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

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

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

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

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.

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.

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.

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.

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

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

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

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

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

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

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

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

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

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

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

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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

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

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.

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.

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