Bipolar JunctionBipolar Junction Transistors (1) Transistors (1)

Dr. Wojciech Jadwisienczak EE314

Introduction  Your goal is to explain the transistor.  It is assumed that EE314 students to which this presentation is aimed, have not a clue to how these little Buggers  work and/or how to use them. A real problem with previous explanations: for the sake of "fidelity" authors' include confusing details until the concept, or thread--of how they actually work & how to use them--is lost.  The following presentation is comprised of several different explanations. You should read chapter 13 and this presentation several times, because any insight gained from one will help in understanding another.

Chapter 13: Bipolar Junction Transistors

pp. 584-624

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

The transistor was probably the most important invention of the 20th Century, and the story behind the invention is one of clashing egos and top secret research. 

First - BJTs

Reference:Bell Labs MuseumB. G. Streetman & S. Banerjee ‘Solid State Electronic Devices’, Prentice Hall 1999.

Picture from previous slide shows the workbench of John Bardeen and Walter Brattain at Bell Laboratories. They were supposed to be doing fundamental research about crystal surfaces. The experimental results hadn't been very good, though, and there's a rumor that their boss, William Shockley, came near to canceling the project.  But in 1947, working alone, they switched to using tremendously pure materials.  It dawned on them that they could build the circuit in the picture. It was a working amplifier!  John and Walter submitted a patent for the first working point contact transistor.  Shockley was furious and took their work and invented the junction transistor and submitted a patent for it 9 days later. The three shared a Nobel Prize. Bardeen and Brattain continued in research (and Bardeen later won another Nobel). Shockley quit to start a semiconductor company in Palo Alto. It folded, but its staff went on to invent the integrated circuit (the "chip") and to found Intel Corporation. By 1960, all important computers used transistors for logic, and ferrite cores for memory.

Interesting story…

Point-Contact Transistor – first transistor ever made

Qualitative basic operation of point-contact transistor

Problems with first transistor…

First Bipolar Junction TransistorsW. Shockley invented the p-n junction transistorThe physically relevant region is moved to the bulk of the material

Understanding of BJT

force – voltage/currentwater flow – current - amplification

Basic models of BJT

Diode

Diode

Diode

Diode

npn transistor

pnp transistor

Qualitative basic operation of BJTs

Basic models of BJT

BJTs – Basic ConfigurationsFluid Flow AnalogyDifference between FET (field effect transistor) and BJTTechnology of BJTs

pnp BJT npn BJT

BJTs – Practical Aspects

Heat sink

BJTs – Testing

BJTs – Testing

A little bit of physics…

A little bit of physics…

A little bit of physics…

A little bit of physics…

A little bit of physics…

More accurate physical description…

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 !

Large-Signal dc Analysis: Active-Region Model

Important: a current-controlled current source models the dependence of the collector current on the base current

The constrains for IB and VCE must be satisfy to keep BJT in the active-mode

VBE

forward bias

VCB

reverse bias ?

?

Large-Signal dc Analysis: Saturation-Region Model

VBE

forward bias

VCB

forward bias?

?

Large-Signal dc Analysis: Cutoff-Region Model

VCB

reverse bias

VBE

reverse bias

?

?

If small forward-bias voltage of up to about 0.5 V are applied, the currents are often negligible and we use the cutoff-region model.

Large-Signal dc Analysis: characteristics of an npn BJT

Large-Signal dc Analysis

Procedure: (1) select the operation mode of the BJT (2) use selected model for the device to solve the circuit

and determine IC, IB, VBE, and VCE (3) check to see if the solution satisfies the constrains for

the region, if so the analysis is done (4) if not, assume operation in a different region and repeat until a valid solution is found

This procedure is very important in the analysis and design of the bias circuit for BJT amplifier.

The objective of the bias circuit is to place the operating point inthe active region.

Bias point – it is important to select IC, IB, VBE, and VCE

independent of the and operation temperature.

Example 13.4, 13.5, 13.6

Large-Signal dc Analysis: Bias Circuit

From Example 13.6

Remember: that the Q point should be independent of the stability issue) VBB & VCC provide this stability, however this impractical solutionOther approach is necessary to solve this problem-resistor network

VBB acts as a short circuit for ac signals

Large-Signal dc Analysis: Four-Resistor Bias Circuit

1

2

3

4

Thevenin equivalent

21 RRRB 212 / RRRVV CCB

Equivalent circuit for active-region model

Solution of the bias problem:

Input Outpu

t

EEBEBBB IRVIRV

BE II 1 VVBE 7.0

EB

BEBB RR

VVI

1

EECCCCCE IRIRVV

npn

BJTs – Practical Aspects

R

VI

http://www.4p8.com/eric.brasseur/vtranen.html