Introduction & Basic Semiconductor Physics

Dr Hanady Hussien Issa

EC238 Electronics I

Lecture 1

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Agenda Semiconductor definition. Intrinsic semiconductors Extrinsic semiconductors Diode (p-n junction).

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Semiconductor Materials A Conductor is a material that

supports a generous flow of charge when a voltage source of limited magnitude is applied across its terminals.

An Insulator is a material that offers a very low level of conductivity when a voltage source is applied across its terminals.

A Semiconductor is a material that has a conductivity level somewhere between an insulator and a conductor

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Semiconductor Materials

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Energy Levels

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Covalent Bonding and IntrinsicMaterials

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Covalent Bonding and IntrinsicMaterials

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Covalent Bonding and IntrinsicMaterials

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For silicon at 300°K,ni 1.5 X 1010 electrons/cm3

Covalent Bonding and IntrinsicMaterials

(Law of mass action)

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Semiconductor Energy Band Model

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Electron and Hole Current

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Electron and Hole Current

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Extrinsic Semiconductors

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Extrinsic Semiconductorsn-type materials

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Extrinsic Semiconductorsn-type materials

ND > NA

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Extrinsic Semiconductorsp-type materials

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Extrinsic Semiconductorsp-type materials

ND < NA

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Extrinsic Semiconductors

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Current in Semiconductors At T≠ 0 the free carriers are in constant

random motion due to their thermal energy The net motion in any particular direction is

zero,Total current I = 0

• There are two principle mechanisms by which charge move in a particular direction, thus creating an electric current: Drift. Diffusion.

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Drift Current Applying an electric field across a

semiconductor material, results in both types of carrier moving in opposite directions thus creating current flow.

E EHole motion

Current direction

Electron motion

Current direction

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The magnitude of the electric field in volts/cm is given by:

And the effective velocity of the carrier moving by the drift action of an applied electric filed is given by:

where n = 1350 cm2/V-s and p = 480 cm2/V-s are the electron and hole mobility constants respectively.

Drift Current

L

VE

Ennv Evpp

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It is a measure of the material’s ability to carry electric current.

It is given by:

where is the material resistivity which measured by .cm

Conductivity

S/cm .cm1/

1

pn pnq

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Current Density Current per unit cross-sectional area.

EvdA

QAt

QA

IJ

Enq

/

A/cm2

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Diffusion Current Diffusion current occurs because of the

physical principle that, over time particles undergoing random motion will show a movement from a region of high concentration to a region of lower concentration.

Diffusion Current Density

Current density is directly proportional to the gradient of carrier concentration.

Dn and Dp are the diffusion constants for electrons and holes respectively.

dx

dnqDJ nn

dx

dpqDJ pp