Department of Information Engineering256 Semiconductor Conduction is possible only if the electrons...

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Semiconductor

• Conduction is possible only if the electrons are free to move

– But electrons are bound to their parent atoms

• To be freed, the electrons need energy

– Conductor: need small energy

– Insulator: need large energy

– Semiconductor: need moderate energy

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Semiconductor

• Silicon, germanium

– IV column in the periodic table

– 4 electrons at the outermost shell

• Most stable structure

– each atom has 8 electrons at the outer most orbit (by sharing electrons with neighboring atoms)

+4 +4

+4

+4

+4

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Semiconductor

• At low temperature

– Not enough thermal energy

– very few free electrons

• At higher temperature (room temperature)

– electrons randomly receive thermal energy

– electrons with high enough energy are freed

• create an electron-hole pair

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Electron-hole pair

+4 +4

+4

+4

+4

free electron

leaving a hole behind

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Hole as carrier

• Why holes can be used to conduct electricity?

– region around the hole is more positively charged

– attract neighboring electron to fill up the hole

– The movement of the hole appears like a positively charged carrier

+4 +4 +4 +4

Movementof hole

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Recombination

• When a free electron fills up a hole, then the electron returns to its initial resting state

– We lost two carriers

• a free electron and a hole

+4 +4 +4 +4

free electronrecombination

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Doping

• Pure semiconductor

– thermal excitation produces only a few free electrons and holes

– Poor conductor

• Doping

– to create more holes or free electrons by adding impurity

– to create more holes, add Group III material

– to create more free electrons, add Group V material

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P-type semiconductor

• doped with group III material– 3 electrons at the outer-most shell

• Thermal energy creates electron-hole pairs– But the number of electrons is small

• Doping can create a large number of holes

• majority carrier - holes• minority carrier - electrons (created by thermal energy)

+3 +4+4+4

hole

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N-type semiconductor

• dope with group V material

– 5 electrons in the outer-most shell

– majority carrier - electrons

– minority carrier - holes (created by thermal energy)

+5 +4+4+4

free electron

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p-n junction

• What happen if we join a p-type and a n-type material together?

– p-type - lots of free holes

– n-type - lots of free electrons

P N

- - - -- - - -

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Diffusion

• Diffusion

– holes and free electrons move randomly

– statistically it is more likely that carriers will move from higher concentration to lower concentration

– this process is called diffusion

• Direction of diffusion

– Holes (from p-side to n-side)

– electrons (from n-side to p-side)

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Diffusion current

• Diffusion current– movement of charges = current

• Direction of holes?– From, p to n, direction of current

• Direction of electrons?– From n to p, opposite to the direction of current

• Direction of diffusion current?– The current produced by holes and electrons are in

the same direction– Total current = hole current + electron current

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PN junction

• Large number of holes move from P to N side

• Large number of electrons move from N to P side

• The holes and electrons meet at the junction between P and N

– What happens when a hole meets an electron?

– Recombination !

+3 +5

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PN junction

• An electron from N side finds a hole in the P side

– Recombination

– P side is more –ve charged !

• Similarly, a hole recombines with an electron in the N side

– N side is more +ve charged

+3 +5+3 +5

-ve +ve

P NP N

0 0

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Some critical properties of the PN junction

• The build-up charges create a potential barrier

---

+++

P N

potential barrier

Junction capacitor !

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Some critical properties of the PN junction

• Potential barrier

– Because of the potential barrier, the N side is more +ve charged

– Repel holes coming in from P side back to P side

– Similarly, electrons from N side is repelled back to N side

• Dynamic equilibrium

– number of diffused charge = number of repelled charge

– Net flow of charge = current = 0

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Some critical properties of the PN junction

• Why the number of diffused charge = number of repelled charge?

• If the potential barrier is too weak, so that the number of diffused charge > the number of repelled charge

– More charges diffuse across the junction

– Recombination

– N side is more +ve charged (P side more –ve charged)

– Barrier increases until the number of diffused charge is exactly the same as the number of repelled charge

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Some critical properties of the PN junction

• Depletion region

– At the junction, electrons and holes are recombined, therefore this region has NO carriers

– High resistance

Depletion layer

P N

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Diode

• Diode is simply a p-n junction

P N

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Diode

• property of a diode

– one-way street

– current flows in one direction only

• vD > 0: short circuit (conducting)

• vD < 0: open circuit (non-conducting)

vD

iD

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Forward bias ( vD > 0 )

• Most of the external voltage applies to the PN junction because it has the highest resistance

• Voltage is applied in a direction that reduces the potential barrier

P N

new potential barrier

barrier at equilibrium

vD

vD

vD

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Forward bias ( vD > 0 )

• Forward bias lowered the potential barrier

– The force of diffusion > potential barrier

– More carriers can cross the barrier

– Once crosses the barrier, the carriers are collected by the terminals

• Holes diffuse from p to n and are collected by the –ve terminal

• Electrons diffuse from n to p and are collected by the +ve terminal

• A small reduction in barrier leads to exponential increase in current

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Reverse bias ( vD < 0 )

• Reverse bias makes the potential at p-side more -ve

– Increases the potential barrier

P N

potential barrier

barrier at equilibrium

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Reverse bias ( vD < 0 )

• Increased potential barrier– A tiny current due to minority carriers still flow

• Minority carriers – Carriers created by thermal energy,– electrons on P side, holes on N side (minority)– Minority carriers can move across the barrier easily !

• Electrons on P side attracted by the +ve potential at N side

• This is known as the reverse saturation current– A very small current carried by the minority carrier– The diode acts like a large resistor

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Diode characteristic

• Forward biased – exponential curve

• Backward biased – no current until diode breakdown

Break down voltageConstant voltage over avery wide range of current.Perfect voltage source !!

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Diode current model

• An equation that approximates the diode current

–

– vD is the voltage across the diode

– I0 is the reverse saturation current

– k is Boltzmann constant

– T is temperature in unit of Kelvin

– q is charge

0 [exp( / ) 1]

/ ~ 25D D T

T

I I v v

where v kT q mV at room temp

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Simple model

• The diode conducts vigorously if VD > 0.6V

• Voltage drop across diode ~ 0.6V

Half-wave rectifier

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Application - full-wave bridge rectifier

D1

D3

D1

D3

+

-e.g simplified circuitduring positive cycle

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Zener diodes

• For ordinary diode, if the reverse-biased voltage is too large, the diode breaks down, conducts large current

• The diode will be burnt

• Zener diode

– Break down at a very precise voltage, but will not be destroyed

– Makes excellent voltage reference source

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Zener diodes