NV6106 Semiconductor Energy Gap Measurement

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Semiconductor Energy Band Gap

Measurement Trainer

NV6106

Learning MaterialVer 1.1

Designed & Manufactured by:

141-B, Electronic Complex, Pardesipura, Indore- 452 010 India,Tel.:91-731- 4211500,

Telefax:91-731-4202959,Toll free:1800-103-5050,E-mail:[email protected]

Website:www.nvistech.com
mailto:[email protected]://www.nvistech.com/http://www.nvistech.com/mailto:[email protected]


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Nvis Technolo ies Pvt. Ltd.

Semiconductor Energy Band Gap Measurement Trainer

NV6106

Table of Contents

1. Introduction 3

2. Features 4

3. Technical Specifications 5

4. Theory 6

5. Experiments

Experiment 1

Finding the energy band gap of semiconductor material of a

P-N junction of diode.

34

Experiment 2

Study of the reverse characteristic of OA79 P-N junction

Semiconductor diode.

37

6. Warranty 38

7. List of Accessories 38


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Introduction

NV6106 Semiconductor Energy Band-Gap Measurement Trainer introduces

you to a very useful nonlinear electronic device: diode. This trainer

familiarizes the characteristic and the energy band gap of semiconductor diode.Semiconductor diode is an important element of most electronic devices, and is

tremendously important and plays an essential role in modern technology,

because of their relevant conductivity. This trainer provides the crucial

framework, which needed to understand the concept of an energy band

gap and characteristic of a diode. Conductivity is directly related to energy

bands and it is necessary for any detailed description of semiconductor devices.

This trainer is based upon reverse diode characteristic, in which conductivitydepends on temperature. This trainer comprises power supply, Oven and digital

panel meter. Oven is mounted on trainer, which provides ambient temperature

range. This trainer not only measures conductivity on various temperature but

also helps to draw a reverse characteristic of diode.

Diode is an important element of most electronic device, from a simple rectifierto integrated circuit. Therefore, understanding of the diode is essential to the

study of more complicated semiconductor device and thus has attracted aconsiderable interest in the application of diode in all electronic devices. The

study of the band gap structure of semiconductors is also important

because it is directly related to its electrical properties.

A diode is a semiconductor, which in its simplest form converts acin to dc and

allows current to flow only in one direction, with far greater ease than in other.

The most common kind of diode in modern circuit design is the semiconductor

diode.

Semiconductors are tremendously important in modern technology.Semiconductor devices, electronic components made-up of semiconductor

materials, are essential in modern electrical devices, from computers cellular

phones to a digital audio player. Silicon diode is used to create most

semiconductors devices commercially, because ofitsgreater ease of processing

and more stable temperature characteristics However, germanium diodes have

the advantage of an intrinsically low forward voltage drop, typically 0.3 volts;

this low forward voltage drop results in a low power loss and more efficient

diode, making it superior in many ways to the silicon diode. A silicon diode

forward voltage drop, by comparison, is typically 0.7 volts. This lower voltage

drop for germanium becomes important in very low signal environments (signal

detection from audio to FM frequencies) and in low level logic circuits. As aresult germanium diodes are finding increasing application in low level digital

circuits.


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Features

Design with all safety standards

Easy understanding circuit arrangement, with diagram interpretation

Adjustable voltage Oven produces isothermal heat with regulation

Onboard voltage and current measurement

Exhaustive learning material

2 Year Warranty


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Technical Specifications

DC Power Supply : +15V, 1.5A

+6V, 0.5 A

Diode : OA79 P-N Junction Germanium type

Switch : 1 Pole, 2 WaysToggle type

Display

Type : LCD

Oven

Height : 66mm

Width : 74mm

Coil : Nichrome Wire

Fuse : 0.5A

Mains : 230 V 10%, 50 Hz

Dimensions (mm) : W 250 x D 130 x H 30


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Theory

I. Materials and concept of energy bands:

Materials can be categorized into conductors, semiconductors or insulators by theirability to conduct electricity. Free electrons are responsible for the conduction of

electricity. Free electrons are those outermost electrons, which are most weakly

bound with atoms. Hence these electrons get separated from their atoms and move

freely inside the entire substance from one atom to another atom. These free electrons

act as the charge carrier. Insulators do not conduct electricity because their valence

electrons are not free to wander throughout the material. Metals conduct electricity

easily because the energy levels between the conduction and valence band are closely

spaced or there are more energy levels available, than there are electrons to fill them

so very little energy is required to find new energies for electrons to occupy. The

band theory of materials explains qualitatively the difference between these types of

materials. Electrons occupy energy levels from the lowest energies to upwards.

