8/3/2019 CP0614_27-Sep-2011_RM01
1/38
Transistors
Unit-4
8/3/2019 CP0614_27-Sep-2011_RM01
2/38
WHAT IS A TRANSISTOR?
A Transistor is a electronic device composed
of layers of a semiconductor material which
regulates current or voltage flow and acts asa switch or gate for electronic circuit.
8/3/2019 CP0614_27-Sep-2011_RM01
3/38
APPLICATIONS
AMPLIFICATION
SWITCHING
OSCILATING CIRCUIT
SENSORS
8/3/2019 CP0614_27-Sep-2011_RM01
4/38
8/3/2019 CP0614_27-Sep-2011_RM01
5/38
Bipolar Junction Transistor
First created in 1948 by Bell
Telephone
Similar to the Mosfet
Reliable under severeconditions dominant in
automobiles
NPN and PNP
Digital logic circuits-Boolean
8/3/2019 CP0614_27-Sep-2011_RM01
6/38
Introduction
The basic of electronic system nowadays is
semiconductor device.
The famous and common use of this device isBJTs
(Bipolar Junction Transistors).
It can be use as amplifier and logic switches.
BJT consists of three terminal:
collector : C
base : B
emitter : E
Two types of BJT : pnp and npn
8/3/2019 CP0614_27-Sep-2011_RM01
7/38
Transistor Construction
3 layer semiconductor device consisting:
2 n- and 1 p-type layers of material npn transistor
2 p- and 1 n-type layers of material pnp transistor
The term bipolar reflects the fact that holes andelectrons participate in the injection process into the
oppositely polarized material A single pn junction has two different types of bias:
forward bias
reverse bias
Thus, a two-pn-junction device has four types of bias.
8/3/2019 CP0614_27-Sep-2011_RM01
8/38
Position of the terminals and symbolof BJT.
Base is located at the middleand more thin from the levelofcollectorandemitter
The emitter and collectorterminals are made of thesame type of semiconductormaterial, while the base of theother type of material
8/3/2019 CP0614_27-Sep-2011_RM01
9/38
Transistor currents
-The arrow is always drawn
on the emitter
-The arrow always point
toward the n-type
-The arrow indicates the
direction of the emitter
current:
pnp:E B
npn: B
E
IC=the collector current
IB= the base currentIE= the emitter current
8/3/2019 CP0614_27-Sep-2011_RM01
10/38
By imaging the analogy of diode, transistor can be
construct like two diodes that connetecd together. It can be conclude that the work of transistor is base on
work of diode.
8/3/2019 CP0614_27-Sep-2011_RM01
11/38
Transistor Operation
The basic operation will be described using the pnptransistor. The operation of the pnp transistor isexactly the same if the roles played by the electronand hole are interchanged.
One p-n junction of a transistor is reverse-biased,whereas the other is forward-biased.
Forward-biased junctionof a pnp transistor
Reverse-biased junctionof a pnp transistor
8/3/2019 CP0614_27-Sep-2011_RM01
12/38
Both biasing potentials have been applied to a pnptransistor and resulting majority and minority carrierflows indicated.
Majority carriers (+) will diffuse across the forward-biased p-n junction into the n-type material.
A very small number of carriers (+) will through n-typematerial to the base terminal. Resulting IB is typically inorder of microamperes.
The large number of majority carriers will diffuse acrossthe reverse-biased junction into the p-type material
connected to the collector terminal.
8/3/2019 CP0614_27-Sep-2011_RM01
13/38
Majority carriers can cross the reverse-biased
junction because the injected majority carriers willappear as minority carriers in the n-type material.
Applying KCL to the transistor :
IE= IC+ IB
The comprises of two components the majority
and minority carriers
IC= ICmajority+ ICOminority
ICO ICcurrent with emitter terminal open and is
called leakage current.
8/3/2019 CP0614_27-Sep-2011_RM01
14/38
Common-Emitter Configuration
It is called common-emitter configuration since :
- emitter is common or reference to both input and
output terminals.
