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Today • Course overview and information 09/16/2010 © 2010 NTUST Silicon and Germanium Atoms • Two types of semiconductors are silicon (Si) and germanium (Ge) • Both the Si and Ge atoms have four valence electrons • Si has 14 protons in its nucleus and Ge has 32 Semiconductors • Semiconductors are crystalline materials that are characterized by specific energy bands for electrons. Energy • Between the bands are gaps; these gaps represent energies that Conduction band electrons cannot posses. Energy gap • The last energy band is the conduction band, where electrons are mobile. • The next to the last band is the valence band, which is the energy level associated with electrons involved in bonding. Valence band Energy gap Second band Energy gap First band Nucleus Only occurred in 0 K Atomic Bonding Atomic Bonding Electron and Hole • 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. ElectronEnergy • For every electron in the conduction band, a hole is left behind in the valence band. hole pair Conduction band Energy gap Valence band Heat energy Electron and Hole • 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 Free electron electrons move from hole-to-hole in the valence band, effectively creating “hole current” shown by gray arrows. 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 III IV V band electrons, pentavalent impurities B C N are added, forming an n-type Al Si P semiconductor. These are elements to the right of Si on the Periodic Table. Ga Ge As • To increase the number of holes, In Sn Sb trivalent impurities are added, forming a p-type semiconductor. These are elements to the left of Si on the Periodic Table. N-Type Semiconductor • To increaseSemiconductor number of free electrons in intrinsic silicon pentavalent N-Type atoms are added • These are atoms with five valence electrons • Each pentavalent atom forms covalent bonds with four adjacent silicon atoms N-Type Semiconductor • Four of a pentavalent atoms’s valence electrons are used to form the covalent bonds with silicon atoms, leaving extra electron • This extra electron becomes a free electron because it is not attached to any atom. • Since most of the current carriers are electrons, silicon doped in this way is an n-type semiconductor. • The n stands for the negative charge on an electron P-Type Semiconductor • To increase number of holes in intrinsic silicon trivalent atoms are added • These are atoms with three valence electrons • Each trivalent atom forms covalent bonds with four adjacent silicon atoms P-Type Semiconductor • Since four electrons are required, a hole is formed with each trivalent atom. • Holes can be thought of as positive charges • Since most of the current carriers are holes, silicon doped in this way is an p-type semiconductor. • The p stands for the positive charge on an electron Distinction between Conductor, Semiconductor and Insulator Diode Diode 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 pmaterial. 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 nregion p n • The barrier potential in the depletion region must be overcome R in order for the external source to cause current. For a silicon diode, + VBIAS this is about 0.7 V. • The forward-bias causes the depletion region to be narrow. - Forward Bias Forward Bias Formation of the Depletion Region Formation of the Depletion Region 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 p n insulator. A relatively few electrons R manage to diffuse across the junction, creating only a tiny + VBIAS reverse current. • The reverse-bias causes the depletion region to widen. Reverse Bias Diode Characteristics • The forward and reverse characteristics are shown on a V-I characteristic curve. IF • In the forward bias region, current increases dramatically Forward after the barrier potential (0.7 V bias V (breakdown) for Si) is reached. The voltage VR 0.7 V across the diode remains Reverse Barrier approximately equal to the bias potential barrier potential. • The reverse-biased diode IR effectively acts as an insulator until breakdown is reached. BR VF Diode Characteristic Curve Diode Characteristic Curve Diode Symbol Ideal Diode Model Practical Diode Mode Practical Diode Model Diode Models • The characteristic curve for a diode can be approximated by various models of diode behavior. The model you will use IF depends on your requirements. • The ideal model assumes the diode is either an open or closed switch. VR • The practical model includes the barrier voltage in the approximation. • The complete model includes the forward resistance of the diode. Forward bias 0.7 V Reverse bias IR VF 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. • The half-wave rectifier converts ac to pulsating dc by acting as a closed switch during the positive alteration. • The diode acts as an open switch during the negative alteration. + D RL D - + RL Half-wave Rectifier Examples • Determine the peak output voltage and the average value of the output voltage of the rectifier Vout Vin - 0.7 5 - 0.7 4.3 VAVG Vout 1.37V 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 D1 F time. Vsec 2 Vsec 2 D2 RL 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 D2 D1 D4 RL Peak Inverse Voltage Peakmust inverse voltage • Diodes 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. 0V 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. 0V Vsec Examples Example (a). Determine the peak output voltage for the bridge recitfier (b). What minimum PIV rating is required for the diodes Vout Vin n 25V PIV Vout 25V Special-Purpose Diodes 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 Selected Key Terms Majority carrier The most numerous charge carrier in a doped semiconductor material (either free electrons or holes. Minority carrier The least numerous charge carrier in a doped semiconductor material (either free electrons or holes. PN junction The boundary between n-type and p-type semiconductive materials. Diode An electronic device that permits current in only one direction. Selected Key Terms Barrier potential The inherent voltage across the depletion region of a pn junction diode. Forward bias The condition in which a diode conducts current. Reverse bias The condition in which a diode prevents current. Full-wave rectifier 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 A type of full-wave rectifier consisting of diodes arranged in a four corner configuration. Zener diode A type of diode that operates in reverse breakdown (called zener breakdown) to provide a voltage reference. Varactor A diode used as a voltage-variable capacitor. Photodiode 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. a poor conductor of electricity. b. an n-type of material. c. a p-type of material. d. 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. III IV V B C N Al Si P Ga Ge As In Sn Sb 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 6. b 2. a 7. c 3. c 8. a 4. b 9. b 5. d 10. d