However, some energy levels are forbidden. The allowed energy levels tend to formbands. In metals, there is no forbidden gap; the conduction band and the valence band

overlap, allowing free electrons to participate in the conduction process. Insulators

have an energy gap that is far greater than the thermal energy of the electron, while

semiconductor materials the energy gap is typically around 1eV.

Energy bands :

Electron of single atom occupies atomic orbital, which forms a discrete energy level.

If several atoms brought together in to a molecule, their atomic orbitals re-arrange

themselves in a space or in allowed degree of freedom. When a large number of

atoms (order 1020

or more) are brought together to form a solid, number of orbital

becomes exceedingly large and difference in energy between them become very small,

so the level forms band of energy rather than the discrete energy level. So energyband consists of a large number of closely spaced energy levels. The bands can be

thought of as the collection of the individual energy levels of electrons surrounding

each atom. The wave function of the individual electron, however, overlaps with

those of electrons confined to neighboring atoms. The Pauli Exclusion Principle does

not allow the electron energy levels to be the same so that one obtains a set of closely

spaced energy levels, forming an energy band.

There are two approaches to find the electron energies associated with atoms bound

electron and unbound or free electron. Here figure (1) shows energy levels of two

atoms, six atoms and solid of N atoms.


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The valence band is the lower band of allowed states and a highest range of electron

energy at absolute zero temperature. Since electrons have a tendency to fill the lowest

available energy states, the valence band is always nearly completely filled with

electrons, especially as the temperature falls toward 0K. Electrons in the valence band

do not participate in the conduction process. (See Valence band in figure 3)

Conduction band :

The conduction band is the upper band of allowed states and is generally empty. Inreference to conductivity in semiconductors, it is the band that accepts the electrons

from the valence band. The conduction band can be seen in the figure 3 below. It is

the range of electron energy, higher than that of valence band sufficient to make the

electrons free to accelerate under the influence of an applied electric field and thus

constitute an electric current. Semiconductors may cross this conduction band when

they are excited.


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Energy band gap :

The energy gap between the top of the valence band and bottom of the conduction

band is known as energy band gap.

Eg= EcEv

This region between the two energy bands is known as forbidden band gap. So energy

band gap of semiconductor may be determined using a semiconductor crystal or

semiconductor diode. Generally Germanium crystal or P-N junction diode is used to

determine the band gap of semiconductor.

Figure (3)

II. Band theory in solid, its application and concept of thermal runaway:

Band theory in solid :

a) When atoms are distinct, as in vapor phase, energy levels are well defined.

b) Atoms come together in a solid, atoms affect each other and energy levels

become a band.

c) Example: 1022

atoms of sodium 1s, 2s, 2p bands are full; 3s band is half

filled; 3p band is empty.

Application of band theory :

a) Sodium is a good conductor because many energy levels are open and

easily accessible.

b) Characteristic of insulator are: large band gap between highest filled

(valence) and lowest unfilled (conduction) band.

c) Semiconductor with small band gap (near 1 eV)d) Magnesium has a filled 3s shell. Atomic theory would suggest a bad

conductor. Band theory calculations however show that 3s and 3p in

magnesium overlap, creating a band that is only filled; therefore, Mg is

a good conductor.

e) 2p shell of carbon is only 1/3 filled, so atomic theory suggests carbon is a

good conductor. However, band theory calculations for carbon shows that


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the 2s and 2p bands overlap but split into bands that contain 4 electron per

atom, which makes it weaker

f) Germanium and silicon are similar to carbon but equilibrium atomic

spacing are larger and band gap is smaller (near 1 eV)

g) Band gap is smaller in germanium than silicon; germanium is more

susceptible to thermal runaway

Thermal run away :

When diode is blocking (suppose its associated supply has a fault in short circuit

mode) than the diode has to operate in reverse mode with high junction temperature

(due to preceding forward losses) and so with relatively high reverse current. This

high reverse current can generate high reverse losses and so increase junction

temperature and so reverse current as well as increases. This is called thermal

runaway phenomena.

III. Semiconductor, and concept of electrons and holes in it :

Semiconductor :

In this class of material, two energy bands are distinctly separate with no overlapping;

the forbidden gap is nearly 1 eV. At absolute zero of temperature, no electron has

energy even to jump the forbidden gap and reach the conduction band. Therefore,

substance is an insulator at zero degree temperature. But at room temperature, some

valence electrons acquire thermal energy greater than the energy gap and move to the

conduction band where they are free to move under the influence of even a weak

electric field. Higher the temperature, greater are the chances of electron to jump to

conduction bands and greater the conductivity.

Electrons and holes in semiconductor :In a semiconductor, as an electron leaves the valence band, it leaves some energy

level in band as unfilled. Such unfilled regions are termed as holes in valence band.

