Concept of Semiconductor Physics - Department of physics ...€¦ · Concept of Semiconductor...

Concept of Semiconductor Physics

Prof. (Dr.) Pradeep Kumar Sharma

Department of PhysicsUniversity of Engineering & Management, Jaipur

Introduction to Semiconductor -Chapter Outline :1.1 Atomic Structures1.2 Semiconductors, Conductors, and Insulators1.3 Covalent Bonds1.4 Conduction in Semiconductor1.5 N-Type and P-Type Semiconductor1.6 The Diode1.7 Biasing the Diode1.8 Voltage Current Characteristic of a Diode1.9 Diode Models1.10 Testing a Diode

Introduction to Semiconductor -Chapter Objectives :

Discuss basic operation of a diode

Discuss the basic structure of atoms

Discuss properties of insulators, conductors and semiconductors

Discuss covalent bonding

Describe the properties of both p and n type materials

Discuss both forward and reverse biasing of a p-n junction

1.1 Atomic Structures History of Semiconductor

ATOM

basicstructure

Atomicnumber

Electron shells

Valence electron

Free electronionization

1.1 Atomic Structures

smallest particle of an element contain 3 basic particles:

Protons(positive charge)

Neutrons(uncharged)

Nucleus(core of atom)

Electrons(negative charge)

ATOM


Atomic Number Element in periodic table are arranged according to atomic number Atomic number = number of protons in nucleus


Electron Shells and Orbits- In an atom, the orbits are group into energy bands – shells- Diff. in energy level within a shell << diff. an energy between shells- Energy increases as the distance from the nucleus increases.


Valence Electrons- Electrons with the highest energy levels exist in the outermost shell. - Electron in the valence shell called valence electrons.- The term valence is used to indicate the potential required to removed any one

of these electrons.


This model was proposed by Niels Bohr in 1915.

• electrons circle the nucleus.

• nucleus made of:

i) +protons

ii) Neutral:neutron

Bohr model of an atom


• Atom can be represented by the valence shell and a core• A core consists of all the inner shell and the nucleus

Carbon atom:-valence shell – 4 e-inner shell – 2 eNucleus:-6 protons-6 neutrons

+6 for the nucleus and -2 for the twoinner-shell electrons

1.2 Semiconductors, Conductors and Insulators

Conductorsmaterial that easily conducts electrical current.

The best conductors are single-element material (copper, silver, gold, aluminum)

One valence electron very loosely bound to the atom- free electron

Insulators material does not conduct electric current

valence electron are tightly bound to the atom – less free electron


Semiconductors material between conductors and insulators in its ability to conduct electric current

in its pure (intrinsic) state is neither a good conductor nor a good insulator

most commonly use semiconductor ; silicon(Si), germanium(Ge), and carbon(C).

contains four valence electrons


Energy Bands


Energy Bands

•Energy gap-the difference between the energy levels of any two orbital shells•Band-another name for an orbital shell (valence shell=valence band)•Conduction band –the band outside the valence shell


Energy Bands

at room temperature 25°

eV (electron volt) – the energy absorbed by an electron when it is subjected to a 1V difference of potential


Comparison of a Semiconductor Atom & Conductor Atom

A Copper atom:•only 1 valence electron•a good conductor•Electron conf.:2:8:18:1

A Silicon atom:•4 valence electrons•a semiconductor•Electron conf.: 2:8:4

14 protons14 nucleus10 electrons

in inner shell

29 protons29 nucleus28 electrons in

inner shell


1-3 Covalent BondingCovalent bonding – holding atoms together by sharing valence electrons

To form Si crystalsharing of valence electronproduce the covalent bond

1.3 Covalent Bonding

The result of the bonding:

1. The atom are held together forming a solid substrate2. The atoms are all electrically stable, because their valence

shells are complete3. The complete valence shells cause the silicon to act as an

insulator-intrinsic (pure) silicon is a very poor conductor


Certain atoms will combine in this way to form a crystal structure. Silicon and Germanium atoms combine in this way in their intrinsic or pure state.

Covalent bonds in a 3-D silicon crystal


FIGURE 1-10 Energy band diagram for a pure (intrinsic) silicon crystal with unexcited atoms. There are no electrons in the conduction band.

1.4 Conduction in Semiconductor (Conduction Electron and holes)

FIGURE 1-11 Creation of electron-hole pairs in a silicon crystal. Electrons in the conduction band are free.

