EEE-201

Lecture 2

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EXTRINSIC MATERIAL

• The characteristic of semiconductor can be altered by adding impurity through doping process (extrinsic material)

• Two type:– N-type– P-type

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N-TYPE• N-type is created by

introducing impurity elements that have five valence electrons (pentavalent) – antimony, arsenic, phosphorus

• Note that four covalent bonds are still present, however there is additional fifth electron due to impurity atom

• The remaining electron is free to move within the newly formed n-type material

• Diffused impurities with five valence electrons are called donor atoms

Figure 1.9 Antimony impurity in n-type material

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P-TYPE• P-type is created by doping

with impurity atoms having three valence electrons – boron gallium, indium

• Note that there are insufficient number of electrons to complete covalent bonds resulting a hole

• This hole is ready to accept a free electron

• The diffused impurities with three valence electrons are called acceptor atoms.

Figure 1.11 Boron impurity in p-type material

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Electron versus Hole Flow

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Majority and Minority Carriers

• In an n-type material - electron is called majority carrier and hole the minority carrier

• In a p-type material – hole is majority carrier and electron is the minority carrier

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Semiconductor Diode• Diode is formed by bringing these two material together p- and

n-type• Electrons and holes at joined region will combine, resulting in a

lack of carriers in the region near the junction (depletion region)

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• Since the diode is two-terminal device, the application of a voltage across its terminals leaves three possibilities:– No bias (VD = 0V)

– Foreard bias (VD > 0V)

– Reversed bias (VD < 0V)

• Each condition will result in a response

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No Applied Bias (VD = 0V)

• Under no-bias conditions, any minority carries (holes) in the n-type material find themselves within the depletion region will pass directly into p-type material

• Majority carriers (electrons) of n-type material must overcome the attractive forces of the layer of positive ions in n-type material and the shield of negative ions in p-type material to migrate into the area beyond the depletion region of p-type material.

• In the absence of an applied bias voltage, the net flow of charge in any one direction for semiconductor diode is zero

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Figure 1.14 p-n junction with no external bias

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Reverse-Bias Condition (VD < 0V)

• The number of uncovered positive ions in the depletion region of n-type will increase due to large number of free electrons drawn to the positive potential

• The number of uncovered negative ions will increase in p-type resulting widening of depletion region

• This region established great barrier for the majority carriers to overcome – resulting Imajority = 0

• The number pf minority carriers find themselves entering the depletion region will not change resulting in minority-carrier flow vectors of the same magnitude

• The current exists under reverse-bias conditions is called the reverse saturation current and represented by Is

• Therefore, ID= -Is www.fida.com.bd

Figure 1-16 Reverse-biased p-n junctionwww.fida.com.bd

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Forward-Bias Condition (VD = 0V)• A semiconductor diode is forward-biased when the

association p-type and positive and n-type and negative has been established

• The application of forward-bias potential will pressure the electrons in n-type and hole in p-type to recombine with ions near the boundary and reduce the width of depletion region

• The resulting minority-carrier flow of electrons from p-type to n-type has not changed in magnitude, but the reduction in width of depletion region has resulted in a heavy majority flow across the junction

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Figure 1.18 Forward-biased p-n junction

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(1.4)

For Forward-bias and Reverse-bias

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Figure 1.19 Silicon semiconductor diode characteristicswww.fida.com.bd

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