Download base region

College Name : Shree Swami Atmanand Saraswati Institute Of
Technology(SSASIT)(076)
Year : 2nd Year(3rd Sem) EC-2015
Subject Name : Electronic Devices &Circuits(EDC) (2131006)
Topic Name : TRANSISTOR FUNDAMENTALS
Prepared By :
Desai Jay H. (140760111008)
Guided By : Prof.Avni P. Lakhlani
BJT STRUCTURE
Basic structure of the bipolar junction transistor (BJT) determines its
operating characteristics.
Constructed with 3 doped semiconductor regions called emitter, base,
and collector, which separated by two pn junctions.
2 types of BJT;
(1) npn: Two n regions separated by a p region
(2) pnp: Two p regions separated by an n region.
BIPOLAR:
refers to the use
of both holes &
electrons as
current carriers
in the transistor
structure.
Base-emitter junction: the pn junction joining the base region & the
emitter region.
Base-collector junction: the pn junction joining the base region & the
collector region.
A wire lead connects to each of the 3 regions. These leads labeled as;
E: emitter
B: base
C: collector
BASE REGION: lightly doped, & very thin
EMITTER REGION: heavily doped
COLLECTOR REGION: moderately doped
STANDARD BJT SYMBOLS
BASIC BJT OPERATION
For a BJT to operate properly as an amplifier, the two pn junctions
must be correctly biased with external dc voltages.
Figure: shows a bias arrangement for npn BJTs for operation as an
amplifier.
In both cases, BE junction is forward-biased & the BC junction is
reverse-biased.  called forward-reverse bias.
Look at this one circuit as two separate
circuits, the base-emitter(left side) circuit and
the collector-emitter(right side) circuit. Note
that the emitter leg serves as a conductor for
both circuits. The amount of current flow in
the base-emitter circuit controls the amount of
current that flows in the collector circuit.
Small changes in base-emitter current
yields a large change in collector-current.
The heavily doped n-type emitter region has a very high density of
conduction-band (free) electrons.
These free electrons easily diffuse through the forward-based BE
junction into the lightly doped & very thin p-type base region
(indicated by wide arrow).
The base has a low density of holes, which are the majority carriers
(represented by the white circles).
A small percentage of the total number of free electrons injected into
the base region recombine with holes & move as valence electrons
through the base region & into the emitter region as hole current
(indicated by red arrows).
BJT OPERATION SHOWING ELECTRON FLOW.
When the electrons that have recombined with holes as valence
electrons leave the crystalline structure of the base, they become free
electrons in the metallic base lead & produce the external base
current.
Most of the free electrons that have entered the base do not recombine
with holes because the base is very thin.
As the free electrons move toward the reverse-biased BC junction,
they are swept across into the collector region by the attraction of the
positive collector supply voltage.
The free electrons move through the collector region, into the external
circuit, & then return into the emitter region along with the base
current.
The emitter current is slightly greater than the collector current
because of the small base current that splits off from the total current
injected into the base region from the emitter.
TRANSISTOR CURRENTS
The directions of the currents in both npn and pnp transistors and their
schematic symbol are shown in Figure (a) and (b). Arrow on the emitter
of the transistor symbols points in the direction of conventional
current. These diagrams show that the emitter current (IE) is the sum of
the collector current (IC) and the base current (IB), expressed as follows:
IE = I C + I B
BJT CHARACTERISTICS & PARAMETERS
Figure shows the proper bias
arrangement for npn
transistor for active
operation as an amplifier.
Notice that the base-emitter
(BE) junction is forwardbiased by VBB and the basecollector (BC) junction is
reverse-biased by VCC. The dc
current gain of a transistor is
