1. ECA1212 Introduction to Electrical & Electronics Engineering Chapter 4: Basic Semiconductor and Diode by Muhazam Mustapha, October 2011

2. Learning Outcome By the end of this chapter students are expected to:
Explain some basic theory about charge transport in semiconductor
Explain diode circuit operation

3. Chapter Content Physics of Semiconductor PN Junction and Diode
Diode Circuits

4. Physics of Semiconductor

5. Three Categories of Material
Based on their electrical conductivity, material can be categorized into 3 groups:
Conductor
Non-conductor
Semiconductor
This conductivity property is determined by the electronic structure in the outer most shell
Electronic structure in the outer most shell, in turn, will determine the amount of energy needed by the outer most electron to be freed from the atom
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6. Three Categories of Material
In a system of a large number of atoms come close together – in a compound or crystal, for example – the energy level of the outer shells will merge together to form BANDS
For a material to conduct electricity, its electron in the outer band (VALENCE) must be able to go up to the CONDUCTION band
The energy distance (gap) between the valence band and the conduction band is what determines the conductivity of the previous 3 categories of material
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7. Three Categories of Material
Small Gap
Overlap
Conduction Band
Electron Energy
Big Gap
Valance Band
Conductor
Semiconductor
Non-conductor
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8. Conductor
In a conductor, the conduction band and the valance band are overlapping
This allows the electrons in outer most shell (valance band) to freely move among the system of atoms
This free movement of electron is permitted without any external energy (or excitation)
Metals are the material that posses this kind of conductivity
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9. Non-Conductor
In a non- conductor, the conduction band and the valance band are far apart
This disallows the electrons in outer most shell to freely move among the system of atoms
It is almost impossible to push the electrons up to the conduction band without damaging the material structure
Most of non-metallic material are non-conductor
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10. Semiconductor
In between conductor and non-conductor, there exist a special type of material that possesses an intermediate electronic structure property
The conduction band and the valance band are not overlapping but not far apart either
This allows the electrons in its valance band to jump into the conduction band if they acquire enough energy
The source of such energy could be from heat, electromagnetic rays, direct hit by another electron, etc
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11. Semiconductor
The elements in the Periodic Table Group IV are the most common semiconductors
The examples are: Carbon, Silicon and Germanium
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12. Electron Transport in Semiconductor
We may view the crystalline structure of Group IV elements as follows:
valance electron bonding 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+
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13. Electron Transport in Semiconductor
Some of the electrons in valance band may gain some energy and become free
free electrons 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+
holes 4+ 4+ 4+ 4+ 4+ 4+
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14. Electron Transport in Semiconductor
The free electrons contribute to electric conductivity in the semiconductor material
The covalent bond from where the electrons come out will now be lacking of an electron and become another electron transport medium called HOLES
Holes made electron transport possible by allowing an electron in a neighboring covalent bond to jump into it and effectively create electrical movement
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15. Electron Transport in Semiconductor
Holes transport phenomenon only exists in semiconductor material
Under the influence of the same electric field, holes make a net movement in an opposite direction of electrons movement
Electric field
Free electrons
Valance band electrons
Holes
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16. Dopant Pure semiconductor material is called intrinsic
In intrinsic semiconductor, the no. free electrons and holes will be balanced
Dopant can be added to intrinsic semiconductor to alter the no. one of the transport carrier – either free electrons or holes
Doped semiconductor is called extrinsic
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17. N-type Semiconductor
Elements of Periodic Table Group V can be added as dopant (phosphorus, arsenic)
These elements have an extra electron that cannot contribute to the covalent bond, hence it is freed
These electrons do not need extra energy to be freed, hence they behave like free electrons in conductors
This type of semiconductor with more free electrons than holes is called N-type semiconductor
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18. N-type Semiconductor
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19. N-type Semiconductor CO1 extra free electrons 4+ 4+ 5+ 4+ 4+ 4+ 4+ 4+

20. N-type Semiconductor In N-type semiconductor:
The majority carrier is free electrons
The atom (element) that contribute to the extra free electron is called DONOR atom
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21. P-type Semiconductor
Elements of Periodic Table Group III can be added as dopant (boron, gallium)
These elements lack an electron in the outer shell hence cannot create a complete covalent bond
These bonds are effectively created with holes
This type of semiconductor with more holes than free electrons is called P-type semiconductor
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22. P-type Semiconductor
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23. P-type Semiconductor CO1 extra holes 4+ 4+ 3+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+

