An intrinsic semiconductor is a pure semiconductor material with no added impurities. Its conductivity depends on its temperature. An extrinsic semiconductor is a doped semiconductor, where impurities have been added to increase either electron or hole concentration.
The depletion region is a region near the p-n junction where mobile charge carriers (electrons and holes) have been depleted, leaving behind immobile ions. This creates a potential barrier.
Energy bands represent the allowed energy levels that electrons can occupy in a solid. The gaps between these bands are called band gaps.
The Zener breakdown occurs in heavily doped p-n junctions under a strong reverse bias. The high electric field across the junction pulls electrons from the covalent bonds, creating a large number of charge carriers and allowing current to flow.
The solar cell does not require current for its operation; instead, it supplies current to the load. Since the generated EMF is positive while the current is negative, the graph appears in the fourth quadrant. This is why the I-V characteristics of a solar cell are plotted in the fourth quadrant.
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Electronic Devices
Electronic devices are powered by components like semiconductors, diodes, transistor, and integrated circuits. For instance, diodes function as rectifiers, transistors amplify signals, LEDs emit light, solar cells convert sunlight into energy, and logic gates process digital signals. These components are vital for driving the technology we use daily, including smartphones and computers.
1.0Classification Of Materials (Solids)
On the basis of Conductivity & Resistivity
Conductors
Insulators
Semiconductors
Abundance of free e−
Very few free e–
Few free e−
High conductivity
(σ∼102−108Sm−1)
Low conductivity
(σ∼10−11−10−19Sm−1)
Intermediate conductivity
(σ∼105−10−6Sm−1)
Low resistivity
(ρ∼10−2−10−8Ωm)
High resistivity
(ρ∼1011−1019Ωm)
Intermediate resistivity
(ρ∼10−5−106Ωm)
Effect of temperature
(T↑R↑ρ↑σ↓)
(At very high temperature)
(T↑R↓ρ↓σ↑)
(At room temperature)
(T↑R↓ρ↓σ↑)
α=+ve
α=−ve
α=−ve
(α=Temp.Coefficient)
E.g. : Metals
E.g.: Rubber, Wood, Plastic,Diamonds etc.
E.g. : Si, Ge, GaAs, CdS, anthracene,
Polypyrole, Polyaniline etc
2.0Energy Band Theory
1. Valence Band: This is a lower-energy band, which contains valence electrons. This band is either partially or completely filled with electrons but never be empty. The electrons in this band are not capable of taking part in the conduction of current.
2. Conduction Band: This is the higher band containing conduction electrons.It is either empty or partially filled with electrons .Electrons present in this band take part in the conduction of current. This band is completely empty. Electrons are forbidden to be in this energy gap.
3.Band Gap or Forbidden Energy Gap (Eg) :The minimum energy required to shift an electron from the valence band to conduction band is called band gap.It depends on the nature of the solid and on the interatomic separation. It also depends on temperature, but this dependence is very weak. Width of forbidden energy gap is represented in eV. As the temperature increases, the forbidden energy gap decreases (very slightly).
Classification of Solids According to Energy Band Theory
Conductor
Insulator
Semiconductor
Overlap of conduction and valence bands, high conduction
No conduction, large energy gap
Small band gap, some thermal excitation to conduction band
ΔEg=0
ΔEg>3eV
ΔEg<3eV
Example:Gold, Silver, Copper
Example:Rubber, Glass
Example:Silicon, Germanium
3.0Types of Semiconductor
Intrinsic
Semiconductor
N-type (Pentavalent impurity)
P-type(Trivalent impurity)
Current is due to both
electrons and holes
Mainly due to electrons
Mainly due to holes
ne=nh=ni
ne>>nh(nD≅ne)
nh>>ne(nA≅nh)
I=Ie+Ih
I⋍Ie
I⋍Ih
Entirely neutral
Entirely neutral
Entirely neutral
Quantity of electrons and
holes are equal
Majority – Electrons
Minority – Holes
Majority – Holes
Minority - Electrons
4.0Mass Action Law
At room temperature, most acceptor atoms become ionized, creating holes in the valence band. Therefore, in an extrinsic semiconductor, the density of holes in the valence band is primarily determined by the impurities.
“Rate of generation of charge carriers is equal to rate of recombination of charge carriers.”
ne.nh=ni2
Note: ni depends only on the nature of the semiconductor material and temperature; it does not depend on the doping level., Semiconductor Diode(P-N Diode)& Diode Formation
When a P-type semiconductor is joined to a N-type semiconductor such that the crystal structure remains continuous at the boundary, the resulting arrangement is called a P-N junction diode.
