semi conductors

• Diffusion of charges

  1. Electrons from N-side diffuse to P-side

  2. Holes from P-side diffuse to N-side

  3. Diffusion current produced
• Diffusion current

  Caused by motion of holes from P-side to N-side and electrons from N-side to P-side
• Formation of barrier electric field

  1. Positively charged dopant atoms on N-side

  2. Negatively charged dopant atoms on P-side

  3. Potential difference between N-side and P-side

  4. Barrier electric field formed
• Barrier electric field

  Resists diffusion current, promotes drift current of electrons to N-side and holes to P-side
• When diffusion current = drift current, the diode is ready to be used
• There e and holes produced due to voltic, emission under the influence of the strong existing in the depletion region holer mone towards the poside and electrone move towards the N-side.
• The process of breakdown of a diode is imevaible Breakdown In a highly doped diode Zener Breakdown In a lightly doped diode Avalanche Breakdown
• Breakdown of a diode

  1. Zener Breakdown (highly doped diode)

  2. Avalanche Breakdown (lightly doped diode)
• VI characteristics of a diode

  Relating the voltage applied across the diode with the current procedure in/across the diode
• As the forward voltage (Uforward) increases


  The forward current (Iforward) increases very small initially, but after a certain value it increases heavily
• Reverse current (Ireverse)

  It will be very small, in the range of microamperes (μA)
• Zener Breakdown happens easily in a Zener diode compared to Avalanche Breakdown
• In a Zener diode, the Ebarrier (strong) already exists in the PN Junction even in the unbiased state
• In a Zener diode, even a small reverse voltage (VR) will be enough to breakdown the diode
• Zener Diode

  Shows an irreversible breakdown at a certain potential (Zener Breakdown potential, UB)
• Zener diode works as a voltage regulator; the potential across it will be constant at VB
• Solar cell

  Similar working to a photodiode, but the diode is unbiased
• To facilitate photons to fall in the depletion region and produce e-h pairs, the n-side of the solar cell is made extremely thin
• Suitable materials for solar cells

  • Eg should be close to 1.5ev
  • High optical absorption
  • High electrical conductivity
• Light Emitting Diode (LED)


  A highly doped diode working in the forward biased state, producing light energy (photons) from electrical energy
• As the forward current (Iforward) increases


  The intensity of light produced increases, but only to a certain extent
• Frequency of photons released

  Depends on the Energy Gap (Eg) of the semiconductor material
• Rectifier

  A device that converts AC to DC
• Half wave rectifier

  1. Diode is forward biased for half the cycle, giving output

  2. Diode is reverse biased for half the cycle, no output
• Full wave rectifier


  Both diodes contribute to the output during the full cycle
• To connect the input with the output, a transformer is used in a rectifier circuit