Extrinsic semiconductors are doped with impurities to modify their electrical properties, making them suitable for electronic devices like diodes and transistors. Extrinsic semiconductors are particularly critical due to their modified electrical properties, which are tailored for specific applications. This article explores the definition, types, and properties of extrinsic semiconductors.
What is Extrinsic Semiconductor?
An extrinsic semiconductor is a type of semiconductor that has been intentionally doped with impurities to modify its electrical properties. Unlike intrinsic semiconductors, which are pure and consist solely of the semiconductor material itself, extrinsic semiconductors have small amounts of other elements added to enhance or change their conductivity.
When adding an impurity to a semiconductor, it is important to choose a dopant that will not disrupt the original structural lattice of the material. This means that the size of the dopant atom must be the same as the size of the atom of the original material, as this is a necessary condition for successful doping.
Extrinsic semiconductors make use of two types of dopants.
- Pentavalent (valency 5): arsenic, antimony, phosphorus, etc.
- Trivalent (valency 3): indium, boron, aluminum, etc.
The use of pentavalent and trivalent elements in doping creates two types of extrinsic semiconductors: n-type and p-type.
Types of Extrinsic Semiconductors
They are classified into two types based on the nature of the dopant added:
- N-Type Semiconductors: These are created by doping the semiconductor with an element that has more valence electrons than the semiconductor itself, typically five. Common n-type dopants include phosphorus and arsenic for silicon-based semiconductors. The extra electron provided by each dopant atom becomes a free electron, which increases the number of negative charge carriers in the semiconductor.
- P-Type Semiconductors: These are made by doping the semiconductor with an element that has fewer valence electrons, typically three. Common p-type dopants include boron and gallium. This creates “holes” or spots where an electron is missing in the semiconductor’s crystal lattice. These holes can move through the lattice and act as positive charge carriers.
Properties of Extrinsic Semiconductors
Extrinsic semiconductors exhibit several distinct properties that differ significantly from those of intrinsic semiconductors:
- Enhanced Conductivity: The addition of impurities increases the number of free charge carriers, either electrons or holes, thereby significantly enhancing the material’s conductivity compared to its intrinsic counterpart.
- Controlled Conductivity: By adjusting the type and amount of dopant, the electrical properties of the semiconductor can be precisely controlled, allowing for the design of electronic components with specific characteristics.
- Temperature Dependence: While the conductivity of extrinsic semiconductors is still temperature-dependent, it is more stable than intrinsic semiconductors because the presence of impurities provides a larger number of charge carriers even at lower temperatures.
- Dependence on Impurity Levels: The electrical behavior of extrinsic semiconductors highly depends on the concentration and type of impurities introduced. This allows for a wide range of applications but also requires careful control during manufacturing.
- Electrical Characteristics: The type of impurity determines whether the semiconductor will have a surplus of electrons (n-type) or holes (p-type), which influences how the semiconductor will interact with other components in an electronic circuit.
Applications
Extrinsic semiconductors are used in virtually all electronic devices, from diodes and transistors to solar cells and integrated circuits. The ability to control their properties through doping makes them adaptable to various applications, ensuring that devices meet specific needs such as voltage, current, and power specifications.