- emitter is usually the terminal closest to or at
groundpotential.
Almost amplifier design is using connection of CE due
to the high gain for current and voltage.
Two set of characteristics are necessary to describethe behavior for CE ;input (base terminal) and output
(collector terminal) parameters.
8/3/2019 CP0614_27-Sep-2011_RM01
15/38
Proper Biasing common-emitter configuration in active region
8/3/2019 CP0614_27-Sep-2011_RM01
16/38
Input characteristics for acommon-emitter NPN transistor
IB is microamperes compared
to miliamperes of IC.
IB will flow when VBE > 0.7V
for silicon and 0.3V forgermanium
Before this value IB is verysmall and no IB.
Base-emitter junction isforward bias
Increasing VCE will reduce IB
for different values.
8/3/2019 CP0614_27-Sep-2011_RM01
17/38
Output characteristics for acommon-emitter npntransistor
For small VCE (VCE < VCESAT, IC increase linearly withincreasing of VCE
VCE > VCESAT IC not totally depends on VCE constant IC
IB(uA) is very small compare to IC (mA). Small increase inIB cause big increase in IC
IB=0 A ICEO occur.
Noticing the value when IC=0A. There is still some value of
current flows.
8/3/2019 CP0614_27-Sep-2011_RM01
18/38
8/3/2019 CP0614_27-Sep-2011_RM01
19/38
Beta () or amplification factor The ratio of dc collector current (IC) to the dc base current
(IB) is dc beta (dc ) which is dc current gain where ICand IB are determined at a particular operating point, Q-
point (quiescent point).
Its define by the following equation:
30 < dc < 300 2N3904
On data sheet, dc=hFE with h is derived from ac hybridequivalent cct. FE are derived from forward-current
amplification and common-emitter configuration
respectively.
8/3/2019 CP0614_27-Sep-2011_RM01
20/38
For ac conditions an ac beta has been defined as the
changes of collector current (IC) compared to the
changes of base current (IB) where IC and IB aredetermined at operating point.
On data sheet, ac=hfe
It can defined by the following equation:
8/3/2019 CP0614_27-Sep-2011_RM01
21/38
Relationship analysis between and
8/3/2019 CP0614_27-Sep-2011_RM01
22/38
Transistor Terminal Identification
8/3/2019 CP0614_27-Sep-2011_RM01
23/38
Disadvantage of BJT
Downfalls of BJT- Large base current, ib,
is needed to turn transistor on.
Electrons and holes contribute toconduction which slows down the
switching speed.
8/3/2019 CP0614_27-Sep-2011_RM01
24/38
8/3/2019 CP0614_27-Sep-2011_RM01
25/38
8/3/2019 CP0614_27-Sep-2011_RM01
26/38
8/3/2019 CP0614_27-Sep-2011_RM01
27/38
8/3/2019 CP0614_27-Sep-2011_RM01
28/38
8/3/2019 CP0614_27-Sep-2011_RM01
29/38
8/3/2019 CP0614_27-Sep-2011_RM01
30/38
Construction of n-channel
enhancement MOSFET
8/3/2019 CP0614_27-Sep-2011_RM01
31/38
Basic operation
8/3/2019 CP0614_27-Sep-2011_RM01
32/38
8/3/2019 CP0614_27-Sep-2011_RM01
33/38
Transfer characteristics and Drain
chacracteristics
8/3/2019 CP0614_27-Sep-2011_RM01
34/38
8/3/2019 CP0614_27-Sep-2011_RM01
35/38
CMOS
CMOS-Complementary MOS
P-channel and n-channel is constructed on
the same substrate
Has extensive application in computer logic
design
It is effectively used as an Inverter.
8/3/2019 CP0614_27-Sep-2011_RM01
36/38
ADVANTAGES
Fast switching speed
Low operating power levels
High input impedence
8/3/2019 CP0614_27-Sep-2011_RM01
37/38
8/3/2019 CP0614_27-Sep-2011_RM01
38/38
CMOS -INVERTER