The holes are called positive charge carrier, because when an electron leaves a

particular spot, it is left as positively charged. Any movement of this region is called

as a positive hole moving from one position to another.

Figure 4(a) represents the formation of electron-hole pair on the energy band diagram.

In (a), at absolute zero the valence band of intrinsic semiconductor is completely

filled and the conduction band is completely empty. In this condition semiconductor

behave like an insulator.

In 4(b), when temperature increases, some of the electrons from the valence band getexcited and reach the conduction band and in their place holes are created in valence

band. Thus the number of electrons and holes are equal (i.e., n = p). In the diagram

electrons are shown by and holes by . As temperature increases, the number of

holes in valence band and number of electrons in conduction band increases.


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Formation of electronhole pair

Figure (4)

When an external electric field is applied, the motion of electrons in the conduction

band is in direction opposite to the direction of electric field, while the motion of

holes in the valence band is in the direction of electric field.

Electron-hole pairs that are produced in an intrinsic semiconductor due to thermal

agitation may recombine again due to their random motion. Normally, at each

temperature, the rate of production of electron-hole pairs is equal to the rate of their

recombination. Let Ecbe the lowest energy of the conduction band and Evbe the

highest energy of valence band as shown in figure 4(c) then at absolute zero, the

value of Fermi energy Efwill be in between the lowest energy Ecof conduction band

and highest energy Evof valence band (i.e. Fermi level must be in the forbidden gap

between EcEv), then the valence band will be completely filled (Ec< Ef, hence f (E)

= 1). Figure (5) shows their distribution of Fermi function f (E) with energy E at

absolute zero.

Distribution of Fermi function at absolute zero

Figure 5


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IV. Characteristic of semiconductor and its types :

Characteristic of semiconductor:

a) A semiconductor behaves like an insulator at 0 absolute; and as

temperature rises; it behaves like a conductor its conductivity increases

with the temperature. This is in contrast with the behavior of metals where

conductivity decreases with rise in temperature.

b) While metals confirm to ohms law and that the conductivity is an intrinsic

property of metal, independent of the potential difference applied at its

ends.

c) By doping semiconductor with external impurities; their conductivity is

vastly increased.

Intrinsic semiconductor :

This type of semiconductor is pure semiconductor, no impurities is added it is

absolutely free from impurities. This type of semiconductor, the valence band and the

conduction band are separated by nearly 1 eV. The energy gap in the case of silicon is

1.1 eV. In the case of germanium, it is 0.74 eV.

The electronic configuration of this type of semiconductor is given as

Silicon: Si 14 1s2

2s2

2p6

3s2

3p2

Germanium: Ge32 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p2

In both silicon and germanium, there are four electrons in the outermost shells so both

are called tetravalent crystals. The atoms in, say germanium, are bound together by

covalent bonds as shown in figure (6).


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Due to mutual sharing of electrons, outermost shells of each Ge atoms are

complete octets, thus we get a stable germanium crystal. At low temperature

(say, 0K), all the electrons are rigidly bound to their nuclei. There is no free

charge carriers available, therefore electrical conductivity is zero. That is whysemiconductor acts as insulator at low temperatures.

Figure 7

At room temperature, thermal energy of crystal agitates the all particle ofcrystal lattice. Hence some of the electrons acquire enough energy to overcome

the forbidden gap and separates from covalent bond. Electrons get promoted

from valence band to conduction band.

Extrinsic semiconductor:An extrinsic semiconductor is one in which an impurity with a valency higher

or lower than the valency of the semiconductor atoms is introduced, which

drastically influence the electrical properties of the semiconductor.

Figure 8


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V Distinction between extrinsic and intrinsic semiconductors:

a) Intrinsic semiconductors are pure group IV element where as extrinsic

semiconductors have some impurity in the form of group V or group III

elements externally introduce in the pure semiconductors.

b) In intrinsic semiconductors, the conductivity is only slight; but due to the added

impurity, the extrinsic semiconductor has a greatly increased electrical

conductivity.

c) In intrinsic semiconductor, the conductivity increase in rise in temperature;

while in extrinsic semiconductors, the value of conductivity depends upon the

amount of impurity added to the semiconductor.

d) In intrinsic semiconductor, the no. of holes is always equal to the no. of electron;

while this is not the case in extrinsic semiconductor. In N-type, the no. of

electrons is much greater than the no of holes; and in P-type the no. of holes is

much greater than the no of electron in it.

VI. Concept of Fermi level in semiconductor:

Fermi-level:

Between the valence band energy level and conduction band energy level, there is

another level known as fermi level and it is donated as Ef. Fermi level has a

probability of half of being occupied by an electron.

