Bipolar JunctionBipolar Junction Transistors Transistors

EE314

Chapter 13: Bipolar Junction Transistors

1.History of BJT2.First BJT3.Basic symbols and features 4.A little bit of physics…5.Currents in BJT’6.Basic configurations7.Characteristics

Current Flow in BJT

pnp BJT

1. Injected h+ current from E to B2. e- injected across the forward-biased EB junction (current from

B to E)3. e- supplied by the B contact for recombination with h+

(recombination current)4. h+ reaching the reverse-biased C junction5,6.Thermally generated e- & h+ making up the reverse saturation

current of the C junction

iE

-iB

-iC

-VCE

Now, you can try…

npn BJT

BJTs – Basic configurations

npn BJTs – Operation Modes

Forward & reverse polarized pn junctions

Different operation modes:

npn BJTs – Operation Modes

•When there is no IB current almost no IC flows•When IB current flows, IC can flow•The device is then a current controlled current device

Operational modes can be defined based on VBE and VBC

BJT-Basic operation

npn BJTpnp BJT

(n+), (p+) – heavy doped regions; Doping in E>B>C

Operation mode: vBE is forward & vBC is reverse

The Shockley equation

1exp

T

BEESE V

vIi

IES–saturation I (10-12-10-16A); VT=kT/q -thermal V (26meV) D – diffusion coefficient [cm2/s] – carrier mobility [cm2/Vs]The Kirchhoff’s laws

BCE iii

EiIt is true regardless of the bias conditions of the junction

Useful parameter

B

C

i

i

the common-emitter current gainfor ideal BJT is infinite

0 CEBCBE VVV

BJTs – Current & Voltage Relationships

q

kTD

Einstein relation

1exp

T

BEESC V

vIi

E

C

i

iUseful

parameterthe common-base current gainfor typical BJT is ~0.99

The Shockley equationonce more

If we define the scale current

ESS II

T

BESC V

vIi

A little bit of math… search for iB

EB ii 1

1exp1

T

BEESB V

vIi

1B

C

i

i

BC ii Finally…

BJTs – Current & Voltage Relationships

BJTs – Characteristics

SchematicCommon-Emitter

Input

Output

VBC<0 or equivalently VCE>VBE

If VCE<VBE the B-C junction is forward bias and IC decreasesRemember VBE has to be greater than 0.6-07 V

BC ii

Example 13.1

BJTs – Load line analysisCommon-Emitter Amplifier

)()()( tvtiRtvV BEBBinBB Input loop

if iB=0 inBBBE vVv if vBE=0 BinBBE RvVi /)(

smaller vin(t)

BJTs – Load line analysis

CECCCC viRV Output loop

Common-Emitter Amplifier

Example 13.2

Circuit with BJTs

Our approach: Operating point - dc operating pointAnalysis of the signals - the signals to be amplified

Circuit is divided into: model for large-signal dc analysis of BJT circuitbias circuits for BJT amplifiersmall-signal models used to analyze circuits for signals being amplified

Remember !