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"‫"מעגלים אנלוגיים‬
5 # ‫הרצאה‬
Bipolar Junction Transistors (BJTs):
- Physical structure and Modes Of Operation
- The transistor in the Active Mode
- Equivalent Models
:‫מקורות‬
Sedra/smith: sec 4.1 - 4.4 ; P221 - 238
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BIPOLAR JUNCTION TRANSISTORS
A bipolar junction transistor, BJT, is a single piece of silicon with
two back-to-back P-N junctions.
However, it cannot be made with two independent back-to-back
diodes and is not symmetric.
BJTs can be made either as PNP or as NPN.
They have three regions and three terminals, emitter, base, and
collector represented by E, B, and C respectively.
The arrow indicates the direction of the current in the emitter
when the transistor is conducting normally.
The transistor is operated as:
- A switch in digital electronics applications.
- An amplifier in linear (Analog) circuits.
NPN transistor is used in most bipolar switching applications
Junction voltage bias: forward or reverse biased
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Actual Physical Structure Of Bipolar Junction
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Bipolar Transistor Modes Of Operation
- Clasical selection
- More Detailed selection
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The transistor in Forward-Active Region (NPN Type)
The BE junction is forward biased and the BC junction is
reverse biased.
BE junction - forward bias of the BE junction will cause the
injection of both holes and electrons across the junction.
The holes are of little consequence because the doping
levels are adjusted to minimize the hole current. The
electrons are the carriers of interest.
The electrons are injected into the base region where they
are called the minority carrier.
Base region - The electrons diffuse across the base region
due to the concentration gradient. Some are lost due to
hole-electron recombination, but the majority reaches the
BC junction.
BC junction - The electrons encounter a potential gradient
(due to the depletion region) and are swept across the
junction into the collector region.
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the minority carrier concentration
Sedra’s version
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Components Of The Base Current
(1) – Recombination Current. To maintain charge
neutrality in the Base region, minority diffusing
electrons recombine with holes.
(2) – BE Junction hole current. Small base current is
required to supply the carriers.
(3) - BC Junction hole current. Negligible Saturation
current of the reverse biased BC Junction
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The Transistor Currents & Current Amplification
- The Base Current
Holes injected from B to E:
iB1 
AE qD p ni2
(SEDRA (4.5))
evBE / VT
N D Lp
where:
D p   hole diffusivity in the Emitter
L p   hole diffusion length in the Emitter
Recombination Current Of electrons from E to B:
(SEDRA (4.8))
1 AE qWni2 vBE / VT
e
2 N A b
where:
W   Effective Base width
 b   min ority carrier lifetime
iB2 
And The Total Base Current is: iB = iB1 + iB2
(SEDRA (4.9))
- The Collector Current
Electrons that reach the collector, most of the diffusion
current from B to C:
(SEDRA (4.3-4.4))
IC  I S evBE / VT 
AE qDn ni2 vBE / VT
e
N AW
where:
Dn   electron diffusivity in the Base
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- The Current Amplification (SEDRA (4.12))
IC   I B
D pN A W 1 W 2

D nNDL p 2 D n τ b
β  Common  Emitter Current Gain
where β  1/
- The Emitter Current
(SEDRA (4.13-4.19))
v
I
1 
1 
1
I  I  I  c  Ic 
Ic 
I S e BE / VT 
I
E C B 


 c
Or :
I c  I E
1 
Where:

c
Or :



1
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CUTOFF REGION: Both junction reverse biased
Leakage currents associated with reverse biased junctions
Saturation Region: (Both junctions forward biased)
Both junctions forward biased: VBE>VBC>4VT
>> VCE>0 ≈ 0.1-0.2v
Small current Gain as a result of:
- holes injected into the collector region from the base
limiting the transistor current.
- Decreased minority carier gradient in the Base region
Transistor in the active region, can be transferred to the saturation
mode as a result of collector current increase.
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Equivalent Circuit Models
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CB Configuration
As shown in Figure 5, there are several components of current:
1. Holes injected from B-E. This is small and is ignored.
2. Electrons injected from base to emitter. This is -IE
3. Electrons that reach the collector. This is -FIE .
4. Recombination current. This is -(1 - F)IE.
5. Collector reverse saturation current IC0 , which we will usually neglect.
The term F is the forward current transfer ratio. This term refers to the fraction of
electrons that reach the collector from those that are injected into the base region across
the emitter junction. In most transistors, F is close to unity, typically 0.9-0.99. The
difference between the current that is injected into the base region and that reaches the
collector, becomes the base current. Thus, in most transistors, the base current is quite
small, and the collector and emitter currents are close to the same magnitude.
The sum of currents entering the three terminals must equal zero,
IB + IC + I E = 0 (1)
Also from current components 3 and 5 above,
IC = -FIE + IC0 (2)
These two equations describe the currents in the common-base configuration in the
forward-active region as shown in Fig. 5.
We can go back to the cutoff region from here by reducing the bias voltage between base
and emitter. Since the BE junction is just a P-N junction, this current is essentially a
diode current. Reducing this bias voltage to zero will reduce the emitter current to zero.
In this case, -IB = IC =IC0 . We will hang on to this leakage current for a while yet
before we completely drop it.
REVERSE ACTIVE REGION (BE junction reverse biased, BC junction forward biased)
In this case, the biasing arrangement is just the reverse of the forward active region. The
collector junction is forward biased, while the emitter junction is reverse biased. The
operation is just the same as the forward active region, except all voltage sources, and
hence collector and emitter currents, are the reverse of the forward bias case. Another
difference is that F is replaced by R . The equation corresponding to Eq. 2 is:
IE = -RIC + IE0
This configuration is rarely used because most transistors are doped selectively to give
forward current transfer ratios very near unity, which automatically causes the reverse
current transfer ratio to be very low.
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