Feature
Diffusion
Drift
Charge Carriers
Majority Carriers
Minority Carriers
Cause
Concentration Gradient
Electric Field in Depletion Region
Direction (Carriers)
Holes: p-side to n-side
Electrons: n-side to p-side
Electrons: p-side to n-side
Holes: n-side to p-side
Direction (Current)
p-side to n-side
n-side to p-side
Effect on Electric Field
Increases electric field strength
Influenced by electric field strength
Note:p-n junction under equilibrium, there is no net current .∣IDrift∣=∣IDiffusion∣
Depletion Width
Width of Depletion region ≈ 0.1μm
It depends on temperature, doping and type of material WidthαDoping1
Barrier Potential
The barrier potential is the electric field created across the p-n junction, with the n-side having a higher potential than the p-side. This potential difference creates a barrier that resists the movement of the majority carriers but allows minority carriers to pass. It depends on temperature, doping, and semiconductor material.
Current increases sharply with a slight increase in reverse voltage
Diode As a Rectifier
Rectifier: It is a device which is used for converting alternating current into direct current. A diode can be used as a rectifier as it is a unidirectional device.
Half Wave Rectifier:
Positive Half of Input Signal:
S1 is positive, S2 is negative.
D1 is forward biased, D2 is reverse biased.
Only D1 conducts, current flows through RL from A to B.
Negative Half of Input Signal:
S1 is negative, S2 is positive.
D1 is reverse biased, D2 is forward biased.
Only D2 conducts, current flows through RL
from A to B.
Full-wave Rectifier:
Positive Half of Input Signal:
S1 is positive, S2 is negative.
D1 is forward biased, D2 is reverse biased.
D1 conducts, current flows from A to B through RL
Negative Half of Input Signal:
S1 is negative, S2 is positive.
D1 is reverse biased, D2 is forward biased.
D2 conducts, current flows from A to B through RL
In both positive and negative halves, current always flows from A to B, making the output a full-wave rectified signal.
Zener Diode And Used as a Voltage Regulator
Zener Diode:Invented by C. Zener, heavily doped P-N junction.Operates mainly in reverse bias breakdown region.
6.0I-V Characteristics
Handles large current variations without changing Zener voltage, useful in regulation.
An increase in reverse bias voltage generates a strong electric field in the depletion region, causing a breakdown.
In reverse biasing, breakdown can occur in two ways:
1. Avalanche Breakdown :It occurs in p-n junctions having low doping. Breakdown voltage is very high. Depletion layer width is also more. Charge carriers crossing the depletion region gain enough kinetic energy and make collisions with other atoms, and hence it starts the avalanche effect. Damage to the diode is permanent.
2. Zener Breakdown:Occurs in a heavily doped p-n junction. Breakdown voltage is very small. The depletion layer is very thin.Breaking of covalent bonds is mainly due to the electric field. Hence damage is not permanent.
Zener Diode as Voltage Regulator:In the breakdown region, the Zener voltage stays constant even as current change.Changes in input voltage cause corresponding changes in current through the series resistor (RS) and Zener diode, but the voltage across the load remains constant.
Input Current, IS=RSVin−VZ
Zener current, IZ=IS−IL
Power Dissipation,
PZ=VZ×IZ
Light Emitting Diode(L.E.D)
An LED (Light Emitting Diode) converts electrical current into light. It's a heavily doped p-n junction operated under forward bias. When forward biased, electrons move from N → P and holes from P → N, recombining at the junction and releasing energy as photons.
Light Intensity: Small forward current = low light intensity, large current = max intensity, then intensity decreases with further increase.
Material: LED semiconductors have a band gap of 1.8 eV to 3 eV (e.g., GaAs₁₋ₓPₓ).
Photodiode
A photo-diode converts light into electrical current or voltage. It’s a moderately doped p-n junction operated under reverse bias with a transparent outer material to allow photons to enter.
As light intensity increases, the photocurrent increases.Example: Used in video cameras to detect light intensity.
Solar Cell
A photovoltaic cell converts light energy into electrical energy, similar to a photo-diode but operates without bias. The thin p-region allows photons to easily reach the junction.
Generation: Light generates electron-hole pairs at the junction (h>Eg)
Separation: The electric field of the depletion region separates the electrons (toward n-side) and holes (toward p-side).
Collection: This creates a photovoltage, with the n-side becoming negative and the p-side positive.
Logic Gates
It is a digital circuit that processes input signals to produce output based on logical operations, using semiconductors like diodes and transistors.
