09/16/2010© 2010 NTUST Today Course overview and information

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 Silicon and Germanium Atoms

Energy Band

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

Atomic Bonding

Electron-Hole Pair

At room temperature, some electrons have enough energy to jump into the conduction band. Valence band Conduction band Energy gap Energy After jumping the gap, these electrons are free to drift throughout the material and form electron current when a voltage is applied. Heat energy Electron- hole pair For every electron in the conduction band, a hole is left behind in the valence band. 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 electrons move from hole-to-hole in the valence band, effectively creating “hole current” shown by gray arrows. Si Free electron Electron and Hole

By adding certain impurities to pure (intrinsic) silicon, more holes or more electrons can be produced within the crystal. 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. 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. Impurities

Doping

N-Type Semiconductor To increase number of free electrons in intrinsic silicon pentavalent 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 N-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 P-Type Semiconductor

Distinction between Conductor, Semiconductor and Insulator

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. The PN Junction Diode

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. 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-regionn- region pn +  R V BIAS The forward-bias causes the depletion region to be narrow. Forward Bias

Formation of the Depletion Region

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

The forward and reverse characteristics are shown on a V-I characteristic curve. 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. VRVR VFVF IFIF IRIR Reverse bias Forward bias 0.7 V Barrier potential The reverse-biased diode effectively acts as an insulator until breakdown is reached. V BR (breakdown) Diode Characteristics

Diode Characteristic Curve

Diode Symbol

Ideal Diode Model

Practical Diode Model Practical Diode Mode

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

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 D RLRL RLRL +   + Half-wave Rectifier

Determine the peak output voltage and the average value of the output voltage of the rectifier Examples

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. F D1D1 D2D2 RLRL V sec 2 2 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. Full-wave 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. F D1D1 D2D2 RLRL At any instant, two of the diodes are conducting and two are off. D3D3 D4D4 Bridge Rectifier

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. 0 V V sec Notice that V p(sec) = 2V p(out) for the full-wave circuit because the output is referenced to the center tap. 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. 0 V Notice that V p(sec) = V p(out) for the bridge because the output is across the entire secondary. V sec Peak Inverse Voltage

Example (a). Determine the peak output voltage for the bridge recitfier (b). What minimum PIV rating is required for the diodes Examples

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 Special-purpose Diodes

Majority carrier Minority carrier PN junction Diode The most 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. The least numerous charge carrier in a doped semiconductor material (either free electrons or holes. Selected Key Terms

Barrier potential Forward bias Reverse bias 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. The condition in which a diode conducts current. The inherent voltage across the depletion region of a pn junction diode. The condition in which a diode prevents current. Selected Key Terms

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

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. 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 Quiz