the ratio of the dc collector
current (IC) to the dc base
current (IB), and called dc beta
(DC).
DC = IC/IB
The ratio of the dc collector current (IC)
to the dc emitter current (IE) is the dc
alpha.
α DC = IC/IE
Analysis of this transistor circuit to predict the dc voltages and currents
requires use of Ohm’s law, Kirchhoff’s voltage law and the beta for the
transistor;
Application of these laws begins with the base circuit to determine the
amount of base current. Using Kichhoff’s voltage law, subtract the VBE
=0.7 V, and the remaining voltage is dropped across RB .
Thus, VRB = VBB - VBE.
Determining the current for the base with this information is a matter of
applying of Ohm’s law. VRB/RB = IB
The collector current is
determined by
multiplying the base
current by beta.
Thus, IC= βDC * IB
What we ultimately
determine by use of
Kirchhoff’s voltage law
for series circuits is that,
in the base circuit, VBB is
distributed across the
base-emitter junction
and RB in the base
circuit. In the collector
circuit we determine that
VCC is distributed
proportionally across
RC and the
transistor(VCE).
BJT CIRCUIT ANALYSIS
There are three key dc voltages and three key dc currents to be
considered. Note that these measurements are important for
troubleshooting.
IB: dc base current
IE: dc emitter current
IC: dc collector current
VBE: dc voltage across
base-emitter junction
VCB: dc voltage across
collector-base junction
VCE: dc voltage from
collector to emitter
When the base-emitter junction is forward-biased,
VBE ≅ 0.7 V
VRB = IBRB : by Ohm’s law
IBRB = VBB – VBE : substituting for VRB
IB = (VBB – VBE) / RB : solving for IB
VCE = VCC – VRc : voltage at the collector with respect to the
grounded emitter
VRc = ICRC
VCE = VCC – ICRC : voltage at the
collector with
respect to the emitter
The voltage across the reverse-biased
collector-base junction
VCB = VCE – VBE
where IC = βDCIB
COLLECTOR CHARACTERISTIC CURVES
Gives a graphical
illustration of the
relationship of collector
current and VCE with
specified amounts of
base current. With
greater increases of VCC ,
VCE continues to increase
until it reaches
breakdown, but the
current remains about the
same in the linear region
from 0.7V to the
breakdown voltage.
Sketch an ideal family of collector curves for the circuit in Figure for IB = 5 μA to 25 μA in 5
μA increment. Assume βDC = 100 and that VCE does not exceed breakdown.
SKETCH AN IDEAL FAMILY OF COLLECTOR CURVES FOR THE CIRCUIT IN
FIGURE FOR IB = 5 ΜA TO 25 ΜA IN 5 ΜA INCREMENT. ASSUME ΒDC = 100 AND
THAT VCE DOES NOT EXCEED BREAKDOWN.
IC = βDC IB
IB
5 μA
10 μA
15 μA
20 μA
25 μA
IC
0.5 mA
1.0 mA
1.5 mA
2.0 mA
2.5 mA
CUTOFF
With no IB , the transistor is in the cutoff region and just as the
name implies there is practically no current flow in the
collector part of the circuit. With the transistor in a cutoff state,
the full VCC can be measured across the collector and
emitter(VCE).
Cutoff: Collector leakage current (ICEO) is extremely small and is usually
neglected. Base-emitter and base-collector junctions are reverse-biased.
SATURATION
Once VCE reaches its maximum value, the transistor is said to be in
saturation.
Saturation: As IB increases due to increasing VBB, IC also increases and VCE
decreases due to the increased voltage drop across RC. When the transistor reaches
saturation, IC can increase no further regardless of further increase in IB. Baseemitter and base-collector junctions are forward-biased.
DC LOAD LINE
The dc load line graphically illustrates IC(sat) and cutoff for a transistor.
Active
region of
the
transistor’s
operation.
DC load line on a family of collector characteristic curves illustrating the
cutoff and saturation conditions.
MAXIMUM TRANSISTOR RATINGS
A transistor has limitations on its operation. The product of VCE
and IC cannot be maximum at the same time. If VCE is
maximum, IC can be calculated as
IC 
PD (max)
VCE
Ex 4-5 A certain transistor is to be operated with VCE = 6 V. If
its maximum power rating is 250 mW, what is the most collector
current that it can handle?
IC 
PD (max)
VCE
250 mW

