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Ch 10. BJT Fundamentals

(a) It will be from the one-junction/two-terminal diode to the two-junction/three-terminal

transistor ( the bipolar junction transistor; BJT)

(b) The BJT is a semiconductor device containing three adjoining, alternatively doped regions,

with the middle region being very narrow-compared to the minority carrier diffusion length in the

region.

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① Transistor–transistor logic (TTL) is a class of digital circuits built from bipolar

junction transistors (BJT) and resistors.

② It is called transistor–transistor logic because both the logic gating function (e.g.,

AND) and the amplifying function are performed by transistors (contrast with RTL

and DTL).

③ TTL is notable for being a widespread integrated circuit (IC) family used in many

applications such as computers, industrial controls, test equipment and

instrumentation, consumer electronics, synthesizers, etc.

④ After their introduction in integrated circuit form in 1963 by Sylvania, TTL

integrated circuits were manufactured by several semiconductor companies, with

the 7400 series (also called 74xx) by Texas Instruments becoming particularly

popular.

⑤ TTL became the foundation of computers and other digital electronics. Even after

much larger scale integrated circuits made multiple-circuit-board processors

obsolete, TTL devices still found extensive use as the "glue" logic interfacing more

densely integrated components.

(1) Application 1: TTL

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Two-input TTL NAND gate with a simple output stage (simplified).

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① Complementary metal–oxide–semiconductor (CMOS) is a technology for

constructing integrated circuits.

② CMOS technology is used in microprocessors, microcontrollers, static RAM, and

other digital logic circuits.

③ CMOS technology is also used for several analog circuits such as image sensors

(CMOS sensor), data converters, and highly integrated transceivers for many types of

communication. In 1963, while working for Fairchild Semiconductor, Frank Wanlass

patented CMOS (US patent 3,356,858).

④ Two important characteristics of CMOS devices are high noise immunity and low

static power consumption.

⑤ CMOS devices do not produce as much waste heat as other forms of logic, for

example transistor–transistor logic (TTL) or NMOS logic, which normally have some

standing current even when not changing state.

⑥ CMOS also allows a high density of logic functions on a chip. It was primarily for

this reason that CMOS became the most used technology to be implemented in VLSI

chips.

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(2) Application 2: CMOS

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CMOS inverter (NOT logic gate)

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(3) BJT circuit

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. where

0

(-). base and )(emitter ebetween th drop voltagedc theis V example,For EB

ECCE

CEBCEB

CBECEB

CBE

VV

VVV

VVV

III

(4) Biasing modes (pnp)

Biasing mode VEB(E-B junction) VCB(C-B junction)

saturation forward forward

active forward reverse

inverted reverse forward

cutoff reverse reverse

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http://www.electronics-tutorials.ws/transistor/tran_4.html

http://www.electronics-tutorials.ws/amplifier/amp_2.html

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(5) BJT structure

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(6) BJT properties

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reverse B-C forward, B-E :

doping.collector doping basedopingemitter mode. activeunder r transistopnp a (b)

m.equilibriuunder stor npn transi a (a) 10.1)Ex

widthbase alquasineutr the:

W

xxwW nCBnEBB

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(7) Introductory operational considerations:

Let us consider a pnp transistor under active mode biasing.

a. The primary carrier activity in the vicinity of the forward-biased E-B junction is majority

carrier injection across the junction into the opposite-side quasineutral regions.

b. Naturally, the p+-n nature of the junction leads to many more holes being injected from the

emitter into the base than electrons being injected from the base into the emitter.

c. The key to the transistor action is what subsequently happens to the carriers that are injected

into the base.

d. If the quasineutral base width were much larger than a minority carrier diffusion length, the

injected holes would simply recombine in the n-type base and there would be no interaction

between the two junctions.

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e. However, the BJT is a structure where the base is narrow compared to a minority

carrier diffusion length.

f. Thus, the vast majority of injected holes diffuse completely through the quasineutral

base and enter the C-B depletion region.

g. The accelerating electric field in the C-B depletion region then rapidly sweeps the

carriers into the collection.

h. When active mode biased, the emitter functions as a source of carriers, emitting

carriers into the base.

i. Conversely, the collector of the reverse-biased C-B junction acts like a sink,

collecting the emitted carriers after they pass through the base region.

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CnBEnBBBB

CEB

CE

CpCEpE

EpEn

En

CnCpCEnEpE

EpCp

IIIIIII

III

II

IIII

II

I

IIIIII

II

Since

2321

CEB

CpCn

CnEn

Cn

biasing. mode activeunder I and I tocompared small very is I

And

,(2) and (1)

(2) II

current, bias-reverse aFor

.components hole respective the tocompared small are I and IBoth *

region.depletion B-C theintor that wandecollector in the electronscarrier minority thefrom arises I *

(1)

And emitter. theinto base thefrominjection electron with associatedcurrent theis where

,

base, in theion recombinatby lost are holes injected theof few very

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(8) Performance parameters

CnCBTdc

CnETCnCpC

ETEpTCp

CBEdcC

Ep

Cp

T

EnEp

Ep

E

Ep

II

IIIII

III

III

I

I

II

I

I

I

a

0

ECB0

dc

0

BC

,(10.10) and (10.8)

(10.10)

)6.10()7.10(

0.I whereflowsat current thcollector the:I

gain.current d.c. basecommon the:

(10.8)

0)(-V mode active At the

gaincurrent d.c. baseCommon (c)

BJT. pnpfor

factor transportBase (b)

BJT. pnpfor

efficiencyemitter The )(

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1 1, ,II

(10.18)

1

(10.17) 1

,(10.16) 1

(10.15) and (10.13) From

(10.15) 11

)1(

)()8.10(

0.I whereflowsat current thcollector the:I

gaincurrent dcemitter common the:

(10.13)

gaincurrent d.c.emitter Common (d)

dcdcBC

0

0

00

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00

BCE0

dc

0

*

I

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III

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IIIIII

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B

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dc

CBBdcC

dc

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dc

dcdc

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dc

dcC

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