Intrinsic Semiconductor

The movement of charge carriers, specifically electrons, is crucial in determining the current flow within a semiconductor. In an intrinsic semiconductor, some electrons gain enough energy to break free from their atomic bonds through various mechanisms such as thermal excitation or exposure to light. These free electrons become mobile and contribute to the conductivity of the material.

However, every electron that breaks free and becomes mobile leaves behind an unfilled space called a “hole”. Holes can be considered positive charges that move in the opposite direction to the flow of electrons. While they do not represent real particles like electrons, holes facilitate current flow by behaving like positive charge carriers.

When an electric field is applied across a semiconductor material, it causes both the mobile electrons and holes to move in opposite directions. The electric field accelerates the movement of electrons toward one end (the positive terminal) while attracting holes toward the other (the negative terminal). This collective motion constitutes an electric current within the semiconductor. It is the fundamental conduction mechanism of intrinsic semiconductors.

The ability to conduct electricity is fundamentally based on a semiconductor’s inherent material properties. Unlike conductors, where a high density of free electrons allows for efficient electrical conduction, intrinsic semiconductors maintain a balance between free electrons and holes. This delicate equilibrium results in a limited number of charge carriers, making intrinsic semiconductors relatively poor conductors compared to metals.

For an intrinsic semiconductor, the concentration of electrons in the conduction band is equal to the concentration of holes in the valence band. Suppose n_e is the intrinsic electron concentration, and n_h is the intrinsic hole concentration. Then,

n_e = n_h = n_i

Where n_i is the intrinsic carrier concentration, which refers to either the intrinsic electron or hole concentration.

The table below highlights the intrinsic carrier concentrations of a few semiconductors at room temperature, juxtaposed with the free electron density of copper for comparison.

The energy band diagram is a crucial concept in semiconductor physics, providing insights into the material’s electrical properties. In a pure semiconductor crystal, such as silicon or germanium, the arrangement of electrons and their energy levels is organized into distinct bands, namely the valence band and the conduction band.

The valence band is the highest energy band in which electrons are typically present at absolute zero temperature. These electrons are tightly bound to the atomic nuclei within the crystal lattice. As electrons gain energy through external stimuli like light or thermal energy, they can move from the valence band to the higher energy conduction band, leaving behind holes in the valence band, as shown in the image below.

The conduction band is above the valence band. It represents the energy levels at which electrons can move freely and conduct electric current. The band gap is the energy difference between the valence and conduction bands. Its value depends on the temperature. The table below shows the room temperature and absolute zero values. The relatively small values mean electrons can transition across the band gap by absorbing thermal energy or photons, contributing to the material’s inherent conductivity.

The donor and acceptor levels represent specific energy states where electrons bind to donor ions, or holes attach to acceptor ions. The energy required to ionize donors corresponds to the difference in energy between the donor level and the conduction band, which happens to be minimal due to its small value.