Semiconductor devices

Semiconductor Devices

Dr G.S. VirdiDirector R & D Former Director Grade Scientist GGS College of Modern Technology CSIR- Central Electronic EngineeringKharar Mohali Research Institute- Pilani

In the modern world no other technology permeates every nook and cranny of our existence as does electronics.

Application of electronics are : Televisions, radios, stereo equipment, computers, scanners, electronic control systems (in cars for example) etc.

Introduction to Electronic Devices

Outline Presentation Introduction Diodes Electrical Properties of Solids Semiconductors P-n Junctions Semiconductor Diodes Diode Circuits

Introduction This course adopts a top-down approach to the

subject and so far we have taken a ‘black-box’ view of active components

It is now time to look ‘inside the box’ we will start by looking at diodes and

semiconductors then progress to transistors later we will look at more detailed aspects of

circuit design

Electronics Components

The p-n junction is at the heart of electronics technology. Most electronics is silicon based, that is, the devices are made of silicon. Silicon wafers are subjected to special procedures which result in what is called p-type silicon material and n-type silicon material. Where these two types of materials meet we have a p-n junction.

p-n junction

Diodes

An ideal diode passing electricity in one direction but not the other

Diodes One application of diodes is in rectification

the example below shows a half-wave rectifier

Inpractice, no real diode has ideal characteristics but semiconductor p-n junctions make good diodes

To understand such devices we need to look at some properties of materials

Atomic Structure of atom,

Semiconductors are materials whose electrical conductivities are higher than those of insulators but lower that those of conductors. Silicon, Germanium, Gallium, Arsenide, Indium, Antimonide and cadmium sulphide are some commonly used semiconductors.

Semiconductors have negative temperature coefficients of resistance, i.e. as temperature increases resistivity deceases

Electrical Properties of Solids

Energy Band Diagram Conduction band Ec

Ev

Eg

Band gap

Valence band

Energy band diagram shows the bottom edge of conduction band, Ec , and top edge of valence band, Ev .

Ec and Ev are separated by the band gap energy, Eg .

Energy Bands in Insulators and Conductors

Energy Bands in Semiconductors Forbidden band small for

semiconductors.● Less energy required

for electron to move from valence to conduction band.

● A vacancy (hole) remains when an electron leaves the valence band.

● Hole acts as a positive charge carrier to conduction band

Energy gap in a conductor, semi conductor, and insulator?.Conductor - no energy gapSemi Conductor - 1. 1 ev.Insulator 6 -9 ev.

Energy Bands in Insulators & Semiconductors and Metals

Electrical Properties of Solids Conductors

e.g. copper or aluminium have a cloud of free electrons (at all temperatures

above absolute zero). If an electric field is applied electrons will flow causing an electric current

Insulators e.g. polythene electrons are tightly bound to atoms so few can

break free to conduct electricity


Semiconductors e.g. silicon or germanium at very low temperatures these have the properties of

insulators as the material warms up some electrons break free

and can move about, and it takes on the properties of a conductor - albeit a poor one

however, semiconductors have several properties that make them distinct from conductors and insulators

Silicon Crystal Structure

Unit cell of silicon crystal is cubic.

Each Si atom has 4 nearest neighbors.


Electrical Properties of Solids Pure semiconductors

thermal vibration results in some bonds being broken generating free electrons which move about

these leave behind holes which accept electrons from adjacent atoms and therefore also move about

electrons are negative charge carriers holes are positive charge carriers At room temperatures there are few charge carriers pure semiconductors are poor conductors this is intrinsic conduction


Doping the addition of small amounts of impurities

drastically affects its properties some materials form an excess of electrons

and produce an n-type semiconductor some materials form an excess of holes and

produce a p-type semiconductor both n-type and p-type materials have much

greater conductivity than pure semiconductors this is extrinsic conduction

Extrinsic Semiconductors / Doping

The electron or hole concentration can be greatly increased by adding controlled amounts of certain impurities

For silicon, it is desirable to use impurities from the group III and V.

N-type Semiconductor can be created by adding phosphorus or arsenic.

P -type Semiconductor can be created by adding Boron or Gallium

Donors: P, As, Sb Acceptors: B, Al, Ga, In

By substituting a Si atom with a special impurity atom (Column Vor Column III element), a conduction electron or hole is created.

Doping


The dominant charge carriers in a doped semiconductor (e.g. electrons in n-type material) are called majority charge carriers. Other type are minority charge carriers

The overall doped material is electrically neutral

p-type material

Semiconductor material doped with acceptors.

Material has high hole concentration

Concentration of free electrons in p-type material is very low.

n-type material

Semiconductor material doped with donors.

Material has high concentration of free electrons.

Concentration of holes in n-type material is very low.

