Electronics, Power Electronics, Optoelectronics, Microwaves ...

1 -24 Electronics, Power Electronics, Optoelectronics, Microwaves, Electromagnetics, and Radarwhich is compensated with aseries-connected forward-biased diode with anegative T C .The disadvantage ofZener diodes is arelatively large electronic noise.Varactor DiodesThe varactor diode is an ordinarypn-diode that uses the voltage-dependent variable capacitanceofthe diode. Thevaractor diode is widely used as avoltage-dependent capacitor in electronically tuned radio receivers and in TV.Tunnel DiodesThe tunnel diode is an ordinary pn-junction diode with very heavy doped nand pregions. Because thejunction is very thin, atunnel effect takes place. An electron can tunnel through the thin depletion layer fromthe conduction band of the nregion directly into the valence band of the pregion. Tunnel diodes create anegative differential resistance in the forward direction due to the tunnel effect. Tunnel diodes are used asmixers, oscillators, amplifiers, and detectors. They operate at very high frequencies in the gigahertz bands.Photodiodes and Solar CellsPhotodiodes are ordinary pn-diodes that generate hole–electron pairs when exposed to light. Aphotocurrentflows across the junction, if the diode is reverse biased. Silicon pn-junctions are used to sense light at nearinfraredand visible spectra around 0.9 m m. Other materials are used for different spectra.Solar cells utilize the pn-junction to convert light energy into electrical energy. Hole–electron pairs aregenerated in the semiconductor material by light photons. The carriers are separated by the high electric fieldin the depletion region across the pn-junction.The electric field forces the holes into the pregion and theelectrons into the nregion. This displacement of mobile charges creates avoltage difference between the twosemiconductor regions. Electric power is generated in an external load connected between the terminals to thepand nregions. The conversion efficiency is relatively low, around 10 to 12%. With the use of new materials,an efficiency of about 30% has been reported. Efficiency up to 45% was achievedbyusing monochromatic light.Schottky Barrier DiodeThe Schottky barrier diode is ametal-semiconductor diode. Majority carriers carry the electric current. Nominority carrier injection takes place. When the diode is forward biased, carriers are injected into the metal,where they reside as majority carriers at an energy level that is higher than the Fermi level inmetals. The I–Vcharacteristic is similar to conventional diodes. The barrier voltage is small, about 0.2 Vfor silicon. Since nominoritycarrier charge exists, the Schottky barrier diodes are very fast. They are used in high-speed electroniccircuitry.Defining TermsAcceptor: Ionized, negative-charged immobile dopant atom (ion) in ap-type semiconductor after therelease of ahole.Avalanche breakdown: In the reverse-biased diode, hole–electron pairs are generated in the depletionregion by ionization, thus by the lattice collision with energetic electrons and holes.Bandgap energy: Energydifference between the conduction band and the valence band in asemiconductor.Barrier voltage: Avoltage which develops across the junction due to uncovered immobile ions on bothsides of the junction. Ions are uncovered due to the diffusion of mobile carriers across the junction.Boltzmann relation: Relates the density of particles in one region to that in an adjacent region, with thepotential energy between both regions.Carrier lifetime: Time an injected minority carrier travels beforeits recombination with amajoritycarrier.Concentration gradient: Difference in carrier concentration.