For an intrinsic semi-conductor:

The concentration of electrons in the conduction band is equal to the concentration of

holes in the valence band and hence the fermi level lies in the middle of the band gap

as shown in figure (9)

Figure 9


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For N-type semiconductor :

Because this type of semiconductor is doped with a pentavalent impurity so it has a

very high concentration of electrons in the conduction band and. In this type of

semiconductor, there is a high concentration of electrons in conduction band ascompared to the hole concentration in valence band, the fermi level therefore lies near

the conduction band below the bottom of the band as shown figure (10)

For P-type semiconductor :

Figure 10

Because this type of semiconductor is doped with a trivalent impurity so it has a very

high concentration of holes in the valence band. In this type of semiconductor there is

high concentration of holes in the valence band as compared to the electron

concentration in conduction band, the fermi therefore lies near the valence band

above the top of the band as shown in figure 11.

Figure 11


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The holes in this type semiconductor are the majority current carriers since

they are present in the greatest quantity, while the electrons are the minority

current carriers.

Figure 13

A trivalent impurity element is used to dope germanium. In this case, the

impurity is 1 electron short of the required amount of electrons needed to

establish covalent bonds with 4 neighboring atoms. Thus, in a single covalent

bond, there will be only 1 electron instead of 2. This arrangement leaves a hole

in that covalent bond. Figure (13) illustrates this, by showing what happens

when germanium is doped with an indium (In) atom. Notice, the indium atom in

the figure (13)

P-type of material contains charge carriers, which are of a positive polarity and

are known as holes.

2. N-type- semiconductor :

N-type semiconductor is a semi-conducting material, which is doped with a

donor atom (pentavalent impurities). Four of the donors atom electron bindscovalently with the semi conducting material, while the fifth is free to move into

the conduction band if given a small amount of thermal energy. The N-type

impurity loses its extra valence electron easily when added to a semiconductor

material, and in so doing, increases the conductivity of the material by

contributing a free electron. This type of impurity has 5 valence electrons and is

called a pentavalent impurity. Arsenic, antimony, bismuth, and phosphorous are

pentavalent impurities. Because these materials give or donate one electron to

the doped material, they are also called donor impurities. The electrons in this

type semiconductor are the majority current carriers, since they are present in

the greatest quantity, while the holes are the minority current carriers

Pentavalent impurities: As, At, Bi, P

Majority charge carriers: Electrons

Minority charge carriers: Holes


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Figure 14

When a pentavalent impurity, like arsenic, is added to germanium, it will form

covalent bonds with the germanium atoms. Figure 14 illustrates this by showing an

arsenic atom (As) in a germanium (Ge) lattice structure. Notice the arsenic atom in

the center of the lattice. It has 5 valence electrons in its outer shell but uses only 4

of them to form covalent bonds with the germanium atoms, leaving1 electron relatively free in the crystal structure.

Pure germanium may be converted into an N-type semiconductor by "doping". This

type of material contains charge carrier, which are of a negative polarity and are

known as electrons.

P-N Junction :

The junction is region, which has no charge carriers and is known as depletion region.

When a P-type and N-type semiconductor are joined together, the majority charge

carriers from each type will naturally diffuse into each other. This is illustrated in

Figure 15. The electrons diffusing to the left combine with holes and holes diffusing

to the right combine with electrons in the vicinity of the junction. In this way, adepletion zone around the junction is formed. N-type and P-type material remains

overall neutral. Equilibrium state is achieved when the diffusion current to the left is

balanced by the drift current of the minority carriers to the right. The minority carriers

drift through the junction as a result of the electric field. It should be noted that the

potential difference at the junction does not appear across the ends of the P-N

junction, when it is open circuited. Junction (depletion) region has a physical

thickness that varies with the applied voltage.


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Figure 15

1. A forward bias decreases the thickness of the depletion region :

When a forward bias is applied to the P-N junction the diffusion current due to

majority charge carrier electron flowing from N-type to P-type and majority

charge carrier holes from P to N raises rapidly, which compress depletion region

and lowers the potential barrier.


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2. A reverse bias increases the thickness of depletion region :

When a reverse bias is applied to the P-N junction majority charge carrier

electron attracts toward negative end of battery and holes attracts towards

positive terminal of battery, which increase the thickness of depletion regionand increase the potential barrier.

There is a potential hill associated with the junction approximately 0.3 volt for

germanium and 0.7 volt for silicon diode in forward bias.

VIII. Biasing of diode and its characteristic :

Biasing of a diode :

Figure 16

Bias refers to the application of external voltage to a semiconductor. There are two

ways a P-N junction can be biased.

