1. electronics fundamentals
circuits, devices, and applications
THOMAS L. FLOYD
DAVID M. BUCHLA
chapter 16

2. Semiconductors
Semiconductors are crystalline materials that are characterized by specific energy bands for electrons.
Between the bands are gaps; these gaps represent energies that electrons cannot posses.
Energy
Conduction band
The last energy band is the conduction band, where electrons are mobile.
Energy gap
Valence band
Energy gap
The next to the last band is the valence band, which is the energy level associated with electrons involved in bonding.
Second band
Energy gap
First band
Nucleus

3. Electron and hole current
At room temperature, some electrons have enough energy to jump into the conduction band.
After jumping the gap, these electrons are free to drift throughout the material and form electron current when a voltage is applied.
Electron-hole pair
Energy
For every electron in the conduction band, a hole is left behind in the valence band.
Conduction band
Heat energy
Energy gap
Valence band

4. Electron and hole current
The electrons in the conduction band and the holes in the valence band are the charge carriers. In other words, current in the conduction band is by electrons; current in the valence band is by holes.
When an electron jumps to the conduction band, valence electrons move from hole-to-hole in the valence band, effectively creating “hole current” shown by gray arrows.
Free electron Si Si Si

5. Impurities
By adding certain impurities to pure (intrinsic) silicon, more holes or more electrons can be produced within the crystal.
To increase the number of conduction band electrons, pentavalent impurities are added, forming an n-type semiconductor. These are elements to the right of Si on the Periodic Table.
To increase the number of holes, trivalent impurities are added, forming a p-type semiconductor. These are elements to the left of Si on the Periodic Table.

6. The pn junction diode
When a pn junction is formed, electrons in the n-material diffuse across the junction and recombine with holes in the p-material. This action continues until the voltage of the barrier repels further diffusion. Further diffusion across the barrier requires the application of a voltage.
The pn junction is basically a diode, which is a device that allows current in only one direction. A few typical diodes are shown.

7. Forward bias
When a pn junction is forward-biased, current is permitted. The bias voltage pushes conduction-band electrons in the n-region and holes in the p-region toward the junction where they combine.
p-region
n-region
The barrier potential in the depletion region must be overcome in order for the external source to cause current. For a silicon diode, this is about 0.7 V. p n R + -
VBIAS
The forward-bias causes the depletion region to be narrow.

8. Reverse bias
When a pn junction is reverse-biased, the bias voltage moves conduction-band electrons and holes away from the junction, so current is prevented.
p-region
n-region
The diode effectively acts as an insulator. A relatively few electrons manage to diffuse across the junction, creating only a tiny reverse current. p n R - +
VBIAS
The reverse-bias causes the depletion region to widen.

9. Diode characteristics
The forward and reverse characteristics are shown on a V-I characteristic curve. IF
In the forward bias region, current increases dramatically after the barrier potential (0.7 V for Si) is reached. The voltage across the diode remains approximately equal to the barrier potential.
Forward bias
VBR (breakdown) VR VF
0.7 V
Barrier potential
Reverse bias
The reverse-biased diode effectively acts as an insulator until breakdown is reached. IR

10. Diode models
The characteristic curve for a diode can be approximated by various models of diode behavior. The model you will use depends on your requirements. IF
The ideal model assumes the diode is either an open or closed switch.
Forward bias
The practical model includes the barrier voltage in the approximation. VR VF
0.7 V
Reverse bias
The complete model includes the forward resistance of the diode. IR

11. Half-wave Rectifier
Rectifiers are circuits that convert ac to dc. Special diodes, called rectifier diodes, are designed to handle the higher current requirements in these circuits. D
The half-wave rectifier converts ac to pulsating dc by acting as a closed switch during the positive alteration. RL D
- +
The diode acts as an open switch during the negative alteration. RL

12. Full-wave Rectifier
The full-wave rectifier allows unidirectional current on both alterations of the input. The center-tapped full-wave rectifier uses two diodes and a center-tapped transformer.
The ac on each side of the center-tap is ½ of the total secondary voltage. Only one diode will be biased on at a time. F D1
Vsec 2
Vsec 2 RL D2

13. Bridge Rectifier
The bridge rectifier is a type of full-wave circuit that uses four diodes. The bridge rectifier does not require a center-tapped transformer.
At any instant, two of the diodes are conducting and two are off. F D3 D1 RL D2 D4

14. Peak inverse voltage
Diodes must be able to withstand a reverse voltage when they are reverse biased. This is called the peak inverse voltage (PIV). The PIV depends on the type of rectifier circuit and the maximum secondary voltage.
For example, in a full-wave circuit, if one diode is conducting (assuming 0 V drop), the other diode has the secondary voltage across it as you can see from applying KVL around the green path.
Notice that Vp(sec) = 2Vp(out) for the full-wave circuit because the output is referenced to the center tap.
0 V
Vsec

15. Peak inverse voltage
For the bridge rectifier, KVL can be applied to a loop that includes two of the diodes. Assume the top diode is conducting (ideally, 0 V) and the lower diode is off. The secondary voltage will appear across the non-conducting diode in the loop.
Notice that Vp(sec) = Vp(out) for the bridge because the output is across the entire secondary.
0 V
Vsec