Absorbs enough energy (thermal energy)to jumps

a free electron andits matching valence band hole


FIGURE 1-12 Electron-hole pairs in a silicon crystal. Free electrons are being generated continuously while some recombine with holes.


FIGURE 1-13 Electron current in intrinsic silicon is produced by the movement of thermally generated free electrons.

Electron current

Apply voltage

freeelectrons

1.4 Conduction in Semiconductor (Electron and holes currents)

FIGURE 1-14 Hole current in intrinsic silicon.

movementof holes

1.4 Conduction in Semiconductor (Electron and holes currents)

Trivalent Impurities:

•Aluminum (Al)

•Gallium (Ga)

•Boron (B)

•Indium (In)

Pentavalent Impurites:

•Phosphorus (P)

•Arsenic (As)

•Antimony (Sb)

•Bismuth (Bi)

Doping -the process of creating N and P type materials-by adding impurity atoms to intrinsic Si or Ge to imporove the conductivity of the semiconductor-Two types of doping – trivalent (3 valence e-) & pentavalent (5 valence e-)

p-type material – a semiconductor that has added trivalent impuritiesn-type material – a semiconductor that has added pentavalent impurities

1.5 N-types and P-types Semiconductors (Doping)

N-type semiconductor:- Pentavalent impurities are added to Si or Ge, the result is an

increase the free electrons- Extra electrons becomes a conduction electrons because it is not

attached to any atom- No. of conduction electrons can be controlled by the no. of impurity atoms- Pentavalent atom gives up an electron -call a donor atom- Current carries in n-type are electrons – majority carries- Holes – minority carries

Pentavalent impurity atom in a Si crystal

Sb impurity atom

1.5 N-types and P-types Semiconductors

P-type semiconductor:- Trivalent impurities are added to Si or Ge to create a deficiency of

electrons or hole charges- The holes created by doping process- The no. of holes can be controlled by the no. of trivalent impurity atoms- The trivalent atom can take an electron- acceptor atom- Current carries in p-type are holes – majority carries- electrons – minority carries

Trivalent impurity atom in a Si crystal

B impurity atom

1.5 N-types and P-types Semiconductors

-n-type material & p-type material become extremely useful when

joined together to form a pn junction – then diode is created

-p region- holes (majority carriers), e- (minority carriers)

-n region- e- (majority carriers), holes (minority carriers)

-before the pn junction is formed -no net charge (neutral)

1.6 The Diode

1.6 The Diode (The Depletion Region)

Summary:

When an n-type material is joined with a p-type material:1. A small amount of diffusion occurs across the junction.2. When e- diffuse into p-region, they give up their energy and fall into the holes in the

valance band covalent bonds.3. Since the n-region have lost an electron, they have an overall +ve charge.4. Since the p-region have gained an electron, they have an overall –ve charge.5 The difference in charges on the two sides of the junction is called the barrier potential.

(typically in the mV range)

Barrier Potential:• The buildup of –ve charge on the p-region of the junction and of +ve charge on the

n-region of the junction-therefore difference of potential between the two sides of the junction is exist.

• The forces between the opposite charges form a “field of forces "called an electric field.• This electric field is a barrier to the free electrons in the n-region-need energy to move an e-

through the electric field.• The potential difference of electric field across the depletion region is the amount of voltage

required to move e- through the electric field. [ unit: V ]• Depend on: type of semicon. material, amount of doping, temperature. (e.g : 0.7V for Si

and 0.3 V for Ge at 25°C)

1.6 The Diode (The Depletion Region)

Energy level for n-type (Valence and Cond. Band) << p- type material(difference in atomic characteristic : pentavalent & trivalent)

After cross the junction, the e- lose energy & fall into the holes in p-region valence band. As the diffusion continues, the depletion region begins to form and the energy level of

n-region conduction band decrease. Soon, no more electrons left in n-region conduction band with enough energy to cross

the junction to p-region conduction band. Figure (b), the junction is at equilibrium state, the depletion region is complete diffusion

has ceased (stop). Create an energy gradient –energy ‘hill’ – electron at n-region must climbto get to the p-region.

The energy gap between valence & cond. band – remains the same

1.6 The Diode (Energy Diagram of the PN Junction and

Depletion Region)

No electron move through the pn-junction at equilibrium state.

