Semiconductor

1
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

2
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)

3
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

4
Electron-hole pair
4
free electron
4
4
leaving a hole behind
5
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

Movement of hole
4
4
4
4
6
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

free electron
recombination
4
4
4
4
7
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

8
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)

9
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)

free electron
5
4
4
10
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
- - - - - - - -
11
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)

12
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

13
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 !

14
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

P N
P N
3
ve
-ve
0
0
15
Some critical properties of the PN junction

• The build-up charges create a potential barrier

Junction capacitor !
- - -

N
P
potential barrier
16
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

17
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 gt 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

18
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

P
N
Depletion layer
19
Diode

• Diode is simply a p-n junction

P N
20
Diode

• property of a diode
• one-way street
• current flows in one direction only
• vD gt 0 short circuit (conducting)
• vD lt 0 open circuit (non-conducting)

iD
vD
21
Forward bias ( vD gt 0 )
vD

• 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
vD
barrier at equilibrium
vD
new potential barrier
22
Forward bias ( vD gt 0 )

• Forward bias lowered the potential barrier
• The force of diffusion gt 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

23
Reverse bias ( vD lt 0 )

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

P N
barrier at equilibrium
potential barrier
24
Reverse bias ( vD lt 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

25
Diode characteristic

• Forward biased exponential curve
• Backward biased no current until diode breakdown

Break down voltage Constant voltage over a very
wide range of current. Perfect voltage source !!
26
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

27
Simple model

• The diode conducts vigorously if VD gt 0.6V
• Voltage drop across diode 0.6V

Half-wave rectifier
28
Application - full-wave bridge rectifier
D1
D3
D1

e.g simplified circuit during positive cycle
D3
-
29
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

30
Zener diodes