1. Forward bias :

In forward bias of diode a positive voltage is applied to P-type materials and the

negative voltage is applied to N-type material. As a result, an electric field isproduced from P to N due to which holes in P region get repelled from the

positive electrode and move in the direction of electric field (i.e. towards

junction) and electrons in N region get repelled from the negative electrode and

move in direction opposite to the electric field (i.e. towards junction)

Figure 17


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A forward bias results current to flow through the diode in a mili-ampere range.

Then the different types of charge will merge at the junction and a full current

will flow around the circuit. The Bands will be brought to the same energies and

electrons and holes will be able to cross from one type to another. Electrons willthen fall into holes and a current is established. The external electric field now

cancels the internal field.

2. Reverse bias :

In reverse bias of diode a positive voltage is applied to N-type material and the

negative voltage is applied to P-type material. Reverse biases the diode. The

positive holes are attracted away from the junction. Similarly the negative

electrons are also attracted away from this region.

(a) Reverse bias of P-N junction (b) Increase in barrier energy in reverse biasFigure 18

We have reinforced the internal E Field with the imposed external field. A large

energy difference for electrons in the conduction band will now appear across

the junction and electrons will fail to cross and merge with holes - it will fail to

conduct. (There is a breakdown point however, diodes deliberately designed to

use this are called "Zener diodes" and are used to lock voltages in circuits.)

Similarly a large energy difference in the Valence Band will appear for holes so

they will not cross the junction.

Characteristic of a P-N junction diode :

Circuit diagram for the characteristics of a P-N junction diode in forward biascondition is shown in figure (19)


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Figure 19In forward bias, a low voltage battery B with variable supply across the junction

diode such that P is connected to the positive terminal and N to negative terminal.

Potential across diode can read by voltmeter V and diode current is measured by the

ammeter(A). Forward voltage is gradually increased and corresponding current is

noted. Graph is plotted by taking the current on the Y-axis and voltage on X-axis. The

graph so obtained is called the characteristic curve in forward bias, which is shown in

figure (20)

Figure 20

From graph it is clear that diode current first increases slowly with the increase of

forward voltage till the barrier potential (= 0.3 for germanium and 0.7 for silicon) is

reached, then with further increase in voltage, the current increases rapidly. The

voltage at which the current starts increasing rapidly is called the knee voltage or


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offset voltage, which is equal to the barrier voltage. Flow of current is due to the

majority charge carriers.

In reverse bias figure 21, the terminal of the battery B are reversed (P is connected to

the negative terminal and N is connected to the positive terminal) and ammeter is setto micrometer range.

Figure 21

The current is very low, since the junction offers a high resistance and it is due to the

flow of minority charge carriers. The current is independent of the voltage applied

and depends only on the temperature of diode. The current is thus called the

saturation current (which is more in Ge diode as compared to that in Si diode). At a

large reverse voltage, the breakdown occurs at B as shown in figure (22) and current

abruptly increases. The point B is called the zener point.

Reverse characteristic of P-N junction diode

Figure 22


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IX. Breakdown voltage and knee voltage in semiconductor :

Breakdown voltage :The reverse voltage, at which P-N junction breaks down with a

sudden rise in reverse current, is known as the breakdown voltage.

Figure 23

A very little reverse current flows through a P-N junction at normal reverse voltage.

But if the reverse voltage is increased to a high value, the junction may breakdown

with sudden increases in reverse current.

When a high reverse voltage is applied, these electrons get enough velocity to

dislodge valence electrons from semiconductor atoms as shown in figure (23 b)

Knee voltage :The forward voltage, at which the current through the junction begins

to increase rapidly, is known as knee voltage.

Figure 24

When the forward bias is applied to a diode, it conducts very slowly until it

overcomes the potential barrier. As shown in figure (24), the knee voltage for silicon

diode is 0.7V and 0.3 for germanium diode.


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When the forward voltage larger than the knee voltage is applied, the current starts

rising rapidly. The applied voltage must be more than the knee voltage to obtain the

significant current.

X. Breakdown in semiconductor junction :

Zener breakdown :

Zener breakdown takes place when reverse bias field across P-N junction is such that

the field may exerts a strong force on the bound electron to detach it out from

covalent bond. Thus a large number of electron hole pair is generated and hence

reverse current increases abruptly. When doping is large in semiconductor then Fermi

level of P-N junction comes close to the valence band of P and N region as shown in

figure (25).

Figure 25

Potential barrier increases on applying reverse bias, therefore in the reverse bias

condition, conduction band of N region comes in line of valence band of P region as

shown in figure (26).


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Figure 26

In this condition, electrons of conduction band of N region reach in their

corresponding energy states in P region through the tunnel in the potential barrier.

Thus a large number of conduction electrons of N-region cross the junction and reach

in P-region. As a consequence, the current, current through the junction abruptly

increases and junction gets breakdown. Zener breakdown voltage decreases with the

increase in temperature.