24. P-type Semiconductor In P-type semiconductor:
The majority carrier is holes
The atom (element) that contribute to the extra holes is called ACCEPTOR atom
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25. PN Junction
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26. PN Junction
What will happen if a P-type semiconductor is fabricated next to an N-type semiconductor?
Will the extra free electrons in the N-type area cross over into the P-type and neutralize the extra holes?
As a matter of fact, they do
However, the crossing over causes charge imbalance
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27. PN Junction P N Depletion Region − − + + − − + + − − + + − − + + − − +
Electron migration
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28. PN Junction
The N-type area that loses electron will have more positive charge, and vice versa
This will then create a voltage difference at the interface between the P- and N-type material in the polarity that is the same direction to the electrons migration from N to P
This voltage difference creates an electric field that will then prevent further migration of electrons to P-type when it reaches certain voltage value
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29. PN Junction
The electron-hole neutralized area (but with effective positive or negative charge), is called depletion region
The width of the depletion region is very small but it depends on the concentration of dopant
The voltage different across the junction is around 0.75V – depletion voltage (VD)
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30. Diode
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31. Diode By creating a PN junction we basically create a diode
Diode symbol in circuit:
Diode is mostly used in rectifier circuits (circuits that allow current to flow only in 1 direction)
The direction of current flow is the same as the direction of the triangle (explained in succeeding slides)
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32. Reverse Biasing Diode
Reverse biasing a diode means we are applying voltage across its to make the depletion region larger
This is done by applying a +ve potential to the N side of the diode and –ve potential to the P side of the diode
The +ve potential will then attract the electrons in N-type side toward the terminal, and the –ve potential will attract the holes in P-type side toward the other terminal
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33. Depletion Region Increased
Reverse Biasing Diode
Depletion Region Increased − − − + + + e- − − − + + + e- − − − + + + e- − − − + + + P N − − − + + + e- − − − + + + e- − − − + + + − − − + + + e- − − − + + +
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34. Reverse Biasing Diode
Since the holes and electrons are attracted toward the opposite terminals, the area of depletion region increased
The whole process is like the process of charging a capacitor because the electrons and the holes attracted to the terminals are like the charges that accumulated on capacitor plates
Hence, at final stage, there is no current flow through the diode – just like capacitor
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35. Forward Biasing Diode
Forward biasing a diode means we are applying voltage across its to make the depletion region smaller
This is done by applying a –ve potential to the N side of the diode and +ve potential to the P side of the diode
The –ve potential will then repel the electrons in N-type side toward the depletion region, and the +ve potential will repel the holes in P-type side toward the depletion region from the side
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36. Depletion Region Decreased
Forward Biasing Diode
Depletion Region Decreased − + e- − + e- − + e- − + P N − + e- − + e- − + − + e- − +
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37. Forward Biasing Diode
Even though the depletion region is smaller now, but there is will still no current flowing until the 0.75V voltage (barrier) due to the electron migration is offset by the external potential
Once the voltage barrier is passed, the depletion region vanishes, the current then flows with very little resistance
Hence, when current is flowing through the diode at forward biased, the diode is basically a short circuit
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38. Forward Biasing Diode P N anode cathode current flow e- e- e- e- e- e-
> 0.75V
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39. I-V Characteristics
On reverse bias voltage, there is zero current flowing
On forward bias, there will be current flowing after the 0.75V voltage barrier is overcome
At a very high reverse bias voltage, a junction breakdown will take place and current will flow in reverse direction – beyond the scope of this course
Refer to the graph in the next slide
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40. I-V Characteristics I Breakdown Reverse Bias Forward Bias V CO1
VD = 0.75V V
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41. Ideal Model I Reverse Bias Forward Bias V ON (Forward Bias)
OFF (Reverse Bias)
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42. Offset Model + − I Reverse Bias Forward Bias V ON (Forward Bias)VD − I
Reverse Bias
Forward Bias VD V
ON (Forward Bias)
OFF (Reverse Bias) + VD −
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43. Diode Circuits
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44. Half-Wave Rectifier
Rectifier is a circuit that changes AC current to DC
The process involves diodes as diodes only allows current to flow in one direction
Half-wave rectifier only allows the positive (half) part of an AC current to flow through it
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45. Half-Wave Rectifier + + vS RL vL − − vS vL t t
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46. Full-Wave Rectifier
Full-wave rectifier requires two diodes and a transformer with a center tap
The diodes still allow only one way current through them – one diode allows one half wave
The arrangement of the two diodes channels the current to flow into the load in same direction
Center tap makes the transferred power is half, but since both half wave are allowed to flow, it doubles back the power – making it the same power as the half wave
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47. Full-Wave Rectifier + vS RL − vS vL − + + vS vL t t
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48. Bridge Rectifier
Bridge rectifier is a full wave rectifier that doesn’t require a center tapped transformer
Four diodes are arranged as shown in the next slide
Only two diodes flowing current during each half wave
The two half waves are channeled through the load in the same direction making the load to experience the same voltage polarity through it
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49. Bridge Rectifier + RL vS + vL − −
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50. Bridge Rectifier RL + vS vL − + − vS vL t t
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51. Bridge Rectifier + RL vS − + vL − vS vL t t
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52. Bridge Rectifier Capacitor can be used to reduce ripple in rectifiers+ + vL vS RL − − vL
Capacitor discharge
Capacitor re-charge t
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