Bipolar Junction Transistor

BJT Basics

ROCHESTER INSTITUTE OF TECHNOLOGY

MICROELECTRONIC ENGINEERING

Bipolar Junction Transistor - Basics

Dr. Lynn Fuller

Webpage: http://people.rit.edu/lffeee/

Microelectronic Engineering

Rochester Institute of Technology

82 Lomb Memorial Drive

Rochester, NY 14623-5604

Tel (585) 475-2035

Email: Lynn.Fuller@rit.edu

MicroE webpage: http://www.microe.rit.edu

Rochester Institute of Technology

Microelectronic Engineering

© December 3, 2010 Dr. Lynn Fuller, Professor

12-3-10 BJT_Basics.ppt

Page 1

BJT Basics

OUTLINE

Definitions

Schematic Symbols

Theory

Integrated BJT Structure

Modes of Operation

IC-VCE Family of Curves

Modifications

References

Homework Questions




Page 2

BJT Basics

DEFINITIONS

Bipolar Junction Transistor - (BJT) Both holes and electrons participate in the conduction of current, hence the name bipolar.

Minority carrier - In a p-type semiconductor electrons are the minority carrier type, in an n-type semiconductor holes are the minority carrier type.

Emitter - Emits minority carriers into the base region of a BJT. For example, in an

NPN BJT the n-type emitter, emits electrons into the p-type base. The emitter usually has the highest doping levels of the three regions of a BJT.

Base - Thin region which is used to control the flow of minority carriers from the emitter to the collector

Collector -Collects the minority carriers that make it through the base from the emitter. The collector usually has the lightest doping concentrations of the three regions.

DC Beta (

AC Beta (

β

β dc ac

) - The ratio of the collector current to the base current. the base current. β ac

= ∆ I

C

/ ∆ I

B


β dc

= I

C

/ I

) - The ratio of the change in the collector current to the change in

B



Page 3

BJT Basics

BJT - BIPOLAR JUNCTION TRANSISTOR

Label

Flat

1

2

3

Emitter

Base

Collector




Page 4

BJT Basics

SCHEMATIC SYMBOLS npn

Base

Collector n p n

Emitter

Collector pnp

Base

Collector p n p

Emitter

Collector

Base Base

Emitter



Emitter

The arrow on the emitter is in the direction that current will flow in the Base Emitter pn junction


Page 5

BJT Basics

IDEALIZED STRUCTURE p-type

Emitter

N

SC

P

SC n-type

Base

N n-type




Page 6

Collector

BJT Basics

ELECTRON CONCENTRATIONS IN AN NPN BJT

Emitter

Base

Collector n~Nde

E n~Ndc n~very small but not zero

~ni2/Nab

BE

Space Charge Layer

BC x

With the B-E junction forward biased, and B-C junction reverse biased. There is a concentration gradient in the base that forces electrons to flow toward the collector.




Page 7

BJT Basics

COMMENTS

1. The concentration of electrons in n-type silicon is ~ doping concentration in that region.

2. In p-type silicon the number of electrons is almost zero

3. A forward biased pn junction means more carriers of both types can cross the potential barrier. So a forward biased base-emitter junction (in an npn BJT) means more electrons on the base side than in equilibrium (no bias).

4. A reverse biased pn junction means less carriers of both types can cross the potential barrier. So a reverse biased base-collector junction

(in an npn BJT) means less electrons on the base side than in equilibrium (no bias). Even closer to zero electrons in p-type base at the edge of the B-C space charge layer.

5. The base is so narrow that few electrons are lost as they diffuse across the base width. Diffusion is driven by a concentration gradient. So electrons move towards the collector and current flows in the opposite direction.




Page 8

BJT Basics

INTEGRATED BJT STRUCTURE

Collector (n) Base (p) Emitter (n + )

~ 10 18 cm -3 ~ 10 18 cm -3 p-type ~ 10 16 cm -3 electrons current

Lightly doped (~10 15 cm -3 ) n-type silicon wafer

Since the emitter is more heavily doped compared to the base than the collector, the emitter-base junction has a lower breakdown voltage than the base-collector junction.


Microelectronic Engineering n + means heavily doped n-type n means lightly doped n-type p + means heavily doped p-type p means lightly doped p-type


Page 9

BJT Basics

BJT TERMINAL CURRENTS

From device physics

IE = I

SE e V

BE

/V

T where I

SE

= AqDni2/N

A

W and V

T

= KT/q

IC = α IE where α represents the fraction of carriers from the emitter that make it to the collector

IC = β IB where β represents the ratio of collector current to base current

We can show that β = α/(1−α) or α = β / (β+1)

With the B-E junction forward biased, and B-C junction



Page 10

BJT Basics

LARGE SIGNAL MODEL IN FORWARD ACTIVE MODE

Modes

Cutoff

Active

Inverse

Base/Emitter Base/Collector

Reverse

Forward

Reverse

Saturation Forward

Reverse

Reverse

Forward

Forward

With the B-E junction forward biased, and B-C junction reverse biased. Base

IB

Collector

IC

α IE = Is e V

BE

/V

T




IE

Emitter

Page 11

BJT Basics

EBERS-MOLL MODEL OF NPN BJT

This type of model works in all four regions of operation

Collector

IC

IC = α

F iDe – iDc

IE = -iDe + α

R iDc

The diode currents are: iDc = I

SC iDe = I

SE

(e Vbc/VT

(e Vbe/VT

-1)

