Electronics devices and circuits class notes (Semiconductor)

Semiconductors

 

 

N-Type semiconductors

 

n (majority carrier concentration in no of electron in CB/volume) 

p (minority carrier concentration in no of holes in VB/volume)

\({ N }_{ D }\) (donor concentration per unit volume)

 

P-Type semiconductors

 

p (majority carrier concentration in no of holes in VB/volume) 

n (minority carrier concentration in no of electrons in CB/volume)

\({ N }_{ A }\) (donor concentration per unit volume)

 

NOTE –  CB (conduction band)

VB (valance band)

 

 

Mass Action Law

 

Mass action law states that Under thermal equilibrium the product of the free electron concentration and the free hole concentration is equal to a constant equal to the square of intrinsic carrier concentration.

It is also known as low of conservation of charges

so mass action law is \(n.p={ n }_{ i }^{ 2 }\)  —————(1)

 

 

Charge Neutrality Equation  

 

Charge neutrality occurs when all the charge in a volume adds to zero, it is neutral, neither positive or negative.

no of +ve charge = no of -ve charge

\(n+{ N }_{ A }=p+{ N }_{ D }\)   —————(2)

 

Using mass action law and charge neutrality equation we can find the minority charge carrier concentration

 

For n-type semiconductor (n>>p)

 

so using (2) equation   \(n\quad \simeq \quad { N }_{ D }\)

\({ N }_{ A }=0\)

Now by using mass action law

\(p\simeq \frac { { n }_{ i }^{ 2 } }{ { N }_{ D } } \)

 

For p-type semiconductor (p>>n)

 

so using (2) equation \(p\quad \simeq \quad { N }_{ A }\)

\({ N }_{ D}=0\)

Now by using mass action law

\(n\simeq \frac { { n }_{ i }^{ 2 } }{ { N }_{ A} } \)

 

Current component in semiconductor

 

Basically there are two types of current component in semiconductors

  1.  Drift current density
  2.  Diffusion current density

 

Drift current density

 

  • Drift current density is due to free charges or potential gradient

\(E=-\frac { { dv } }{ dx } \)

  • It is also known as conduction current density

 

Diffusion current density

 

  • Diffusion current density is due to presence of concentration gradient
  • It is not present in metal or conductors
  • It is only present in semiconductors

 

 

we will continue this topic on our upcoming post keep visiting

 

 

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June 18, 2018

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