Extrinsic Semiconductors

p-type material

Contains NEGATIVELY charged acceptors (immovable) and POSITIVELY charged holes (free).

Total charge = 0

n-type material

Contains POSITIVELY charged donors (immovable) and NEGATIVELY charged free electrons.

Total charge = 0

Extrinsic Semiconductors

donor: impurity atom that increases n

acceptor: impurity atom that increases p

n-type material: contains more electrons than holes

p-type material: contains more holes than

electrons

majority carrier: the most abundant carrier

minority carrier: the least abundant carrier

intrinsic semiconductor: n = p = ni

extrinsic semiconductor: doped semiconductor

Semiconductor Terminology

The p-n junction is at the heart of electronics technology. Most electronics is silicon based, that is, the devices are made of silicon. Silicon wafers are subjected to special procedures which result in what is called p-type silicon material and n-type silicon material. Where these two types of materials meet we have a p-n junction.

On its own a p-type or n-type semiconductor is not very useful. However when combined very useful devices can be made.

The formation of p-n junction

The p-n junction is the basic element of all bipolar devices. Its main electrical property is that it rectifies (allow current to flow easily in one direction only).The p-n junction is often just called a DIODE. Applications;

>photodiode, light sensitive diode,>LED- ligth emitting diode,>varactor diode-variable capacitance diode

transistors and integrated circuits


The p-n junction can be formed by pushing a piece of p-type silicon into close contact with a piecce of n-type silicon. But forming a p-n junction is not so simply. Because;

There will only be very few points of contact and any current flow would be restricted to these few points instead of the whole surface area of the junction.

Silicon that has been exposed to the air always has a thin oxide coating on its surface called the “native oxide”. This oxide is a very good insulator and will prevent current flow.

Bonding arrangement is interrupted at the surface; dangling bonds.


A P-N Junction cannot be produced by simply pushing two pieces together or by welding etc…..Because it gives rise to discontinuities across the crystal structure.

Special fabrication techniques are adopted to form P-N Junction, e.g. Crystal Preparation , Masking, Photolithographic Process , Deposition ,Implantation , Diffusion ,Oxidation ,Epitaxy ,Contacts, Interconnects ,Metallization and Planarization.


To overcome these surface states problems

p-n junction can be formed in the bulk of the semiconductor, away from the

surface as much as possible.

Surface states



PN junction is present in perhaps every semiconductor device.

diodesymbol

N P

VI

– + Building Blocks of the PN Junction Theory

V

I

Reverse bias Forward bias

Donor ions

N-type

P-type


When p-type and n-type materials are joined this forms a pn junction majority charge carriers on each side diffuse

across the junction where they combine with (and remove) charge carriers of the opposite polarity

hence around the junction there are few free charge carriers and we have a depletion layer (also called a space-charge layer)

DEPLETION REGION Free electrons on the n-side and free holes on the p-side can initially

diffuse across the junction. Uncovered charges are left in the neighborhood of the junction.

This region is depleted of mobile carriers and is called the

DEPLETION REGION (thickness 0.5 – 1.0 µm).



The diffusion of positive charge in one direction and negative charge in the other produces a charge imbalance this results in a

potential barrier across the junction

++++++++++++++++++++++++++++++++++++++++

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Hole Movement

ElectronMovement

++++++++++++

Fixed positive space-charge

- - - -- - - -- - - -

Fixed negativespace-charge

Ohmic end-contact

n-type p-type

Metallurgical junction



Potential barrier the barrier opposes the flow of majority charge

carriers and only a small number have enough energy to surmount it this generates a small diffusion current

the barrier encourages the flow of minority carriers and any that come close to it will be swept over this generates a small drift current

for an isolated junction these two currents must balance each other and the net current is zero

Forward bias to p-n junction

When an external voltage is applied to the P-N junction making the P side positive with respect to the N side the diode is said to be forward biased (F.B). The barrier p.d. is decreased by the external applied voltage. The depletion

band narrows which urges majority carriers to flow across the junction.

A F.B. diode has a very low resistance.

Forward bias to p-n junction

Forward bias if the p-type side is made positive with respect to

the n-type side the height of the barrier is reduced

more majority charge carriers have sufficient energy to surmount it

the diffusion current therefore increases while the drift current remains the same

there is thus a net current flow across the junction which increases with the applied voltage

When an external voltage is applied to the PN junction making the P side negative with respect to the N side the diode is said to be Reverse Biased (R.B.).

The barrier p.d. increases. The depletion band widens preventing the movement of majority carriers across the junction.

A R.B. diode has a very high resistance.

