Unit 5

• Semiconductor devices

  Devices made from semiconductor materials, such as transistors and diodes
• The invention of the transistor or semiconductor was probably the most important development that lead to the personal computers amazing growth and what we know of as modern day computers
• Diode

  A two-terminal electronic component that only conducts current in one direction
• Ideal diode

  • Zero resistance in one direction, infinite resistance in the reverse direction
• 1. N junction diode

  A semiconductor device that comprises a single p-n junction, formed by joining p-type and n-type semiconductor materials
• 1. N junction formation

  1. Electrons from n-region diffuse across junction and combine with holes

  2. Filling a hole makes a negative ion and leaves behind a positive ion on n-side

  3. A space charge builds up, creating a depletion region
• Depletion region

  The zone around the junction that controls the behavior of the diode
• Forward biasing

  1. Drives holes to the junction from p-type material and electrons to the junction from n-type material

  2. Electrons and holes combine so a continuous current can be maintained
• Reverse biasing

  • Causes a transient current to flow as both electrons and holes are pulled away from the junction
  • When the potential formed by the widened depletion layer equals the applied voltage, the current will cease except for the small thermal current
• Zener diode

  A specially designed, highly doped PN junction diode that operates in reverse biased mode
• Zener diode working

  1. Operates like a normal diode when forward-biased

  2. When reverse-biased, a small leakage current flows until the breakdown voltage is reached, then current increases to a maximum and stabilizes
• Avalanche breakdown

  Occurs in Zener diodes with Zener voltage greater than 6V, where free electrons gain sufficient energy and accelerate at high velocities, knocking off more electrons and rapidly increasing current
• Zener breakdown

  Occurs when the applied reverse bias voltage reaches closer to the Zener voltage, the electric field in the depletion region gets strong enough to pull electrons from their valence band
• Tunnel diode

  A heavily doped p-n junction diode that shows negative resistance, where current flow decreases as voltage increases, based on the tunneling effect
• Tunneling effect

  Charge carriers can easily cross the narrow depletion region by punching through the junction, without needing kinetic energy
• Tunnel diode characteristics

  • Has a negative resistance region where it produces power instead of absorbing it
• Applications of tunnel diode

  • Switch, amplifier, oscillator, high frequency component, logic memory storage device, oscillator circuits, FM receivers
• Light Emitting Diode (LED)

  A semiconductor device that emits light when electrons and holes recombine at the p-n junction
• Direct band-gap semiconductor

  The maximum energy level of the valence band aligns with the minimum energy level of the conduction band with respect to momentum, allowing direct recombination and high efficiency
• Indirect band-gap semiconductor

  The maximum energy level of the valence band are misaligned with the minimum energy level of the conduction band with respect to momentum, requiring conservation of momentum before recombination, reducing efficiency
• Direct band-gap semiconductor

  • Gallium Arsenide (GaAs)
• Indirect band-gap semiconductors

  • Silicon, Germanium
• Indirect band-gap (IBG) semiconductor

  One in which the maximum energy level of the valence band are misaligned with the minimum energy level of the conduction band with respect to momentum
• Direct band-gap (DBG) semiconductor

  A direct recombination takes place with the release of the energy equal to the energy difference between the recombining particles
• Recombination in DBG semiconductor

  First, the momentum is conserved by release of energy and only after both the momenta align themselves, a recombination occurs accompanied with the release of energy
• DBG semiconductor

  • The efficiency factor is much more than that of an IBG semiconductor
• IBG semiconductor

  • The probability of a radiative recombination is much less in comparison to that in case of DBG semiconductors
• DBG semiconductor material

  • Gallium Arsenide (GaAs)
• IBG semiconductors

  • Silicon
  • Germanium
• DBG semiconductors are always preferred over IBG for making optical sources
• IBG semiconductors cannot be used to manufacture optical sources
• Materials used in Solar Cell

  • Silicon
  • GaAs
  • CdTe
  • CuInSe2
• Photodiode

  A semiconductor device with a P-N junction that converts photons (or light) into electrical current
• Photodiode Working

  1. Photons in the form of light affect the generation of electron-hole pairs

  2. If the energy of the falling photons (hv) is greater than the energy gap (Eg) of the semiconductor material, electron-hole pairs are created near the depletion region of the diode

  3. The electron-hole pairs created are separated from each other before recombining due to the electric field of the junction

  4. The direction of the electric field in the diode forces the electrons to move towards the n – side and consequently the holes move towards the p-side

  5. As a result of the increase in the number of electrons on the n – side and holes on the p-side, a rise in the electromotive force are observed

  6. When an external load is connected to the system, a current flow is observed through it

  7. The more the electromotive force created, the greater is the current flow

  8. The magnitude of the electromotive force created depends directly upon the intensity of the incident light
• Photodiodes generate current flow directly depending upon the light intensity received, they can be used as photodetectors to detect optical signals
• Hall Effect

  When a current-carrying conductor or a semiconductor is introduced to a perpendicular magnetic field, a voltage can be measured at the right angle to the current path
• Hall Effect applications

  • Magnetic field sensing equipment
  • Multiplier applications to provide actual multiplications
  • Phase angle measurement
  • Linear or Angular displacement transducers
  • Proximity detectors
  • Hall Effect Sensors and Probes
  • Detecting wheel speed and accordingly assist anti-lock braking system (ABS)
• Magnetoresistance

  The resistance of some of the metal and the semiconductor material varies in the presence of the magnetic field
• How Magnetoresistance works

  1. In the absence of magnetic field, the charges carriers in the material move in a straight path and electric current flows in a straight path

  2. When the magnetic field is applied to the material, the magnetic forces cause the mobile charge carriers (free electrons) to change their direction from direct path to indirect path, increasing the length of electric current path

  3. Hence, large number of free electrons collides with the atoms and loses their energy in the form of heat and only a small number of free electrons flow through the conductive path

  4. The small number of free electrons moving from one place to another place carries the electric current

  5. Therefore, the resistance of the material increases with increasing magnetic field
• Magneto Resistor - Uses and Applications

  • Electromagnetic compass
  • Magnetometers which measure magnetic field intensity and direction
  • Position sensors (angle, rotary or linear)
  • Ferrous metal detection
  • Bio sensors
  • Hard disk drives