1 Department of Information Engineering 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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Department of Information Engineering256 Semiconductor Conduction is possible only if the electrons are free to move –But electrons are bound to their.
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1Department of Information Engineering
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
– 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