AC Bamunuarachchic

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PHYSICAL ELECTRONICS

ECX5239

Assignment 01


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Q1. conductivity difference in different materials, using

energy band diagrams.

Energy Band Diagram

The range of energies that an electron may possess in an

atom is known as the energy band. There Important energy bandsare,

Valence Band

Conduction Band

Forbidden Band


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Valence Band :-The Range of Energy possessed by valence electrons is known as

valence Bands.

Conduction Band :-

The Range of energy passed by these electrons is known as

conduction band.

Forbidden Band (or) Energy Gap :-

The energy band in between the condition band and the valence band

is called forbidden Band.


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Classification of Materials According to Energy Bands

Conductors :-

The materials in which conduction and valence bands overlap as shown in figure are

called conductors.

The overlapping indicates a large number of electrons available for conduction.

Hence the application of a small amount of voltage results a large amount of current.


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Insulators :-

The materials in which the condition band and valence bands are separated by a wide energy gap ( 15eV) as shown in figure.

A wide energy gap means that a large amount of energy is required, to free the electrons, by moving

them from the valence band into the condition band ;

Since at room temperature, the valence electrons of an insulator do not have enough energy to jump

in to the condition, therefore insulator do not have an ability to conduct current. Thus insulators have

very high resistively (or extremely low conductivity) at room temperatures.

However if the temperature is raised, some of the valence electrons may acquire energy and jump in to

the conduction band. It causes the resistively of insulators to decrease.

Therefore an insulator have negative temperature co-efficient of resistance.

Semiconductors :-

The materials, in which the conduction and valence bands are separated by a small energy gap (1eV)

as shown in figure are called semiconductors.

Silicon and germanium are the commonly used semiconductors.

A small energy gap means that a small amount of energy is required to free the electrons by moving

them from the valence band in to the conduction band.

The semiconductors behave4 like insulators at 0K, because no electrons are available in the

conduction band.

If the temperature is further increased, more valence electrons will acquire energy to jump into the

conduction band.

Thus like insulators, semiconductors also have negative temperature co-efficient of resistance.

It means that conductivity of semiconductors increases with the increases temperature.


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Q5:-The resistivity of a doped Si crystal is 9.27 10 and the Hall coefficient is 3.84 10. Assuming conduction by a single type of charge carrier, calculate the density

and the mobility of the carrier.

Resistivity = = 9.27 * 10-3m

Hall Co Efficient = RH = 3.84 * 10-4

m3

c-1

Mobility=

Carrier Density = n Conductivity () = 1/

Mobility of carrier = * RH = 3.84 * 10-4/ 9.27 * 10-3= 0.04142 m2/V.s

Carrier Density = n = / (e );where e = 1.6*1022

n = 1/ (e) =1/ [(9.27 * 10-3) (1.6*10-19) (0.04142)] = 1.628 * 1023m-3


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Q8:- Consider a rectangular sample of a metal with a length L, width W and the

thickness D. A current I is passed along L, perpendicular to the cross section W D.The face W Lis exposed to a magnetic field of density B. A voltmeter is connected

across the width to read the Hall voltage VH. Show that the reading is VH = IB / Den.

Let us take dX small element of length. Therefore dV= WDdX

Total Charge = dQ = enWDdX

But Current I = (dQ/dt) = enWD(dX/dt)

But charge velocity v = (dX/dt)

Therefore; en = I/WDv

Magnetic force = Fm= q (v * B) = ne(v * B) = (I/WDv)(vB) = IB/WD

Fm = IB/WD ---------------------- a


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Electrostatic force = Fe = qE = neE

A potential is develop across W, & is given by VH = EW,

Therefore, Fe = ne VH/W ------------------------ b

In equilibrium, Fe = Fm

Therefore neVH/W = IB/WD

ne VH= IB/D

Therefore, Hall Voltage = VH = IB / Den


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http://en.wikipedia.org

Micro Electronic Devices - 2ndEdition by KeithLeaver

Semiconductor Physics and Devices BasicPrinciples by Donald A. Neamen
http://en.wikipedia.org/http://en.wikipedia.org/http://en.wikipedia.org/http://en.wikipedia.org/

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