# 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)

[latex]{ N }_{ D }[/latex] (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)

[latex]{ N }_{ A }[/latex] (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 [latex]n.p={ n }_{ i }^{ 2 }[/latex] —————(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

[latex]n+{ N }_{ A }=p+{ N }_{ D }[/latex] —————(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 [latex]n\quad \simeq \quad { N }_{ D }[/latex]

[latex]{ N }_{ A }=0[/latex]

Now by using mass action law

[latex]p\simeq \frac { { n }_{ i }^{ 2 } }{ { N }_{ D } } [/latex]

### For p-type semiconductor (p>>n)

so using (2) equation [latex]p\quad \simeq \quad { N }_{ A }[/latex]

[latex]{ N }_{ D}=0[/latex]

Now by using mass action law

[latex]n\simeq \frac { { n }_{ i }^{ 2 } }{ { N }_{ A} } [/latex]

## Current component in semiconductor

Basically there are two types of current component in semiconductors

- Drift current density
- Diffusion current density

### Drift current density

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

[latex]E=-\frac { { dv } }{ dx } [/latex]

- 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

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