Electrical devices

Electrical devices

  • Fermi energy is the maximum possible energy possessed by free electrons of a material at absolute zero temperature. The value of Fermi energy for different materials is different.
  • Fermi level in semiconductor. It is that energy level in energy band diagram of semiconductor for which the probability of occupancy becomes half.
  • The electric conduction in semiconductors is caused by the motion of the electrons in the conduction band and also by the motion of the holes in the valence band. Electrons are majority carriers and holes are minority carriers.
  • N-type doping: when we add pentavalent impurity in a pure semiconductor of Ge or Si, the Fermi level in energy gap shifts very close to conduction band.
  • P-type doping: when we add trivalent impurity in a pure semiconductor of Ge or Si, the Fermi level in energy gap shits very close to valence band.
  • The band separating the valence band and conduction band is called forbidden energy gap.
  • The electrical conductivity fo a metal depends upon the number of free electrons and their drift velocity through the metal on applying the field.
  • The conductivity of semiconductor increases with rise of temperature because more covalent bonds in semiconductor get broken with rise of temperature, providing¬† more number of electrons and holes as through the metal on applying the field.
  • Two important processes occur during the formation of a pn junction: diffusion and drift.
  • Knee voltage: it is the forward voltage beyond which the current through the junction starts increasing rapidly with voltage, showing the linear variation. But below the knee voltage the variation is non-linear.
  • A junction diode cannot work as an amplifier i.e. it cannot increase the amplitude of signal current, voltage or power.
  • Zener diode is design specially to operate in the reverse breakdown voltage region continuously without being damage. It is work as voltage regulator.
  • Photodiode is an optoelectronic device in which current carriers are generated by photons through photo excitation i.e. photoconduction by light.
  • LED converts electrical signal into light signals.
  • Solar cell is a p-n junction device which converts solar energy into electrical energy.
  • A junction transistor is a semiconductor device having two junctions and three terminals.
  • Transistor as a switch. Transistor as a common base amplifier. Transistor as a oscillator.
  • Amplifier is a device which is used for increasing the amplitude of variation of alternating voltage or current or power.
  • 7V is the value of potential barrier of pn junction made of silicon semiconductor.
  • The width of depletion layer of a p-n junction decreases with the decrease in reverse bias.
  • Zener diode is used to regulate the output of an unregulated dc supply.
  • In a transistor base is thin doped and is thin layer, so that it may contain smaller number of majority carriers. In a transistor emitter is always forward biased.
  • Positive feedback is required in an oscillator.
  • Communication is the act of transmission and reception of information. Three essential elements: transmitter, communication channel and receiver.
  • A transducer converts the message signal into electrical form suitable for transmission through the channel.
  • Two basic modes of communication: (a) point to point; (b) Broadcast.
  • The undesirable effects in the course of signal transmission are: attenuation (decrease in signal strength due to energy losses), distortion, interference and noise.
  • Bandwidth refers usually to the range of frequencies over which the communication system works.
  • Modulation is the phenomenon of superimposing the low frequency message signal (called the modulating signal) on a high frequency wave (called the carrier wave). The resulting wave is called the modulated wave.
  • For an AM radio station, band width is generally 10khz.
  • Modulation index determines the strength and quality of the transmitted signal.
  • A modem is a device that modulates and demodulates. It connects one computer to another across ordinary telephone lines.
  • Radio waves are EM waves of frequency ranging from 500khz to about 1000Mhz.
  • Sky wave propagation is a mode of wave propagation in which the radio-waves emitted from the transmitter antenna reach the receiving antenna after reflection by the ionosphere. Radio waves are called sky wave propagation and it is also known as ionospheric propagation.
  • Critical frequency is the highest frequency of radio wave, which when sent straight towards the layer of ionosphere gets reflected from ionosphere and returns to the earth.
  • The frequency range for commercial FM radio broadcasting is 88Mhz to 108 Mhz.
  • Micro waves are the EM waves of frequency in the range of 1 Ghz to 300Ghz, greater than those of TV signals (54Mhz to 890 Mhz). Microwave propagation is called line of sight propagation.
  • Satellite communication is a mode of signal between transmitter and receiver through satellite.
  • It is necessary to use satellites for long distance TV transmission because TV signals being of high frequency are not reflected by the ionosphere.

Leave a Reply

Your email address will not be published. Required fields are marked *