Bias is a potential applied (dc voltage) to a pn junction to obtain a desired

mode of operation – control the width of the depletion layer

Two bias conditions : forward bias & reverse bias

Depletion Layer Width

Junction Resistance

Junction Current

Min Min Max

Max Max Min

The relationship between the width of depletion layer & the junction current

1.7 Biasing The Diode (Bias)

•Voltage source or bias connections are + to the p material and – to the n material•Bias must be greater than barrier potential (0 .3 V for Germanium or 0.7 V for Silicon diodes)•The depletion region narrows.•R – limits the current to prevent damage for diode

Diode connectionFlow of majority carries and the voltage across the depletion region

•The negative side of the bias voltage push the free electrons in the n-region -> pnjunction

•Also provide a continuous flow of electron through the external connection into n-region

•Bias voltage imparts energy to the free e- to move to p-region

•Electrons in p-region loss energy- positive side of bias voltage source attracts the e- left the p-region•Holes in p-region act as medium or pathway for these e- to move through the p-region

1.7 Biasing The Diode ( Forward Bias)

As more electrons flow into the depletion region, the no. of +ve ion is reduced.

As more holes flow into the depletion region on the other side – the no. of –ve

ions is reduced.

Reduction in +ve & -ve ions – causes the depletion region to narrow

1.7 Biasing The Diode ( The Effect of Forward Bias on the Depletion Region)

Electric field between +ve & -ve ions in depletion region creates “energy hill”

-prevent free e- from diffusing at equilibrium state -> barrier potential

When apply forward bias – free e- provided enough energy to climb the hilland cross the depletion region

Electron got the same energy = barrier potential to cross the depletion region

An add. small voltage drop occurs across the p and n regions due to internal resistance of material – called dynamic resistance – very small and can be neglected

1.7 Biasing The Diode ( The Effect of the Barrier Potential during Forward Bias)

•Condition that prevents current through the diode

•Voltage source or bias connections are – to the p material and + to the n material

•Current flow is negligible in most cases.

•The depletion region widens

Diode connectionShot transition time immediately after reverse bias voltage is applied

•+ side of bias pulls the free electrons in the n-region away from pn junction

• cause add. +ve ions are created , widening the depletion region

•In the p-region, e- from – side of the voltage source enter as valence electrons

•e- move from hole to hole toward the depletion region, then created add. –ve ions.

•As the depletion region widens, the availability of majority carriers decrease

1.7 Biasing The Diode ( ReverseBias)

• extremely small current exist – after the transition current dies out• caused by the minority carries in n & p regions that are produced by thermally

generated electron-hole pairs • small number of free minority e- in p region are “pushed” toward the pn junction by the

–ve bias voltage• e- reach wide depletion region – they “fall down the energy hill” combine with minority

holes in n -region as valence e- (flow towards the +ve bias voltage) – create small hole current

• the cond. band in p region is at higher energy level compare to cond. band in n-region e- easily pass through the depletion region

1.7 Biasing The Diode ( Reverse Current)

-When a forward bias voltage is applied – current called forward current,

-In this case with the voltage applied is less than the barrier potential so the diode for all practical purposes is still in a non-conducting state. Current is very small.

-Increase forward bias voltage –current also increase

FI

FIGURE 1-26 Forward-bias measurements show general changes in VF and IF as VBIAS is increased.

1.8 Voltage-Current Characteristic of a Diode( V-I Characteristic for forward bias)

-With the applied voltage exceeding the barrier potential (0.7V), forward current begins increasing rapidly.

-But the voltage across the diode increase only above 0.7 V.

FIGURE 1-26 Forward-bias measurements show general changes in VF and IF as VBIAS is increased.


-Plot the result of measurement in Figure 1-26, you get the V-I characteristic curve for a forward bias diode

- Increase to the right

- increase upward

FFd IVr /'

dynamic resistance r’d decreases as you move up the curve

FV

FI

VVF 7.0

zerobias

VVF 7.0


Reverse Current

Breakdown voltage-not a normal operation of pnjunction devices- the value can be vary for typical Si

1.8 Voltage-Current Characteristic of a Diode( V-I Characteristic for Reverse bias)

Combine-Forward bias& Reverse bias CompleteV-I characteristic curve

1.8 Voltage-Current Characteristic of a Diode( Complete V-I Characteristic curve)

• Forward biased dioed : for a given value of

• For a given

• Barrier potential decrease as Tincrease

• Reverse current breakdown –small & can be neglected

FIT ,

FV

FF VI ,

1.8 Voltage-Current Characteristic of a Diode( Temperature effect on the diode V-I Characteristic)

Directional of current

cathodeanod

1.9 Diode Models( Diode structure and symbol)

DIODE MODEL

The Ideal Diode Model

The Complete Diode Model

The Practical Diode Model

1.9 Diode Models

•Assume •Forward current, by Ohm’s law

Ideal model of diode-simple switch:

