02 - Semiconductor Devices

8/6/2019 02 - Semiconductor Devices

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Digital Circuits I

Semiconductor Devices

University of the PhilippinesElectrical and Electronics Engineering Institute

2Joy Reyes-Madamba@2010

Semiconductor Fundamentals

Conductors solids with high conductivity and

low resistivity

Insulators solids with small or low conductivityand high resistivity

Semiconductors solids with conductivity andresistivity between conductors and insulators

Three types of materials:


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Energy Band Model

EC

EV

EG

EC = conduction energyEV = valence energy EG = energy gap

conduction band

valence band

EC

EV

EG

conduction band

valence band

ECEV

conduction band

valence band

EG

most commonly used:

Silicon (Si), Germanium (Ge)

INSULATOR

SEMICONDUCTOR

CONDUCTOR



Types of Semiconductors

Intrinsic semiconductors semiconductors in

pure state, without chemical impurities

Extrinsic semiconductors impurities (atomsof other elements) are introduced into an intrinsic

semiconductor


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Intrinsic Semiconductors

Few charge carriers

Not good for electronic devices

At 0K, they act like insulators

Conductivity increases with temperature



Semiconductor Structure

Semiconductor atoms form a crystalline structure

similar to that of diamond

At 0K, no free electrons (like an insulator)Si Si Si

Si

Si

Si

Si Si

Si

--

-

-

-

-

- -

-

-

--

-

-

-

-

--

--

-

-

- -

Electrons are

shared

between atoms

using covalent

bonds


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Semiconductor Structure

When an electron transfers to a hole, effect is the

hole has moved

Si Si Si

Si

Si

Si

Si Si

Si

--

-

-

-

-

- -

-

-

-

-

-

-

-

--

-

-

-

- -



Extrinsic Semiconductors

Intrinsic semiconductors to which impurities havebeen added to increase conductivity

Process of adding impurities is called doping

Effect of doping is addition of an energy levelcloser to the conduction or valence band


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Types of impurities

N-typeor donor impurities atoms with five

valence electrons (e.g. arsenic[As], antimony[Sb],

phosphorus[P]); give rise to an n-type

semiconductor

P-typeor acceptor impurities trivalent atoms

(e.g. gallium[Ga], boron[B], indium[In]); results ina p-type semiconductor




Si Si Si

Si

Si

P

Si Si

Si

--

-

-

-

-

- -

-

---

-

-

-

-

--

--

-

-

- -

-

Fifth

electron has

a weaker

bond than

other

electrons

EC

EV

EG

-

- --

Free electron

from impurity

Edonor


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Si Si Si

Si

Si

B

Si Si

Si

--

-

-

-

-

- -

-

-

--

-

-

-

-

--

-

-

-

- -

Acceptor

impurity has

only three

electrons to

share; hole

is created

EC

EV

EG

Free hole

from impurity

Eacceptor+





To increase the conductivity of the intrinsic

semiconductor, a small amount of impurity is

needed Doping produces semiconductors with a

predominance of one type of carrier


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In n-type semiconductors, electrons are majoritycarriers& holes are minority carriers

In p-type semiconductors, vice versa

Doping reduces minority carriers through fasterrecombination



Junction Diode

A two-terminal device resulting from the

combining a p-type and n-type semiconductor

P-N junction diode is the building block of allsemiconductor devices

P N

PN junction


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Diode Circuit Symbol

I

+ V

Typical V-I CurveI

V

Breakdown

region

junction diode lets the

current flow in one direction

only ideally, acts like a switch



Physical Operation of Junction Diodes

There is a greater concentration of holes in theP-region than in the N-region, while a greater

concentration of electrons is in the N-region thanin the P-region

This results in diffusion of carriers.

P N+++


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P N+++

A depletion or space-charge region is formed, where

there are no mobile charges, only immobile ions.

These immobile ions produce an electric field which

results in a potential barrier and a drift current.




The potential barrier prevents the flow of carriers

across the junction.

An external voltage source or bias, V, isconnected to the diode to increase or decrease

the potential barrier.


P-regionN-region


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Forward biasing positive values of V

o Decreases the potential barrier

o A net forward current is seen at the diode

o As V is increased, the current increases

+ V -

P-regionN-region




Reverse biasing negative values of V

o Increases the potential barrier

o A small reverse or saturation current, IS, is seen atthe diode

o An increase in V does not increase the currentsignificantly, until breakdown occurs.

