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Communication Microelectronics (W17)
Lecture 4: Bipolar Junction TransistorDr. Eman Azab
Assistant Professor
Office: C3.315
E-mail: [email protected]
Dr. Eman Azab Electronics Dept., Faculty of IET The German University in Cairo
1
Bipolar Junction Transistor (BJT)
Physical Structure and I-V Characteristics
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
2
BJT Physical Structure
Two back to back PNJunctions
NPN or PNP Transistor
Three terminal device, forNPN: Base (P-Type), Emitterand Collector (N-Type)
Base-Emitter Junction
Base-Collector Junction
Emitter doping is higher thanCollector doping
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
3
NPN BJT Transistor
PNP BJT Transistor
BJT NPN I-V characteristics
1. BJT NPN Transistor in Cutoff Mode
Base-Emitter Junction is Reverse biased
Base-Collector Junction is Reverse biased
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
4
BEJ Reverse
BCJ Reverse
Cutoff Mode
C B EI I I 0 Q
2. BJT NPN Transistor in Active Mode
Base-Emitter Junction is Forward biased
Base-Collector Junction is Reverse biased
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
5
BEJ Forwar
d
BCJ Revers
e
Active Mode
Q
BJT NPN I-V characteristics
2. BJT NPN Transistor in ActiveMode
Base-Emitter Junction is Forwardbiased and electrons passthrough the Base to the collectordue to the Base small area
Electrons from the Emitter arecollected at the Collector side
The transistor’s Collector currentcan be modeled by a currentdependent current source
Is depends on doping and widthof the Base
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
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C F BI I
BEC s
T
VI I exp
V
Q
𝑉𝐵𝐸 ≅ 0.7𝑉
BJT NPN I-V characteristics
3. NPN Transistor in Reverse Active mode
Base-Collector Junction is Forward biased
Base-Emitter Junction is reverse biased
Emitter and collector reverse their roles
However, BJT has an asymmetrical physical structure
The current gain from Base to Emitter is very small
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
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E R BI IR F
BEJ Reverse
BCJ Forward
Reverse Active Mode
Q
𝑉𝐵𝐶 ≅ 0.5𝑉
BJT NPN I-V characteristics
4. NPN Transistor in Saturation Mode
Both junctions are Forward biased
The total current is the EBJ diffusion current subtracted from CBJdiffusion current
BJT could be used as a closed switch in Saturation mode
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
8
BEJ Forward
BCJ Forward
Saturation Mode
𝑉𝐵𝐸 = 0.7𝑉 𝑉𝐵𝐶 = 0.5𝑉
C F BI I CEV 0.2VQ
BJT NPN I-V characteristics
IC versus VBE and VCE
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
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IC = Is expVBEVT
For Active
ONLYQ
BJT NPN I-V characteristics
IC versus VCE
The Early effect
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
10
CEBEC S
T A
vVi I exp( ) 1
V V
C Ao
CE C
i Vr
v I
Q
BJT NPN I-V characteristics
BJT Large Signal Model in Active Mode
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
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Q
BJT PNP Physical Structure
BJT PNP Transistor
PNP is the NPN Complementarystructure
Two back to back PN Junctions
Three terminal device: Base (N-type), Emitter and Collector (P-type)
Emitter-Base Junction
Collector-Base Junction
Emitter doping is higher thanCollector doping
Same I-V Characteristics as NPNTransistor
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
12
Q
PNP BJT Transistor
BJT PNP Physical Structure
BJT PNP Transistor in Active Mode
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
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EBJ Forwar
d
CBJ Reverse
Active Mode
Q
BJT Modes of OperationElectrical Equations of BJT
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
14
BJT NPN Modes of Operation
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
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Mode BEJ BCJ Equations Condition
Cutoff Reverse Reverse 𝐼𝐶 = 𝐼𝐸 = 𝐼𝐵=0𝑉𝐵𝐸 < 0.7𝑉𝐵𝐶 < 0.5
Active (Forward)
Forward
Reverse
𝑉𝐵𝐸 = 0.7𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
𝐼𝐶 = 𝛽𝐹𝐼𝐵 = 𝛼𝐹𝐼𝐸
𝛼𝐹 =𝛽𝐹
1 + 𝛽𝐹
𝑉𝐵𝐶 < 0.5Or
𝑉𝐶𝐸 > 0.2
SaturationForward
Forward
𝑉𝐵𝐸 = 0.7𝑉𝐵𝐶 = 0.5𝑉𝐶𝐸 = 0.2𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
𝐼𝐶 < 𝛽𝐹𝐼𝐵
Reverse Active
Reverse
Forward
𝑉𝐵𝐶 = 0.5𝐼𝐶 = 𝐼𝐸 + 𝐼𝐵
𝐼𝐸 = 𝛽𝑅𝐼𝐵 = 𝛼𝑅𝐼𝐶
𝛼𝑅 =𝛽𝑅
1 + 𝛽𝑅
𝑉𝐵𝐸 < 0.7Q
Q
BJT PNP Modes of Operation
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
16
Mode EBJ CBJ Equations Condition
Cutoff Reverse Reverse 𝐼𝐶 = 𝐼𝐸 = 𝐼𝐵=0𝑉𝐸𝐵 < 0.7𝑉𝐶𝐵 < 0.5
Active (Forward)
Forward
Reverse
𝑉𝐸𝐵 = 0.7𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
𝐼𝐶 = 𝛽𝐹𝐼𝐵 = 𝛼𝐹𝐼𝐸
𝛼𝐹 =𝛽𝐹
1 + 𝛽𝐹
𝑉𝐶𝐵 < 0.5Or
𝑉𝐸𝐶 > 0.2
SaturationForward
Forward
𝑉𝐸𝐵 = 0.7𝑉𝐶𝐵 = 0.5𝑉𝐸𝐶 = 0.2𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
𝐼𝐶 < 𝛽𝐹𝐼𝐵
Reverse Active
Reverse
Forward
𝑉𝐶𝐵 = 0.5𝐼𝐶 = 𝐼𝐸 + 𝐼𝐵
𝐼𝐸 = 𝛽𝑅𝐼𝐵 = 𝛼𝑅𝐼𝐶
𝛼𝑅 =𝛽𝑅
1 + 𝛽𝑅
𝑉𝐸𝐵 < 0.7
Q
Q
Calculating DC operating point
Solved Exercise
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
17
Solved Example
Find the DC Operating point of the Transistors?
Given: VBE=0.7V ,β=10
(Ans.: IB=0.023mA, IC=0.23mA, IE=0.253mA, VCE=9.54V, Active )
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
18
Solution Steps:1. Identify the BJT Type
2. Place the terminals name on the circuit
3. Write a KVL in the INPUT Loop
Input loop for BJT is any loop containing VBE
4. Assume the BJT mode (most of the time active)
5. Calculate the currents and voltages
6. Write KVL in the OUTPUT loop
Output loop for BJT is any loop containing VCE
7. Verify your assumption!
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
19
Example
Find the DC Operating point of the Transistors?
Given: VEB=0.7V ,β=10
(Ans.: IB=93µA, IC=0.93mA, IE=1.023mA, VEC=8.14V, Active )
Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo
20