Electronic devices communication systems

In this particular article Electronic devices communication systems, we are going to discuss about various important topics of physics like energy band in solids, classification of solids. Classification of semiconductor, p-n junction, diffusion current, drift current, biasing in p-n junction, I-V characteristics, diode as a Rectifier, full wave Rectifier, half wave Rectifier etc.

Energy Bands in Solids

  • Range of energy possessed by electron in a solid is known as energy bands.
  • There are two types of energy band.
  1. Valence band: Range of energy possessed by valence electron is known as valence band. These electrons are bounded and not responsible for flow of current.
  2. Conduction band: Range of energy possessed by free electron is known as conduction band. These electrons are responsible for flow of current.

Forbidden energy gap

It is the energy gap between the bottom of the conduction band and top of the valence band. No electron exist in this gap.

 

  1. The Width of forbidden energy gap depends on the substance and its nature.
  2. If width is more then valence electron are strongly attached with nucleus.
  3. With the rise in temperature, forbidden energy gap decreases continuously(very slightly)

According to energy band theory there are three types of solids,

  1. Insulators
  2. Conductor
  3. Semiconductor

Classification of Semiconductor

  1. A pure state of semiconductor is a type of intrinsic semiconductor or called intrinsic semiconductor.
  2. A semiconductor doped with suitable impurity to increase its conductivity, is called extrinsic semiconductor.
  3. On the basis of doping impurity in a semiconductor, extrinsic semiconductor can be devided into two ways-
  • n-type semiconductor
  • P-type semiconductor

n-type semiconductor: Extrinsic semiconductor doped with pentavalent impurity like As, Sb, Bi, etc. In which negatively charged electrons works as charge carriers, is called n-type semiconductor. Every pentavalent impurity atom have an excess electron in it. That remains un-bonded. Pentavalent atom donate one electron in the crystal, therefore it is called a donor atom.

P type semiconductor: Extrinsic semiconductor doped with trivalent impurity like Al, B etc. In which positively charged holes is work as charge conductor, is called p-type semiconductor. Every trivalent impurity atom have a tendency to accept one electron. Hence consequently it is called an acceptor atom.

  • In a doped semiconductor nenh = ni² where ne and nh are the number density of electrons and holes and ni is number density of intrinsic carriers, i.e., electrons or holes.
  • Electrical conductivity of extrinsic semiconductor is given by σ = 1/ρ

Where ρ is resistivity.

p-n junction
  • It is a basic unit of all semiconductor devices.
  • p(n) region has majority carrier holes (electrons) and minority carrier electrons (holes).
  • The layer of immobile positive and negative ions, which have no free electrons And holes. This is called depletion layer.
  • Diffusion current: Because of high carrier concentration difference, holes from p-side try to diffuse n-side, produces diffusion current.
  • Drift current: Because of barrier potential, minority charge carriers force to move and such movement produces drift current.
Biasing of p-n junction

Unbiased

Forward biased

  • Effective barrier potential decreases
  • Depletion width decreases
  • Low resistance offered at junction
  • High current flows through the circuit.

Reverse biased

Electronic devices communication systems

  • Effective barrier potential increases
  • Depletion width in increases
  • High resistance offered at junction
  • Low current flows through the circuit.
I-V characteristics

1. Current flowing through the diode,

I = Io [ eˆeV/nkT -1 ]

Where Io is reverse saturation current and V is potential difference across the diode,

n = constant = { 1 for Ge and 2 for Si

2. Dynamics resistance of p-n junction, r = ΔV/ΔI

3. Static or dc resistance r = Vz/I

4. For an ideal diode,

  • In forward bias
  • In reverse bias
Diode as a Rectifier

It is a device which is used for converting alternating current into direct current. There are two types of rectifiers.

Half wave Rectifier

1. Output frequency = Input frequency

2. Peak current through load

Im = ε / (rf + R)rf = forward resistance of the diode; R= load resistance ; ε = peak of ac voltage.

3. Ripple factor = rms value of ac component/ dc dc component in the rectifier circuit = 1.21

4. Efficiency = dc power of half wave rectifier/ ac power supplied to the rectifier = 40.6%

Full wave Rectifier

1. Peak current through the load

Im = ε/ rf + R

2. Ripple factor = Iac/ Idc = 0.48

3. Efficiency = Pdc/ Pac = 81.2%

(Double the efficiency of the half wave rectifier)

4. Output frequency = 2× Input frequency

5. The pulsing dc obtained in both the rectifiers can be further rectified by using filter circuits.

Conclusion

In this article we have covered many important information of important topics like diodes, types of solids, pn junction etc in easiest way possible.

 

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