-1)

Base iDc

IB iDe

Transistors are modeled by determining appropriate values of: α

F

, α

R

, I

SC and I

SE

Νοτε: β is often given instead of α but α = β /(1+ β )


IE

Emitter

α

F iDe

α

R iDc


Page 12

BJT Basics

EARLY VOLTAGE

Increasing VCE increases the reverse bias on the BC junction increasing the width of the BC space charge layer resulting in a decrease in the base width and increase in concentration gradient and an increase in collector current. To account for this the equation relating the collector current to the V

BE can be modified slightly as shown: VA is the Early voltage after

Dr. Jim Early of Fairchild Semiconductor.

IC

IC = IS 1+

V

CE

VA e V

BE

/V

T

-VA

This is one of the many modifications to make the BJT models more accurate.


Other modifications include resistors to account for series resistance in the collector, base and emitter.

VCE


Page 13

BJT Basics

CHARACTERISTICS OF TWO TERMINAL DEVICES

I

-

+

V

I

V




Page 14

-8

I

BJT Basics

BE JUNCTION, BC JUNCTION, CE

I I

0.7

V

-8

0.7

V

-8

Base

I

+

V

-

Base

Emitter



I

+

V

-

Collector


0.7

V

V

+

-

I

Collector

Emitter

Page 15

BJT Basics

CHARACTERISTICS OF THREE TERMINAL DEVICES

Iin

Iout

Iin

-

+

Vin

Common

-

+

Vout Iout

Vin

Vout

Each trace is for a different value for Iout or Vout

Each trace is for a different value for Iin or Vin




Page 16

BJT Basics

BJT IC-VCE FAMILY OF CURVES

I

C

10 mA

9 mA

8 mA

7 mA

6 mA

5 mA

4 mA

3 mA

2 mA

1 mA

Steps of base current I

B

I

B

= 30 µ A

10 µ A increments dc

I

B

= 20 µ A

∆Ι

C

= 2.5 mA ac

= 10 µ A

I

B

= 10 µ A

V

CE

Beta ( β ac

∆ I

C

2.5x10

-3

10x10 -6

Beta ( β dc ΙΙΙΙ

I

C

5.0x10

-3

20x10 -6



The two Beta values are not always the same!


Page 17

BJT Basics

NPN COMMON EMITTER IC-VCE CHARACTERISTICS

Forward Active Mode

Base-Emitter junction is forward biased

Base-Collector junction is reverse biased npn

I

C

V a

> 0 is a forward biased junction

V a

< 0 is a reverse biased junction

Va is defined as the voltage from p to n base current steps

I

B

B-C junction is reverse biased

V bc negative

base - p

+

0.7 V

+

V be positive

B-E junction is forward biased

-

+5 V S weep from 0 to 5 Volts

Collector - n

Emitter - n

0 V

Inverse Active Mode

Base-Emitter junction is reverse biased

Base-Collector junction is forward biased



Page 18

BJT Basics

TRIPLE DIFFUSED BJT STRUCTURE

Collector Emitter Base p-wafer n+ n+ n-well collector p-base

Doping

Conc n+ emitter

1-D Doping Profile p-base

§ Simple BJT structure

§ Large collector series resistance

§ Large dimensions

§ Isolation issues n-well collector



© December 3, 2010 Dr. Lynn Fuller, Professor depth

W

B

Base Width W

B

~ 0.5

µ m

Page 19

BJT Basics

SHALLOW TRENCH ISOLATION

Collector Emitter Base

SiO

2 n+ plug

SiO2 p-wafer n+ p-base n+ buried layer

SiO

2 n-type epitaxial silicon

§ Process Enhancements

§ Oxide-plug isolation

§ Patterned buried sub-collector

§ Epitaxial silicon

§ Performance Improvements

§ Low collector series resistance

§ Improved collector/emitter isolation

§ smaller geometries




Page 20

BJT Basics

REFERENCES

1. Sedra and Smith, 5.1-5.4

2. Device Electronics for Integrated Circuits, 2nd Edition, Kamins and Muller, John Wiley and Sons, 1986.

3. The Bipolar Junction Transistor, 2nd Edition, Gerald Neudeck,

Addison-Wesley, 1989.




Page 21

BJT Basics

HOMEWORK - BJT’S

1. Why won’t two back to back diodes behave like a BJT?

2. Sketch a figure like that on page 7 showing the hole concentration for a pnp transistor with B-E junction forward biased and B-C junction reverse biased. Show direction of current flow.

3. The Ic versus Vce family of

Vce curves for a 2N3906 BJT is shown.

What is the current gain, Beta, β

4. Look up the 2N3906 and see

Ib what the typical β is.

5. Look up some information

Emitter about John Bardeen. Write a few sentences.




Page 22

Bipolar Junction Transistor

Bipolar Junction Transistor - Basics

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