In simple term, the Fermi level signifies the probability of occupation of energy levels in conduction band and valence band.

In extrinsic semiconductors, the number of electrons in the conduction band and the number of holes in the valence band are not equal.

Thus, the probability of occupation of energy levels in the conduction band and valence band is not equal. Therefore, the Fermi level for the n-type semiconductor lies close to the conduction band and for p-type semiconductors, it lies close to the valence band. You can see the following example, this is the example of the n-type semiconductor, where Fermi level lies close to the conduction band,

n-type:

E_Cn—————————————                                                

E_Fn______________________(Fermi Level)

E_Vn_______________________

In p type semiconductor, it lies close to valence band because holes are majority.

p type:

E_Cp—————————————                                                

E_Fp______________________(Fermi Level)

E_Vp_______________________

This is the concept of Fermi level in semiconductors.

In case of any doubt, please mention in the comment section.

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In the full wave rectifier, two PN junction Diodes (say D₁ and D₂) a connected in the circuit. A load resistance (R_L) is connected in the circuit across which the output is taken.

            Let at any instant S₁ is positive and S₂ is negative. the diode (D₁) is forward biased and diode (D₂) is reversed biased so D₁ will conduct but D₂ will not.

            In the next half cycle, D₁ will be reversed biased whereas D₂ will be forward biased so the out put will be due to D₂ only. This process goes on and it must be noted the current through the load resistance flows in the same direction and ultimately a constant output is obtained which is a uni-directional pulsating d.c.

This is how a full wave rectifier operates.

Also read: Operation of half wave rectifier.

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A rectifier is a device that converts alternating current into direct current.

Half wave rectifier:

In this rectifier, the PN junction diode is connected to an input AC supply with the help of a primary coil, which will induce an emf in the secondary coil and ultimately the output is taken across the load resistance (R_L).

Let at any instant S₁ is positive and S₂ is negative. Due to this polarity the junction diode is forward biased so it will start conducting and output is obtained.

In the next half cycle, S₁ will be –Ve and S₂ will be +Ve. So, the diode will now be in the reversed biased position. So, it will not conduct and hence no output is taken.

In the similar pattern the output is obtained for that half-cycle in which the diode is forward biased so ultimately the output is a disturbed unidirectional pulsating d.c. but this discrepancy, we use a full wave rectifier.

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In a PN junction diode holes are the majority carriers on P side whereas electrons are the majority carriers on n-side. The process of diffusion takes place due to which the majority carriers diffuse from one region to the other so the P-region becomes less positive and the n-region becomes less negative.

                        An imaginary battery is developed across the junction which prevents further movement of majority carriers and the voltage so developed is known as the potential barrier. The layer is known as the depletion layer.

                        A PN junction Diode is said to be reverse biased when P-side is connected to the –Ve terminal and n-side is connected to the +Ve terminal of the battery.

                        In this biasing, the majority carriers are pulled away from the junction and hence no current flows through the junction due to majority carriers. The current due to minority carriers is present but it is of very small value of the order of mA.

                        This reverse current has a very small value at the start but it shoots up at a particular voltage known as the break down voltage. The cause of this rise is that the break down voltage, large no. of covalent bonds break up due to which a large number of charge carriers are generated (Avalanche Breakdown). At this breakdown there is an increase in the value of current as shown.

This is how the depletion layer or potential barrier is formed in the pn junction diode.

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 The combination of the p-type material or semi-conductor with the n-type semi-conductor results in a PN junction Diode.

 Biasing: The biasing of a diode means to make junction operative. In other words biasing means to connect an external to pn junction diode. The battery can be connected by two methods, one is known as forward biasing and second is known as reverse biasing. Let us discuss them one by one:

1.         Forward Biasing

In case of forward biasing, the P side of the pn junction is connected to the positive terminal and N-side to the negative terminal of the battery or voltage source. When the voltage increases, then the depletion layer starts decreasing (depletion layer will be discussed in next article, here I can just say that it means that layer which is depleted from charge carriers).

2. Reverse Biasing

In case of reverse biasing, the P side of the pn junction is connected to the negative terminal and N-side to the positive terminal of the battery or voltage source.

This is the difference between forward biasing and reverse biasing.