Avalanche breakdown :

When doping is low in semiconductor then in reverse bias, tunneling probability of

electrons is negligible and there is no Zener breakdown but in this case, minority

charge carrier suffers collisions with semiconductor atom and separate electron fromthem (i.e., they ionize the atoms of semiconductor). As a consequence, electron-hole

pairs are formed. When reverse bias voltage is increases kinetic energy of minority

charge carrier also increases. Hence large no. of atoms get ionized and large number

of electrons-hole pairs are formed. New electrons so formed get accelerated and

further ionize other atoms. This process continues. Electrons move towards positive

electrodes (N region) and holes towards negative electrodes (P regions). As a result

current rapidly increases and junction gets breakdown. Avalanche breakdown voltage

increasers, with increasing in temperature.


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XI. Space charge capacitance and Diffusion capacitance in semiconductor:

Space charge capacitance: When P-N junction diode is operated in reverse

bias, the width W of depletion region is increased because the holes and

electrons are moved towards the negative and positive polarity of appliedvoltage. the additional ions are uncovered due to this widening in the space

charge region. The existence of opposite charge on either side of the space

charge region is same as a parallel plate capacitor which is given by

T

AC = farad

w

Where

A - Junction area,

W- Width of depletion region

- Permittivity of semiconductor.

Diffusion Capacitance:The width of the depletion region is narrowed due to

migration of holes to N-side and electrons on the P-side in a forward biased

diode. Consequently, in the vicinity of the depletion region holes on N-side andelectrons on P-side are accumulated. This is same as a plate capacitor charged

by a voltage. An electric field exists between the two stored charges that is

maximum at the centre of the junction. Now, on slightly increasing the voltage

toV`, there is a change in charge byQ and their ratio is the capacitance-

Figure 27

D

Q= C = Diffusion Capcitance

V

The capacitance is very high of the order of microfarads due to narrow width

that is million times larger than the CT


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XII. Various types of diode:

Zener diodes :

The Zener diode is designed to have a specific reverse breakdown voltage. Because ofthis, Zener diodes can be used by themselves as voltage-sensitive switches, or in

series with a current-limiting resistor to provide voltage regulation.

Photodiodes :

All P-N junctions are light sensitive; photodiodes are just P-N junctions that are

designed to optimize this effect. Photodiodes can be used two ways - in a

photovoltaic (here it becomes a current source when illuminated - see solar cell), or

photoconductive role. To use a photodiode in its photoconductive mode, the

photodiode is reverse-biased; the photodiode will then allow a current to flow when it

is illuminated.

Light-Emitting Diodes (LEDs) :

All diodes emit some light when forward-biased. LEDs are made from a special

semiconductor (like gallium arsenide phosphide), which optimizes this light output.

Unlike light bulbs, LEDs are rarely burn out unless their current limit is passed. When

current is flowing through an LED the voltage on the positive terminal is about 1.4

volts higher than the voltage on the negative side (this varies with LED type - infrared

LEDs have a lower forward voltage requirement, others may need up to 1.8 V).

Remember that there is very little resistance to limit the current, so a resistor must be

used in series with the LED to avoid destroying it. Also note that LEDs can be used

as photodiodes (though, their sensitivity is relatively low, so they're only useable this

way in very bright conditions).

Flashing LEDs (FLEDs) :

A flashing LED is just an LED with a built-in microcircuit to cause it to flash

periodically. Since the Flash LED draws current when it flashes, we can use Flash

LEDs to drive a number of timing-dependent circuits (via the fact that it periodically

becomes conductive). Note that some Flash LEDs need 3 V minimums to work in).

Tunnel diode :

A tunnel diode is a semiconductor with a negative resistance region that results in

very fast switching speeds, up to 5 GHz. The operation depends upon a quantum

mechanic principle known as "tunneling" wherein the intrinsic voltage barrier (0.3Volt for Germanium junctions) is reduced due to doping levels, which enhance

tunneling. Referring to the curves below, superimposing the tunneling characteristic

upon a conventional P-N junction, we have:


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Combination of Tunneling Current and Conventional PN Junction CurrentFigure 28

1. Typically semiconductor with more than 1019/cm

3impurities.

2. Forward bias: Negative differential conductance.

3. Reverse bias: Large inter-band transport.

4. Large forward bias: thermal current.5. Tunneling phenomena is a majority charge carrier effect.

Figure 29

6. Tunneling: fast process. Therefore, the tunnel diode exhibits ultra-high speed,

low power and low noise.

7. They are used as microwave ampl ification, high speed switching and binary

memory.


