Transistor AE4B34EM - cvut.cz...5. lecture • Bipolar transistor semiconductor material, with at least three • Parameters • Applications Jiří Jakovenko – Electronics and Microelectronics

5. lecture

• Bipolar transistor

• Parameters

• Applications

Jiří Jakovenko – Electronics and Microelectronics - Department of Microelectronics – CTU

Electronics and Microelectronics AE4B34EM

B

C

E

Transistor

Active components are needed in electronics and microelectronics circuits out of passive components (resistors, capacitors, inductors, ...)

A transistor is a semiconductor device used to amplify and switch electronic signals

Transistors are made of a solid piece of semiconductor material, with at least three terminals for connection to an external circuit

The transistors are the fundamental building block of modern electronic devices.



Transistors - types

Transistors

Bipolar Field effect transistors

MESFET JFET MOSFET NPN PNP

En

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nce

me

nt

Channel

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ple

tio

n

Channel

En

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Channel

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ple

tio

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Channel

Channel N

kanál P

NM

OS

PM

OS

NM

OS

PM

OS N P N P

Channel P

N P N P

Bipolar transistors


Consists of 3 alternate layers of n and p type semiconductor called emitter (E), base (B) and collector (C).

Majority of current enters collector, crosses base region and exits through emitter. A small current also enters base terminal, crosses base-emitter junction and exits through emitter.

The BJT – Bipolar Junction Transistor

Note: Normally Emitter layer is heavily

doped, Base layer is lightly doped and Collector layer has Moderate doping.

The Two Types of BJT Transistors:

npn pnp

n++ p+ n E

B

C p++ n+ p E

B

C

Cross Section Cross Section

B

C

E

Schematic Symbol

B

C

E

Schematic Symbol

Collector doping is usually ~ 1015

Base doping is slightly higher ~ (1016 – 1017 )

Emitter doping is much higher ~ 1022


BJT Current & Voltage - Equations

B

C E

IE IC

IB VBE VCB

+ - VCE

B

C E

IE IC

IB VEB VBC

+ - VEC

n p n IE = IB + IC VCE = VCB + VBE

p n p IE = IB + IC VEC = VEB + VBC

n++ p+ n E C p++ n+ p E C


Cross sections and simplified models of discrete and IC npn BJTs


Physical Structure


Various Regions (Modes) of Operation of BJT

There are four basic operation regions


Region Polarization B-E Polarization B-C

Cutoff

Forward-active

Reverse

Saturation

VBE < VT reverse

VBC < 0 reverse

VBC < 0 reverse

VBC > UT forward

VBC > 0 forward

VBE > VT forward

VBE < 0 reverse

VBE > 0 forward

IE

IC

IB

VBE

VCB

VCE

Various Regions (Modes) of Operation of BJT


Cutoff: In cutoff, both junctions reverse biased. There is very little current flow, which corresponds to a logical "off", or an open switch.

Forward-active (or simply, active): The emitter-base junction is forward biased and the base-collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, βf in forward-active mode. If this is the case, the collector-emitter current is approximately proportional to the base current, but many times larger, for small base current variations. Reverse-active (or inverse-active or inverted): By reversing the biasing

conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Since most BJTs are designed to maximize current gain in forward-active mode, the βf in inverted mode is several times smaller. This transistor mode is seldom used. The reverse bias breakdown voltage to the base may be an order of magnitude lower in this region.

Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch.

Two diodes in series

V1 V2

n n p

eV1 eV2 EF

EF Ln Ln

np

np0

V1 V2

forwardI reverseI

No transistor effect


Cutoff region WB

BJT WB

Saturation mode

If base current is so large that electron injection causes flooding of BC junction, its polarization will change in a forward direction

Transistor shows very little resistance

Transistor is used as a switch

Disadvantage - need to divert off a great charge from the base


n n p

VBE = 0.7 V VCB

+ _ VB

IC

UCE = 0,2 V

UCC

Transistor with Schottky diode

Input characteristic

VBE

IC

2 mA

4 mA

6 mA

8 mA

0.7 V

Collector Current:

IC = IES e

VBE/VT

Transconductance: (slope of the curve)

gm = IC / VBE IES = The reverse saturation current of the B-E Junction. VT = kT/q = 26 mV (@ T=300

oK) = the emission coefficient and is usually ~1


P0

Boundary parameters

Max. voltage VCE Max. voltage VBE Max. current IC Max. power dissipation PCMAX = VCE . IC

Max. input current IB


Basic BJT ciruits

Biasing the transistor refers to applying voltages to the transistor

to achieve certain operating conditions: 1. Common-Base Configuration (CB) : input = VEB & IE output = VCB & IC

2. Common-Emitter Configuration (CE): input = VBE & IB output= VCE & IC

3. Common-Collector Configuration (CC): input = VBC & IB (Also known as Emitter follower) output = VEC & IE


Common-Base (CB) Characteristics

Satu

ration R

egio

n

IE

IC

VCB

Active

Region

Cutoff

IE = 0

0.8V 2V 4V 6V 8V

mA

2

4

6

IE=1mA

IE=2mA

Breakdown Reg.


Common-Base (CB) Characteristics

= Common-base current gain

(0.9-0.999; typical 0.99)

Jiří Jakovenko – Elektronika a Mikroelektronika - Katedra mikroelektroniky – ČVUT FEL

T

BEU

U

SC eIi

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BEU

U

SE e

Ii

E

C

i

i

Common-Emitter Configuration (CE)


Collector-Current Curves

VCE

IC

Active Region

IB

Saturation Region Cutoff Region

IB = 0

input output

common terminal

Common-Emitter Configuration (CE)

= Common-emitter current gain (10-1000; typical 50-200)


input output

B

C

i

i

T

BEU

U

SC eIi

T

BEU

U

SB e

Ii

Common-Collector Configuration


Collector-Current Curves

VCE

IC

Active Region

IB

Saturation Region Cutoff Region

IB = 0

input output

společná svorka

The Common-Collector biasing circuit is basically

equivalent to the common-emitter biased circuit

except instead of looking at IC as a function of VCE

and IB we are looking at IE.

Also, since ~ 1, and = IC/IE that means IC~IE

Relation between and

= Common-base current gain

(0.9-0.999; typical 0.99)

= Common-emitter current gain (10-1000; typical 50-200)


Example: = 0.98, β = 49 = 0.99, β = 99 = 0.995, β =199

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Documents

Transistor AE4B34EM - cvut.cz...5. lecture • Bipolar transistor semiconductor material, with at least three • Parameters • Applications Jiří Jakovenko – Electronics and Microelectronics