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Last time we have discussed the conductivity of semiconductor. Let us today derive the expression the expression for conductivity of semiconductor.

In a Semi-Conductor of length (ℓ) and area of Cross-Section. Let n_e and n_h be the no. of electrons and holes with drift velocities V_e and V_h respectively.

So, the total current in the semi-conductor will be the sum of current due to electrons as well as holes,

i.e.        I = I_e + I_h                                                             (i)

As we know that

I = n_e A V_d

\I_e = n_e e A V_e

And I_h = n_h e A V_h

I = n_e e A V_e + n_h e A V_h

I = eA[n_e  V_e + n_h v_h]

                   I/A = e[n_e  V_e + n_h v_h]                  (2)

As we know that

E = V/l      (in magnitude)

Also, R = ρl/A

Or ρ = RA/l

Where ρ is resistivity and R is resistance

Or E/ρ = (V/l)/ ρ

Or E/ρ = (V/l)l/ RA

E/ρ = (V/RA)

E/ρ = I/A                         (3)

Put (3) in (2) we get,

E/ρ = e[n_e  V_e + n_h v_h]

I/ò ρ = e / E[n_e  V_e + n_h v_h]

σ = e [n_e  V_e/E + n_h v_h/E]                  (σ = Conductivity)

Here, mobility (m) = Drift vel./ Electric field

This is the derivation and expression for the conductivity of a semiconductor.

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1. By doping: The addition of impurity atoms to a pure-semi-conductor is known as doping. The doping is done to increase the conductivity of a pure semi-conductor. It must be noted that the impurity atom is about 1 in every 10¹⁰ atoms.

2. By increase in temperature: In case of a Semi-Conductor there is an energy gap between the valence band and the conduction band but at OK no electron has sufficient energy to cover this gap. So, when temperature of Semi-Conductor is increased, covalent bands may break up and some of the electrons may acquire sufficient energy to cover this gap thereby increasing the conductivity. Whereas in case of metals, with the increase in temperature the collisions increase due to which there is more hindrance to the flow of electrons which will ultimately decrease the conductivity.

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Hole in a semiconductor:

In a semi-conductor the energy gap is of the order of 1eV. At OK, the semi-conductor behaves like an insulator but at room temp the electrons acquire sufficient energy to jump to the conductor band thereby creating a vacancy in the valence band.

            This deficiency or vacancy of electron in the valence band is termed as a hole. It is equivalent to 1 unit of +Ve charge. To fill this vacancy, the nearly electrons present in the valence band jumps to that position. So another hole is created and thus it appears as if the holes are moving. So, in R semi-conductor, electrons move in the conduction band whereas holes in the valence band.

Why doping is done in semiconductors?

The addition of impurity atoms to a pure-semi-conductor is known as doping. The doping is done to increase the conductivity of a pure semi-conductor. It must be noted that the impurity atom is about 1 in every 10¹⁰ atoms.

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n-type Semi-Conductor

It is an extrinsic semi-conductor which is obtained by doping the impurity atoms of Vth group of the periodic table to the pure Ge and Si semi-conductor.

The impurity atoms added, provide extra electrons in the structure and are called donor atoms.

The electrons are majority carriers and holes are minority carriers.

The electrons density (n_e) is much greater than the hole density (n_h) i.e.>> n_h.

The donor energy level is close to the conduction band and for away from the valence band.

The Fermi-energy level lies in between the donor energy level and conduction band.

P-type Semi-Conductor

It is an extrinsic semi-conductor which is obtained by doping the impurity atoms of III group of the periodic table to the pure Ge and Si semi-conductor.

The impurity atoms added, create vaccines of electrons (i.e. holes) in the structure and are called acceptor atoms.

The holes are majority carriers and electrons are minority carriers.

The holes density (n_h) is much greater than the electrons density (n_e) i.e.>> n_e.

The acceptor energy level is close to the Valence band and for away from the Conduction band.

The Fermi-energy level lies in between the acceptor energy level and valence band.

These are the differences between n type and p type semiconductors. If you know more, then please discuss.

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These are the differences between intrinsic and extrinsic semiconductors. If you know more, then please discuss. You can also read my articles about semiconductor and difference between n type and p type semiconductors.

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