Electronic Devices and Machines

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2011

MD. HASAN SHAHRIAR

Leonis Technology Inc.

7/9/2011

CONDUCTIVITY AND DOPING



CONDUCTIVITY AND DOPING 2011

MD. HASAN SHAHRIAR

Page 2

E L E C T R O N IC D E V I C E S A N D M A C H I N E S

CONDUCTIVITY AND DOPING

By

MD. HASAN SHAHRIAR

[email protected]

mailto:[email protected]






MD. HASAN SHAHRIAR

Page 3

Types of materials:

At the perspective of conductivity, materials are of three types:

1. Conductor,

2. Semi-conductor and

3. Insulator.

Conductor:

A conductor is a material that will support a generous flow of charge when a voltage

source is applied across its terminal. Insulator:

An insulator is a material that offers a very low level conductivity in case of applied

voltage.

Semi-conductor: A semi-conductor, therefore, is a material that has conductivity somewhere between

the extremes of an insulator and a conductor.

The characteristics of semi-conductor material can be altered significantly by the

addition of certain impurity atoms into relatively pure semi-conductor material.

A semi-conductor that has been subjected to the doping process is called an extrinsic

material.

There are two extrinsic materials:

1) n-type,

2) p-type.

Both n-type and p-type are formed by adding impurity atoms in germanium or silicon

base.

n-type material: The n-type material is created by introducing a pre-determined number of impurity

atoms that have five valance electrons such as- Antimony (Sb), Arsenic (As) and

Phosphorus (P).




MD. HASAN SHAHRIAR

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The effect is indicated in figure with four covalent bonds, an additional fifth electron

due to impurity atoms. The remaining electron is loosely bounded to its parent atom

and relatively free to move in the structure. The diffused impurities with five valance

electrons are called donor.

p-type material: The p-type material is formed by doping a pure Ge or Si crystal with impurity atoms

that have three valance electrons such as- Boron (B), Gallium (Ga), Indium (In).

There is insufficient number of electrons to complete the covalent bonds of the newly

formed lattice. The resulting vacancy is called hole and is represented by a small circle

or a positive sign.




MD. HASAN SHAHRIAR

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Since the resulting vacancy will readily accept a free electron, it is considered to have

positive charge. The diffused impurities with three valance electrons are called

acceptor.

Majority & Minority Carrier:In an n-type material, the number of holes has not changed significantly from its

intrinsic level. The net result, is therefore, is that the number of electrons outweighs

the number of holes. So, in an n-type material, electrons are majority carrier and holes

are minority carrier.

In a p-type material, the number of holes has changed significantly from its intrinsic

level. The net result is therefore, is that the number of holes outweighs the number of

electrons. So, in a p-type material, holes are majority carrier and electrons are minority

carrier.




MD. HASAN SHAHRIAR

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Semi-conductor diode:

The semi-conductor diode is simply formed by bringing p-type and n-type materials

together. At the instant the two materials are joined, the electrons and holes will

combine in the region of the junction, resulting in lack of carriers near the junction.

The region of uncovered positive and negative ion is called the depletion region due

to the depletion of carrier in this region.

Since, diode is a two-terminal device, there are three types of combination possible to

apply voltage:

1. No Bias ( ),

2. Forward Bias (

),

3. Reverse Bias ( ).




MD. HASAN SHAHRIAR

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Diode forward bias i-V relationship:

Here, Reverse saturation current, it should be in the order of Amp,

= Thermal voltage = , K=Boltzmann constant= ⁄ ,

T= Temperature in Kelvin, q = Electron charge =,

= Forward bias voltage.

Approximately the equation becomes,

or,

or,

or,




MD. HASAN SHAHRIAR

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i-V relationship in diode:

Let us consider the forward-bias i-V characteristics, for current if corresponding

diode voltage is , then

. If the voltage is , the forward bias current

becomes

.

Now,

or,

or,

or,

or, (

)

or, (

)

or, (

)

(

)

The equation simply states that, for a change in current, the diode voltage drop

changes by 2.3 , for n=1 which is approximately 60 mV .




MD. HASAN SHAHRIAR

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MD. HASAN SHAHRIAR

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Forward and Reverse Bias p-n junction:

If negative voltage is applied, diode operated in reverse bias mode, diode current iszero and diode said to be cut off.

A positive voltage is applied to the ideal diode, zero voltage drop across diode occurs.

The diode behaves as short circuit in forward region. Forward bias is said to be turnedon.




MD. HASAN SHAHRIAR Page 11

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