What is an N-Type Semiconductor?
A n-type semiconductor is an extrinsic semiconductor — meaning it is created by adding impurities to a pure (intrinsic) semiconductor to modify its electrical properties. When a small amount of pentavalent impurity (Group V element) is added, it donates an extra electron that is free to move in the crystal lattice. These free electrons drastically improve the conductivity of the semiconductor.
In contrast, intrinsic semiconductors have very few free charge carriers and are poor conductors at room temperature. By doping, we control both the number and type of carriers, making the semiconductor much more useful in practical circuits.
- Base Material: Silicon (Si) or Germanium (Ge)
- Doping Element (Donor): Arsenic (As), Phosphorus (P), Antimony (Sb), or Bismuth (Bi)
- Majority Carriers: Electrons
- Minority Carriers: Holes
- Type of Conductivity: Negative (n-type)
These impurities are called donor impurities because each atom donates one free electron to the semiconductor crystal. For instance, when an arsenic atom (atomic number 33) is added to silicon, it forms four covalent bonds with neighboring silicon atoms using four of its five valence electrons. The fifth electron remains loosely bound and can easily be excited into the conduction band with minimal energy. This extra electron increases the number of conduction carriers, thereby enhancing conductivity.
Formation of N-Type Semiconductor
Let’s take an example of doping silicon with arsenic. Silicon has four valence electrons, while arsenic has five. When arsenic atoms replace some silicon atoms in the lattice, four of arsenic’s electrons participate in bonding with neighboring silicon atoms. The fifth electron, however, is only weakly attached to the arsenic nucleus and can be easily freed with a small amount of thermal energy.
Because even a trace amount of arsenic introduces millions of free electrons into the lattice, the overall conductivity of the crystal increases significantly. The resulting semiconductor now behaves as an n-type material.
Energy Band Diagram of N-Type Semiconductor
The energy band diagram of an n-type semiconductor clearly shows how doping alters the energy structure. In a pure semiconductor, the conduction band and valence band are separated by a forbidden energy gap (Eg), and only a few electrons have enough energy to jump into the conduction band. When a pentavalent impurity is introduced, a new donor energy level (ED) forms just below the conduction band (EC).
- The donor level lies only a few tenths of an electron volt below the conduction band.
- Very little energy is needed to excite donor electrons into the conduction band.
- As a result, a large number of electrons are available for conduction even at room temperature.
- The Fermi level (EF) moves closer to the conduction band, reflecting the increased electron population.
This shift of the Fermi level indicates that the probability of electron occupancy near the conduction band is much higher in an n-type semiconductor than in an intrinsic one. Therefore, conduction occurs mainly due to these free electrons.
Conduction Process in N-Type Semiconductor
In the n-type semiconductor, the free electrons provided by donor impurities are the main cause of conduction. When a voltage is applied across the material:
- Electrons (negative charge carriers) move toward the positive terminal of the voltage source.
- The flow of electrons constitutes the electric current through the material.
- Because electrons are negative carriers, the conductivity is referred to as negative or n-type conductivity.
- Holes (positive charge carriers) still exist, but in very small numbers. Their contribution to total current is negligible.
In practical applications, almost all the current in an n-type semiconductor is due to electron motion. This is why n-type materials generally exhibit higher mobility and conductivity than p-type materials, where holes dominate.
Important Properties of N-Type Semiconductors
- Dopant Type: Pentavalent (Donor atoms)
- Majority Carriers: Electrons
- Minority Carriers: Holes
- Carrier Concentration: Electron density (n) ≫ Hole density (p)
- Conductivity: High and mainly due to electrons
- Fermi Level Position: Closer to the conduction band
- Temperature Dependence: Conductivity increases slightly with temperature due to thermal excitation
Mathematically, the relationship between carrier concentrations is expressed as:
n × p = ni2
Since in n-type material n ≫ p, the electron concentration dominates and defines the material’s overall conductivity.
The conductivity (σ) of an n-type semiconductor is given by:
σ = n·e·μn
Where:
- σ = conductivity
- n = number of free electrons
- e = charge of an electron (1.6 × 10⁻¹⁹ C)
- μn = electron mobility
Applications of N-Type Semiconductors
N-type semiconductors play a vital role in nearly every electronic device we use today. Some of their most common applications include:
- Diodes: Used with p-type materials to form p–n junctions that allow current to flow in one direction.
- Transistors: Used as emitter or collector regions in bipolar junction transistors (BJTs).
- MOSFETs: Serve as n-channel layers in field-effect transistors for switching applications.
- Solar Cells: Combine with p-type layers to create photovoltaic junctions that convert sunlight into electricity.
- LEDs and Photodiodes: Form active regions where electrons and holes recombine to emit or detect light.
Advantages of N-Type Semiconductors
- Higher electron mobility compared to holes in p-type materials.
- Greater conductivity at similar doping levels.
- Essential for forming the n-side of p–n junctions in most semiconductor devices.
Conclusion
In summary, an n-type semiconductor is created by doping a pure semiconductor with a small quantity of pentavalent impurity. The added atoms donate free electrons, which act as the majority charge carriers. The donor energy level appears just below the conduction band, allowing electrons to move easily into the conduction band and contribute to current flow.
The result is a material with high conductivity and fast electron mobility, making n-type semiconductors indispensable in all modern electronic and optoelectronic devices. Understanding their energy diagram, conduction mechanism, and behavior under bias is crucial for mastering the fundamentals of semiconductor physics.