Peak inverse voltageor breakdown voltage

(PIV) around 75V for general- purpose diodes


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Cut-inor threshold voltage forward bias voltage

needed for current to flow, typically 0.6V for silicon

diodes.

Forward voltage in normal operation, VF, typically

0.4 0.8V




Special types of diodes:

o Zener diodes typically used in the breakdown

regiono Light-emitting diodes (LED) uses materials other

than germanium and silicon; color depends on energygap of material


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Diode Circuits

Half-wave rectifier

VS Vin Vout RL



Full-wave bridge

Diode Circuits

VSVin

VoutRL


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Vin Vout

Diode Circuits

Clipper circuit

has other variations (see book by Boylestad)



Junction Transistor

Also called bipolar transistor

Three-element device formed from two junctions

Can function as a controlled source or switch Two types: pnpand npntransistors

Arrow of symbols indicate direction of positive

charge flow (or actual current)


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Junction Transistor

Three elements/terminals:

emitter source of mobile carriers

collector collects carriers

base controls flow of carriers from emitter tocollector



PNP transistor

basic structure:

NPN transistor

N P N

E C

B

P N P

E C

Bsymbol:

E

C

B

E

C

B


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Junction Transistor

Convention for polarity of currents: for allterminals, currents entering the transistor arepositive

IB

IC

IEIB

IC

IE

IE = IC + IB



Transistor Operation

Active region

Mode of operation in which transistor acts like acontrolled source

Emitter-base junction is forward-biased andcollector-base junction is reverse-biased

Cutoff region

Emitter-base junction and collector-base junctionare reverse-biased


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Saturation region

Emitter-base junction and collector-base junction areforward-biased

Reverse-active region

Emitter-base junction is reverse-biased and

collector-base junction is forward-biased




Transistor acts as a switch when in saturationand cutoff region

When in cutoff, transistor is OFF

When in saturation, transistor is ON


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PNP Output characteristics

VEB/VBE

VCB/VBC

Forward

active Saturation

Inverted

activeCutoff

PNP/NPN regions ofoperation




Amplifier Types

According to common terminals

common base

common emitter common collector

IE IC

P+ N P

VEB

+

_

VCB

+

_

P

N

P+

IB

IC

VEB

+

_VEC

+

_

P+

N

P

IB

IE

VCB

+

_ VEC

+

_


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Biasing a Transistor

Quiescent point point in

loadline that corresponds toquiescent conditions oftransistor; is usually found in

active region of transistor

Biasing configuring the

circuit to establish thequiescent point



Some Transistor Circuits

Fixed bias circuit

VCC

RB RC

VBE + IBRB = VCC

IB = VCC VBERB

VBE is fixed for Si: 0.7V

VCE + ICRC = VCC

For active region, VCE > 0.2V

IC = IB


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Junction Field EffectTransistor (JFET)

A voltage-operated , unipolar transistor that

functions using only majority carriers

two types

N-channel JFET

P-channel JFET

Gate

Drain

Source

Gate

Drain

Source



Basic JFET Structure

Cross section of basic JFET structure


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Biasing

Biasing conditions





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Metal Oxide Semiconductor

Field Effect Transistor (MOSFET)

Insulated gate FET

Uses a metal gate instead of a PN junction

Oxide electrically isolates metal gate fromsemiconductor channel

Two types

N-type MOSFET (NMOS)

P-type MOSFET (PMOS)


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Modes of Operation

Depletion mode conducts when zero bias is

applied to the gate

Enhancement mode conducts when positive

bias is applied to the gate



MOSFET Symbols

Enhancement Type MOSFETs

Gate

Drain

Source

Substrate G

D

S

SS

G

D

S

G

D

SNMOS PMOS


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MOSFET Symbols

Depletion Type MOSFETs

G

D

S

SSG

D

S

SS

G

D

S

G

D

S

NMOS PMOS



Basic MOSFET Structure

Cross section of basic MOSFET structure


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Phases of MOSFET Operation (VG > VT)

VD = 0

moderate VD

pinch-off

post pinch-off



Digital Device Families

Different logic families are grouped according to

the major circuit element

These families are used to implement differentintegrated circuits (ICs)

ICs can be grouped by standard levels of

complexity (or no. of gates)


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Standard Levels of IC Complexity

Complexity No. of Gates Functions

Small-scaleintegration (SSI)

Fewer than 12 Gates, flip-flops

Medium-scaleintegration (MSI)

12 to 99 Registers, counters,decoders, etc.

Large-scaleintegration (LSI)

100 to 9999 RAMs and ROMs

02 - Semiconductor Devices

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