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XIII. Conductivity and band gap determination:

Conductivity:

Semiconductor is a special class of element having conductivity between the

Conductivity of a good conductor and an insulator. Conductivity ofsemiconductor depends upon temperature. Semiconductor materials have negative

temperature coefficient, so unlike metals conductivity of semiconductor increases

with increase in temperature. As the temperature increases, the thermal energy

of the valence electrons increases, allowing more of them to reach the

energy gap into the conduction band. When an electron gains enough energy to

escape the electrostatic attraction of its parent, it leaves behind a vacancy which

may be filled be another electron. The vacancy produced can be thought of as a

second carrier of positive charge. It is known as a hole. As electrons flow through

the semiconductor, holes flow in the opposite direction. If there are n freeelectrons in an intrinsic semiconductor, then there must also be n holes.

When P-N diode is kept in reverse bias then current flows through junction due to

minority charge carrier. (i.e., In P-region due to electrons and in N-region due to

holes). Concentration of these charge carriers depends upon energy band gap Eg.

Reverse saturation current Is, depends upon, diode temperature and is expressed

by the equation:

Equation 1

Where,

Nn = concentration of electrons in N- region

Np = concentration of holes in P-region

Vn = drift velocity of electrons

Vp = drift velocity of holes

A = area of junction

k = boltzman constant

T = junction temperature

Value of Nn and Vn is evaluated by

Where:

mn= effective mass of electron

mp = effective mass of hole


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Taking both side log of equation (4)

-Eg/kTlog Is = log A Nn ev Np ev en p

, or

-Eg/kTlog Is = log A Nn ev Np ev en p

log Is = log A Nn ev Np ev -Eg/kTn p , or

Eglog Is = C-

10 2.3026 kT

If Eg in electron volt then,

-191.6 10 Eglog Is = C -

10 -232.3026 1.38 X 10 T

-35.036 10

log Is = C- Eg10 T

Here, C = log10[A {Nn evn+ Np ev p}] = a constant

Take saturation reverse current on different-different temperature:

No. Temperature0C

Temperature

T (K)Current Is(inA) 10

3/T Log10Is

1.

2.

3.

4.

5.

6.

60

55

50

45

30

25


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If graph is plotted between log10Is and 103/T, then we get a straight line.

Graph (1)

Whose slope will be,

S = -5.036 Eg

Hence energy band gap of semiconductor

slope of line SEg = eV

5.036

XIV. Factor affecting energy band gap of semiconductor

1. Temperature dependence of the energy band gap:

The energy band gap of semiconductor tends to decrease as the temperature is

increased.Thermal energy increases amplitude of atomic vibration which cause

band gap to shrink.

The temperature dependence of the energy band gap, has been experimentally

determined by following expression for Eg as a function of temperature, T.

Where Eg, and are the fitting parameters. These parameters are listed for

germanium, silicon arsenide in table below:

Parameters Germanium Silicon GaAs

Eg .7437 1.166 1.519

.477 .473 .541

235 636 204


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Parameters used to calculate the energy band gap of germanium, silicon and

gallium arsenide (GaAs) as a function of temperature.

A plot of the resulting band gap versus temperature is shown in graph (3) for

germanium, silicon and gallium arsenide.

Graph (2)

2. Doping dependence of energy band gap:

High doping densities cause the band gap to shrink. This effect is explained by the

fact that the wave functions of the electrons bound to the impurity atoms start tooverlap as the density of the impurities increase. For instance, at a doping density

of 1018 cm-3, the average distance between two impurities is only 10 nm. This

overlap forces the energies to form an energy band rather than a discreet level.

If the impurity level is shallow, this impurity band reduces the energy band of the

host material by:

Where:

N = Doping density,

q = Electronic charge,

= Dielectric constant of the semiconductor,

k =Boltzmann's constant and

T = Temperature in Kelvin.

For silicon (= 11.7) this expression further reduces to:

NEg (N) = -22.5 meV

18 -310 cm


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From this expression we find that the band gap shrinkage can typically be ignored

for doping densities less than 1018

cm-3

.

A plot of the change in band gap energy with doping density is shown in graph (4)

Graph (3)


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Objective:

Experiment 1

Finding the energy band gap of semiconductor material of a P-N junction of

diode.

Procedure :

1. Switch Offthe power switch from trainer board.

2. Switch the toggle switch of +6 V and +15 V power supply towards off

condition.

3. Set potentiometer of +6 V towards anticlockwise position.

4. Connect the mains cord to trainer.

5. Short terminal 2 to 3 and 6 to 7 by using patch cords.

6. Connect DC ammeter between terminals 8 and 9 (+)ve and ()verespectively.

7. Connect DC voltmeter across the terminals 1 and 10 (+)ve and ()ve

respectively.