Reversed bias to p-n junction

Reversed bias to p-n junction

Reverse bias if the p-type side is made negative with respect to the

n-type side the height of the barrier is increased the number of majority charge carriers that have

sufficient energy to surmount it rapidly decreases the diffusion current therefore vanishes while the drift

current remains the same thus the only current is a small leakage current caused

by the (approximately constant) drift current the leakage current is usually negligible (a few nA)

Currents in a pn junction

Currents in a pn junction

Forward and reverse currents

Forward and reverse currents pn junction current is given approximately by

where I is the current, e is the electronic charge, V is the applied voltage, k is Boltzmann’s constant, T is the absolute temperature and (Greek letter eta) is a constant in the range 1 to 2 determined by the junction material

for most purposes we can assume = 1

1exp

ηkTeV

II s

Semiconductor Diodes

Thus

at room temperature e/kT ~ 40 V-1

If V > +0.1 V

If V < -0.1 V

IS is the reverse saturation current

1exp

kTeV

II s

VIkTeV

II ss 40expexp

ss III 10

Semiconductor Diodes

Forward and reverse currents

19.6

Silicon diodes

Silicon diodes generally have a turn-on voltage of about 0.5 V generally have a conduction voltage of about 0.7 V have a breakdown voltage that depends on their

construction perhaps 75 V for a small-signal diode perhaps 400 V for a power device

have a maximum current that depends on their construction perhaps 100 mA for a small-signal diode perhaps many amps for a power device

I-V characteristics of electronic components.

Resistor

The I-V plot represents is the dependence of the current I through the component on the voltage V across it.

VR

IRIV

1;I = V /

R;R = V/I

V

I

R

VI

tg() = 1/R

The I-V characteristic of the resistor

Silicon diodesTurn-on and breakdown voltages for a silicon

device

There is no turn-on voltage because current flows in any case. However , the turn-on voltage can be defined as the forward bias required to produce a given amount of forward current.

If 1 m A is required for the circuit to work, 0.7 volt can be called as turn-on voltage.

VVbb

II00VVbb ; Breakdown voltage

II0 ;0 ; Reverse saturation current

Forward BiasForward BiasReverse BiasReverse Bias

I(current)

V(voltage)

Ge ~ 0.2 – 0.4 V

Si ~ 0.6 – 0.8 V

Applying bias to p-n junction

The experimental I-V characteristic of a Si diode

Small leakagecurrent

Temperature dependence of the volt ampere of a p-n diode

Diode Circuits

Half-wave rectifier peak output voltage

is equal to the peak input voltage minus the conduction voltage of the diode

reservoir capacitor used to produce a steadier output

Diode Circuits

Full-wave rectifier use of a diode

bridge reducesthe time for whichthe capacitor hasto maintain theoutput voltageand thus reducedthe ripple voltage

Diode Circuits Signal rectifier

used to demodulatefull amplitudemodulated signals(full-AM)

also known as anenvelope detector

found in a wide rangeof radio receivers fromcrystal sets to super heterodynes

Diode Circuits

Signal clamping a simple form of

signal conditioning

circuits limit theexcursion of thevoltage waveform

can use a combination of signal and Zenerdiodes

Key Points of Diode

Diodes allow current to flow in only one direction At low temperatures semiconductors act like insulators At higher temperatures they begin to conduct Doping of semiconductors leads to the production of p-

type and n-type materials A junction between p-type and n-type semiconductors

has the properties of a diode Silicon semiconductor diodes approximate the behavior

of ideal diodes but have a conduction voltage of about 0.7 V

There are also a wide range of special purpose diodes Diodes are used in a range of applications

Parameter Germanium Silicon Comments

Depletion layer p.d. 0.15V 0.6VGermanium can be useful for low voltage applications.

Forward currentA few milli-Amperes

Tens of Amperes

Silicon much better for high current applications.

Reverse leakage current A few micro-amperes

A few nano-amperes

Germanium 1000 times more leaky than silicon.

Max. reverse voltage VoltsHundreds of volts

Silicon the only real choice for high voltage applications.

Temperature stability Poor GoodGermanium more sensitive to temperature. Can be a problem or can be useful.

Junction capacitanceVery low (point contact)

Comparatively high

This is a useful feature for high frequency use. Note: low capacitance silicon diodes are also available but their capacitance is still higher than point contact type.

Silicon & Germanium diode Comparison


G.S.VIRDI

• Crystal Preparation

• Masking

• Photolithographic Process

• Deposition

• Implantation

• Diffusion

• Oxidation

• Epitaxy

• Contacts, Interconnect ,Metalization and Planarizatiomn

IC Fabrication Techniques

IC Fabrication LaboratoryG.S.VIRDI


Design of an Integrated Circuit

Thorough Inspection of Design

Fully Processed WaferSTAGES OF IC FABRICATION

G.S.VIRDI


60Micro-Fabrication Facilities, CEERI

Fabricated 4” Silicon Wafer – MEMS Acoustic Sensor

MEMS Devices


G.S.VIRDI

G.S.VIRDI


Semiconductor devices

Technology

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