Communication Microelectronics (W17) - GUCeee.guc.edu.eg/Courses/Electronics/ELCT508...

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

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Bipolar Junction Transistor (BJT)

Physical Structure and I-V Characteristics

Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo

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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


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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


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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


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BEJ Forwar

d

BCJ Revers

e

Active Mode

Q


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


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C F BI I

BEC s

T

VI I exp

V

Q

𝑉𝐵𝐸 ≅ 0.7𝑉


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


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E R BI IR F

BEJ Reverse

BCJ Forward

Reverse Active Mode

Q

𝑉𝐵𝐶 ≅ 0.5𝑉


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


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BEJ Forward

BCJ Forward

Saturation Mode

𝑉𝐵𝐸 = 0.7𝑉 𝑉𝐵𝐶 = 0.5𝑉

C F BI I CEV 0.2VQ


IC versus VBE and VCE


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IC = Is expVBEVT

For Active

ONLYQ


IC versus VCE

The Early effect


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CEBEC S

T A

vVi I exp( ) 1

V V

C Ao

CE C

i Vr

v I

Q


BJT Large Signal Model in Active Mode


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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


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Q

PNP BJT Transistor

BJT PNP Physical Structure

BJT PNP Transistor in Active Mode


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EBJ Forwar

d

CBJ Reverse

Active Mode

Q

BJT Modes of OperationElectrical Equations of BJT


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BJT NPN Modes of Operation


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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


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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


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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 )


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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!


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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 )


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Communication Microelectronics (W17) - GUCeee.guc.edu.eg/Courses/Electronics/ELCT508...

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