A **semiconductor** is a material that can conduct electricity under certain conditions but not always, making it a good bridge between conductors (like metals) and insulators (like rubber). Semiconductors are the backbone of electronic devices such as computers, smartphones, and solar cells. The most commonly used semiconductor is **silicon**.
### Pure Semiconductors (Intrinsic Semiconductors)
A **pure semiconductor** is one without any impurities. In its natural state, every atom in a pure semiconductor, like silicon or germanium, is bonded to four other atoms, creating a stable structure. At room temperature, some of these bonds may break due to heat, freeing up a few electrons to move and carry electric current.
In a pure semiconductor:
- **Electrons** (negatively charged particles) and **holes** (the absence of an electron, acting as a positive charge) are present in equal numbers.
- The ability of a pure semiconductor to conduct electricity is very low because very few electrons are available to carry the current.
### Doped Semiconductors (Extrinsic Semiconductors)
To improve the conductivity, we introduce impurities into a pure semiconductor. This process is called **doping**. Doped semiconductors are called **extrinsic semiconductors**. There are two types of doping:
#### 1. **N-type Semiconductor (Negative Type)**
In this type, an impurity with more electrons than the semiconductor (usually phosphorus or arsenic, which have 5 outer electrons) is added. This extra electron becomes free to move and conduct electricity, increasing the material's conductivity.
- **Example**: If we add phosphorus (P) to silicon (Si), phosphorus has 5 electrons in its outer shell, while silicon has 4. Phosphorus can only bond with 4 silicon atoms, leaving the 5th electron free to move around. This free electron enhances the conductivity.
- **Result**: The semiconductor has more **free electrons** than holes, making electrons the primary charge carriers.
#### 2. **P-type Semiconductor (Positive Type)**
Here, an impurity with fewer electrons than the semiconductor (usually boron or gallium, which have 3 outer electrons) is added. This creates a "hole" where an electron is missing. These holes act like positive charges that can move and allow current to flow.
- **Example**: If boron (B) is added to silicon, boron has only 3 electrons in its outer shell, while silicon has 4. As a result, one bond with silicon will have a "hole," or missing electron.
- **Result**: The semiconductor has more **holes** than free electrons, making holes the primary charge carriers.
### Summary of Differences:
- **Pure (Intrinsic) Semiconductors**: Equal number of electrons and holes, low conductivity.
- **Doped (Extrinsic) Semiconductors**: Conductivity is increased by adding impurities.
- **N-type**: More electrons, electrons are the majority carriers.
- **P-type**: More holes, holes are the majority carriers.
By combining **N-type** and **P-type** semiconductors in devices like diodes and transistors, we can control the flow of electricity in electronics, making modern technology possible.