs
).

Transistor
-

History


The bipolar (point
-
contact) transistor was
invented in December 1947 at the
Bell
Telephone Laboratories

by
John Bardeen

and
Walter Brattain

under the direction of
William
Shockley
.


The junction version, invented by Shockley in
1951, enjoyed three decades as the device of
choice in the design of discrete and
integrated
circuits
.


Nowadays, the use of the BJT has declined in
favour of
CMOS

technology in the design of
digital integrated circuits.

Transistor (The two types of)


Modern transistors are divided into two
main categories:


bipolar junction transistors

(BJTs)


and
field effect transistors

(FETs).

Bi
-
Polar Junction Transistor or BJT




NPN




PNP

Field Effect Transistor or FET




MOSFET




VFET




IGFET

Bipolar junction transistor


The BJT is a 3 terminal component.


The 3 terminals are the Emitter, Collector,
and Base.


The Emitter and Collector are made up of
the same type of semiconductor material
(p
-

or n
-
type), and the base is made up of
the other type of material.

Bipolar junction transistor


There are two types of BJT’s.


The npn transistor has n
-
type emitter and
collector materials, and a p
-
type base.


The pnp transistor has p
-
type emitter and
collector materials, and an n
-
type base.

Bipolar junction transistor

PNP Transistor

NPN Transistor

Bipolar junction transistor


The arrow is always drawn on the emitter
terminal.


The opposite terminal is the collector.


The middle terminal is the base.


The arrow always points towards the n
-
type material.


NPN
-
Arrow Not pointing in


PNP
-
Arrow Pointing in

Bipolar junction transistor


Equivalent Circuit

Collector

Base

Emitter

NPN Type

Collector

Base

Emitter

PNP Type

Bipolar junction transistor


The transistor is a current
-
controlled device, the
values of the collector and emitter currents are
determined primarily by the value of the base
current.



Small changes in the current applied across the
base

emitter terminals causes the current that
flows between the
emitter

and the
collector

to
change significantly. This effect can be used to
amplify the input voltage or current.

NPN Transistor


Most bipolar transistors used today are NPN. So
we will concentrate on the NPN transistor,
however, all the principles covered apply equally
to the PNP transistor.


An NPN transistor can be considered as two
diodes

with a shared
anode

region.


In order for the transistor to be considered on,
the emitter

base
junction

is
forward biased

and
the base

collector junction is
reverse biased
.


NPN Transistor

NPN Transistor


To make the transistor conduct from C to
E, the voltage between B and E must be
above a
threshold voltage

sometimes
referred to as the cut
-
in voltage.


The cut
-
in voltage is usually about 600 mV
for silicon BJTs.


This applied voltage causes the lower p
-
n
junction to 'turn
-
on' allowing a flow of
electrons from the emitter into the base.

NPN Transistor


Because of the electric field existing
between base and collector (caused by
VCE
), the majority of these electrons cross
the upper p
-
n junction into the collector to
form the collector current,
IC
.


The remainder of the electrons recombine
with holes, the majority carriers in the
base, making a current through the base
connection to form the base current,
IB
.


NPN Transistor


In the diagram, the arrows representing current
point in the direction of the electric or
conventional current

the flow of electrons is in
the opposite direction of the arrows since
electrons carry negative
electric charge
.


The ratio of the collector current to the base
current is called the
DC current gain
. This gain is
usually quite large and is often 100 or more.

Transistor Switches


Intended to be either off or on and
saturated.


Low resistance between Collector and
emitter when saturated


High resistance between collector and
emitter when in the off condition


Used to increase either the voltage or
current rating of an electronic signal


Typical Switch Circuit

Input

Common

Load

Load

Power

Supply

BJT used

as

Switch

Pull off Resistor

Switch in OFF Condition

Common

0 Volts

Input

0 Volts

Load

Supply

Voltage

Load

Supply

Voltage

Switch in ON Condition

Common

+5 Volts

Input

0 Volts

Load

Supply

Voltage

Load

Supply

Voltage