•Closed (on) switch -> FB

•Open (off) switch -> RBVVF 0

LIMIT

BIASF R

VI

BIASR

R

VVAI

0

(1-2)

1.9 Diode Models( The ideal Diode model)

•Adds the barrier potential to the ideal switch model

• ‘ is neglected

•From figure (c):

The forward current [by applying Kirchhoff’s voltage low to figure (a)]

Ohm’s Law

dr '

•Equivalent to close switch in series with a small equivalent voltage source equal to the barrier potential 0.7V

•Represent by produced across the pnjunction

FV

•Same as ideal diode model

)(3.0)(7.0

GeVVSiVV

F

F

0LIMITRFBIAS VVV

LIMITFR RIVLIMIT

LIMIT

FBIASF R

VVI

BIASR

R

VVAI

0

(1-3)

1.9 Diode Models ( The Practical Diode model)

Complete model of diode consists:

•Barrier potential

•Dynamic resistance,

•Internal reverse resistance,

•The forward voltage:

•The forward current:

dr '

Rr '•acts as closed switch in series with barrier potential and small dr '

Rr '

•acts as open switch in parallel with the large

'7.0 dFF rIVV

'7.0

dLIMIT

BIASF rR

VVI

(1-4)

(1-5)

1.9 Diode Models ( The Complete Diode model)

10V

1.0kΩ

5V

1.0kΩ

(1) Determine the forward voltage and forward current [forward bias] for each of the diode model also find the voltage across the limiting resistor in each cases. Assumed rd’ = 10 at the determined value of forward current.

1.9 Diode Models ( Example)

a) Ideal Model:

b) Practical Model:

(c) Complete model:

VARIV

mAVR

VI

V

LIMITFR

BIASF

F

LIMIT10)101)(1010(

101000

100

33

VARIV

mAVVR

VVI

VV

LIMITFR

LIMIT

FBIASF

F

LIMIT3.9)101)(103.9(

3.91000

7.010)(7.0

33

VkmARIVmVmAVrIVV

mAVkVV

rRVVI

LIMITFR

dFF

dLIMIT

BIASF

LIMIT21.9)1)(21.9(

792)10)(21.9(7.07.0

21.9101

7.0107.0

'

'

1.9 Diode Models ( Example)

Diodes come in a variety of sizes and shapes. The design and structure isdetermined by what type of circuit they will be used in.

1.9 Diode Models ( Typical Diodes)

Testing a diode is quite simple, particularly if the multimeter used has a diode check function. With the diode check function a specific known voltage is applied from the meter across the diode.

K A A K

With the diode check function a good diode will show approximately .7 V or .3 V when forward biased.

When checking in reverse bias the full applied testing voltage will be seen on the display.

1.10 Testing A Diodes ( By Digital multimeter)

NG DIODE

1.10 Testing A Diodes ( By Digital multimeter)

Select OHMs rangeGood diode:Forward-bias: get low resistance reading (10 to 100 ohm)Reverse-bias: get high reading (0 or infinity)

1.10 Testing A Diodes ( By Analog multimeter – ohm function )

P-materials are doped with trivalent impurities

N-materials are doped with pentavalent impurities

P and N type materials are joined together to form a PN junction.

A diode is nothing more than a PN junction.

At the junction a depletion region is formed. This creates barrier which requires approximately .3 V for a Germanium and .7 V for Silicon for conduction to take place.

Diodes, transistors, and integrated circuits are all made of semiconductor material.

Summary

When reversed biased a diode can only withstand so much applied voltage. The voltage at which avalanche current occurs is called reverse breakdown voltage.

There are three ways of analyzing a diode. These are ideal, practical, and complex. Typically we use a practical diode model.

A diode conducts when forward biased and does not conduct when reverse biased

Summary

1. Describe the difference between:a) n-type and p-type semiconductor materialsb) donor and acceptor impuritiesc) majority and minority carries

2. Predict the voltmeter reading in Figure 2.1. (assumed voltage across the diode is 0.7V, R1= 10kohm, V1 = 5V). Then, calculate current, I.

D11N4148

V15V

R1

10kohm

XMM1

Figure 2.1

voltmeter

I

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