 41.7 mA
6V
THE BJT AS A SWITCH
A transistor when used as a switch is simply being biased so that it
is in cutoff (switched off) or saturation (switched on). Remember
that the VCE in cutoff is VCC and 0V in saturation.
CONDITIONS IN CUTOFF & SATURATION
A transistor is in the cutoff region when the base-emitter junction is not
forward-biased. All of the current are zero, and VCE is equal to VCC
VCE(cutoff) = VCC
When the base-emitter junction is forward-biased and there is enough base
current to produce a maximum collector current, the transistor is saturated.
VCC  VCE ( sat )
The formula
for
I C ( sat )  collector saturation current is
RC
The minimum value of base current
needed to produce saturation is
I B (min) 
I C ( sat )
 DC
Transistor Construction
There are two types of transistors:
• pnp
• npn
pnp
The terminals are labeled:
• E - Emitter
• B - Base
• C - Collector
npn
26
Transistor Operation
With the external sources, VEE and VCC, connected as shown:
• The emitter-base junction is forward biased
• The base-collector junction is reverse biased
27
Currents in a Transistor
Emitter current is the sum of the collector and
base currents:
IE  IC  IB
The collector current is comprised of two
currents:
IC  IC
 I CO
majority
minority
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Operating Regions
•
Active – Operating range of the
amplifier.
•
Cutoff – The amplifier is basically
off. There is voltage, but little
current.
•
Saturation – The amplifier is full on.
There is current, but little voltage.
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Approximations
Emitter and collector currents:
I
C
I
E
Base-emitter voltage:
VBE  0.7 V (for Silicon)
30
Alpha (a)
Alpha (a) is the ratio of IC to IE :
αdc 
IC
IE
Ideally: a = 1
In reality: a is between 0.9 and 0.998
Alpha (a) in the AC mode:
αac 
ΔI C
ΔI E
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Beta ()
 represents the amplification factor of a transistor. ( is
sometimes referred to as hfe, a term used in transistor modeling
calculations)
In DC mode:
βdc
IC

IB
In AC mode:
 ac 
IC
I B
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VCE  constant
Beta ()
Relationship between amplification factors  and a
α
β
β1
α
β
α 1
Relationship Between Currents
I C  βI B
I E  (β  1)I B
33
Power Dissipation
Common-base:
PCmax  VCB I C
Common-emitter:
PCmax  VCE I C
Common-collector:
PCmax  VCE I E
34
LOAD LINE FOR
EMITTER-BIAS CIRCUIT
IC
I C (sat )
IC(sat)
VCC  (VEE ) VCC  VEE


RC  RE
RC  RE
VCE (off )  VCC   VEE   VCC  VEE
VCE(off)
VCE
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FIG 7.28 EMITTER-BIAS CHARACTERISTICS.
(1)
+VCC
IC
Circuit recognition: A split (dualpolairty) power supply and the base
resistor is connected to ground.
RC
IB
Q1
Input
RB
RE
IE
-VEE
Advantage: The circuit Q-point
values are stable against changes in
hFE.
Output
Disadvantage: Requires the use of
dual-polarity power supply.
Applications: Used primarily to
bias linear amplifiers.
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FIG 7.28 EMITTER-BIAS CHARACTERISTICS.
(2)
+VCC
IC
Load line equations:
I C (sat )
RC
VCC  VEE

RC  RE
VCE (off )  VCC  VEE
IB
Q1
Input
Output
Q-point equations:
I CQ
RB
RE
IE
-VEE
VBE  VEE
  hFE 
RB   hFE  1 RE
VCEQ  VCC  I CQ  RC  RE   VEE
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FIG 7.31 EMITTER-FEEDBACK BIAS.
+VCC
RB
IC
RC
VCC  VBE
IB 
RB   hFE  1 RE
I CQ  hFE I B
I E   hFE  1 I B
IB
VCEQ  VCC  I C RC  I E RE
IE
RE
 VCC  I CQ  RC  RE 
38
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