8. Switch Onthe power supply.

9. Select the toggle switch of +6 V power supply towards oncondition.

10.Use potentiometer of +6 V power supply to set voltage across diode to 2V.

11.Select the toggle switch of +15 V power supply towards oncondition.

12.Note down the initial reading of current in micro ampere.13.Wait until temperature reaches up to 65 degree celsius after that switch

Off+15 V power supply and note the reading of current.

Note:Dont increase temperature more than 65 degree celsius.

14.As the temperature decreases to 60 degree celsius, note corresponding

readings of current.

15.Take several readings of current, at the interval of 5 degree celsius

decrement.

16.Tabulate all retrieved data in below table and calculate other factors of the

table.


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Measure the Energy Band Gap of Semiconductor by using software

Procedure:

1. Install the software on your computer.2.Connect USB to Serial cable between serial port of trainer and USB port

of PC.

3. Make connections same as above (from step 1 to step 10).

4. Detect the device on PC by using software.

5. Select the toggle switch of +15V power supply towards oncondition.

6. Wait until temperature reaches up to 65-67 degree celsius after that switch

Offthe +15 V power supply.

7. Now click on theTake Readingcommand button, so that temperate and

corresponding reading of current automatically appears in the table.

8.Wait again until temperature reaches up to 30-35 degree celsius after that

cl ick on the Plot Graphcommand button.

9.Values of the A, B, C, and D blocks show coordinate of two best fitted

points P and Q.

10.When the graph is plotted then clicks on the Energy Band Gap

command button.

Observation Table:

S.No

.Temperature

oC

TemperatureT (K)

=(273+o

C)

103/T

Current

(Isin mA) Log10(Is)

1.

2.

3.

4.

5.

6.

7.

8.9.

25

30

35

40

45

50

55

6065

Take 103/T along the X-axis and Log1 0 ( I s ) along Y-axis; plot a graph between

Log1 0 ( I s ) and 103/T. (the graph will be straight line as shown in graph 4)


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Take two best fitted point P and Q on graph as shown in above figure.

Calculate Slope:

Slope of line S = AB / CD = A-B / C-D

We know that

Energy band gap of diode,

Eg = Slope/5.036 eV

= .eV

Measured Energy Band Gap= eV

Graph (4)


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Experiment 2

Objective:

Study of the reverse characteristic of OA79 P-N junction semiconductor diode.

Procedure:

1. Switch Offthe power supply from trainer board.

2.Select the toggle switch of +6 V and +15 V power supply towards off

condition.

3.Set potentiometer of +6 V power supply towards anticlockwise position.

4.Connect the mains cord to trainer.

5.Short terminals 2 to 3 and 6 to 7 by using patch cords.

6.Connect DC ammeter between terminals 8 and 9 (+)ve and ()ve

respectively.7.Connect DC voltmeter across the terminals 1 and 10 (+)ve and ()ve

respectively.

8.Select potentiometer of +6 V across the diode to 0.5 V.

9.Note and record corresponding reading of current in micro ampere.

10. Increase potential across diode to 1 volt.

11. Note and record corresponding reading of current in DC ammeter.

12. Increase potential up to 3 V in the steps of 0.5 V (small interval can be

taken for better result)

13. Note and record corresponding reading of current and voltage.

14. Take diode current along y-axis and diode voltage along x-axis and

draw a curve.

15. The reverse characteristic of P-N junction diode is shown below.

Match your result with the shown figure.

Graph (5)


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Warranty

1) We guarantee the instrument against all manufacturing defects during 24 months

from the date of sale by us or through our dealers.2) The guarantee covers manufacturing defects in respect of indigenous components

and material limited to the warranty extended to us by the original manufacturer,

and defect will be rectified as far as lies within our control.

3) The guarantee does not cover perishable item like cathode ray tubes, crystals,

batteries, photocells etc. other imported components.

4) The guarantee will becomeINVALID.

a) If the instrument is not operated as per instruction given in the instruction

manual.

b) If the agreed payment terms and other conditions of sale are not followed.

c) If the customer resells the instrument to another party.

d) If any attempt is made to service and modify the instrument.

5) The non-working of the instrument is to be communicated to us immediately giving

full details of the complaints and defects noticed specifically mentioning the type

and sr. no. of the instrument, date of purchase etc.

6) The repair work will be carried out, provided the instrument is dispatched securely

packed and insured with the railways. To and fro charges will be to the account of

the customer.

List of Accessories

1. 2mm Patch Cord8........................................................................10 Nos.

2. Mains Chord......................................... ...........................................1 No.

3. Learning Material CD.....................................................................1 No.

4. USB to Serial Adaptor1 No.

NV6106 Semiconductor Energy Gap Measurement

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