16. Power supplies
By adding a filter and regulator to the basic rectifier, a basic power supply is formed.
Typically, a large electrolytic capacitor is used as a filter before the regulator, with a smaller one following the regulator to complete filtering action.
IC regulator F D3 D1
7805 D2 D4 C1 C2
1000 mF
1 mF

17. Special-purpose diodes
Special purpose diodes include
Zener diodes – used for establishing a reference voltage
Varactor diodes – used as variable capacitors
Light-emitting diodes – used in displays
Photodiodes – used as light sensors

18. Example: Troubleshooting power supplies
Begin troubleshooting by analyzing the symptoms and how it failed. Try to focus on the most likely causes of failure.
Example:
A power supply has no output, but was working until a newly manufactured PC board was connected to it. (a) Analyze possible failures. (b) Form a plan for troubleshooting.
IC regulator F D3 D1
7805 D2 D4 C1 C2
1000 mF
1 mF

19. Troubleshooting power supplies
Analysis:
The supply had been working, so the problem is not likely to be an incorrect part or wiring problem. The failure was linked to the fact that a new PC board was connected to it, which points to a possible overloading problem. If the load was too much for the supply, it is likely a fuse would have blown, or a part would likely have overheated, accounting for the lack of output.
IC regulator F D3 D1
7805 D2 D4 C1 C2
1000 mF
1 mF

20. Troubleshooting power supplies
Based on the analysis, a sample plan is as follows. (It can be modified as circumstances warrant.)
Planning:
1. Disconnect power and check the fuse. If it is bad, replace it. Before reapplying power, remove the load, open the power supply case, and look for evidence of overheating (such as discolored parts or boards). If no evidence of overheating proceed.
2. Check the new pc board (the load) for a short or overloading of the power supply that would cause the fuse to blow. Look for evidence of overheating.
3. Verify operation of the supply with measurements (see next slide).

21. Troubleshooting power supplies
Measurements:
The analysis showed that a likely cause of failure was due to an overload. For the measurement step, it may be as simple as replacing the fuse and confirming that the supply works. After replacing the fuse:
Reapply power to the supply but with no load. If the output is okay, put a resistive test load on the power supply and measure the output to verify it is operational. If the output is correct, the problem is probably with the new pc board. If not, you will need to further refine the analysis and plan, looking for an internal problem.

22. Selected Key Terms Majority carrier
Minority carrier
PN junction
Diode
The most numerous charge carrier in a doped semiconductor material (either free electrons or holes.
The least numerous charge carrier in a doped semiconductor material (either free electrons or holes.
The boundary between n-type and p-type semiconductive materials.
An electronic device that permits current in only one direction.

23. Selected Key Terms Barrier potential
Forward bias
Reverse bias
Full-wave rectifier
The inherent voltage across the depletion region of a pn junction diode.
The condition in which a diode conducts current.
The condition in which a diode prevents current.
A circuit that converts an alternating sine-wave into a pulsating dc consisting of both halves of a sine wave for each input cycle.

24. Selected Key Terms Bridge rectifier
Zener diode
Varactor
Photodiode
A type of full-wave rectifier consisting of diodes arranged in a four corner configuration.
A type of diode that operates in reverse breakdown (called zener breakdown) to provide a voltage reference.
A diode used as a voltage-variable capacitor.
A diode whose reverse resistance changes with incident light.

25. Quiz
1. An energy level in a semiconductor crystal in which electrons are mobile is called the
a. barrier potential.
b. energy band.
c. conduction band.
d. valence band.

26. Quiz 2. A intrinsic silicon crystal is
a poor conductor of electricity.
an n-type of material.
a p-type of material.
an excellent conductor of electricity.

27. Quiz
3. A small portion of the Periodic Table is shown. The elements highlighted in yellow are
a. majority carriers.
b. minority carriers.
c. trivalent elements.
d. pentavalent elements.

28. Quiz 4. At room temperature, free electrons in a p-material
a. are the majority carrier.
b. are the minority carrier.
c. are in the valence band.
d. do not exist.

29. Quiz 5. The breakdown voltage for a silicon diode is reached when
a. the forward bias is 0.7 V.
b. the forward current is greater than 1 A.
c. the reverse bias is 0.7 V.
d. none of the above.

30. Quiz 6. The circuit shown is a a. half-wave rectifier.
b. full-wave rectifier.
c. bridge rectifier.
d. zener regulator.

31. Quiz 7. PIV stands for a. Positive Ion Value.
b. Programmable Input Varactor.
c. Peak Inverse Voltage.
d. Primary Input Voltage.

32. Quiz 8. A type of diode used a a voltage-variable capacitor is a
a. varactor.
b. zener.
c. rectifier.
d. LED.

33. Quiz
9. If one of the four diodes in a bridge rectifier is open, the output will
a. be zero.
b. have ½ as many pulses as normal.
c. have ¼ as many pulses as normal.
d. be unaffected.

34. Quiz
10. When troubleshooting a power supply that has a bridge rectifier, begin by
a. replacing the bridge rectifier.
b. replacing the transformer.
c. making measurements.
d. analyzing the symptoms and how it failed.

35. Quiz
Answers:
1. c
2. a
3. c
4. b
5. d
6. b
7. c
8. a
9. b
10. d