In semiconductor physics, direct and indirect band gap semiconductors are crucial concepts that define how materials absorb and emit light. This article will delve into the definitions of band gap semiconductors and provide a comparative analysis.
Definition
In semiconductors, we call the energy differential between the bottom of the conduction band and the top of the valence band the band gap. An electron needs this energy to move from the valence band to the conduction band. This excitation occurs when heat, light, or electrical energy supplies the required energy. The electron hole in the valence band and the resulting conduction-band electron are free to travel throughout the crystal lattice. And act as charge carriers to carry electric current. Since no states are available when the valence band is full. And the conduction band is empty, electrons cannot move within the solid. Because there is no net charge carrier mobility, no current is produced. However, the current can still flow if some electrons shift from the primarily full valence band to the mostly empty conduction band.
Significance
Each solid material has a unique energy-band structure. This variation in band structure causes a wide range of electrical properties in different materials. Semiconductors are essentially defined as insulators with a band gap energy of less than 3.0 eV (~290 kJ/mol). This cutoff was selected because the conductivity of undoped semiconductors is extremely low at 3.0 eV and decreases exponentially with the band gap energy. Furthermore, substances with larger band gaps (such as SrTiO3, Egap = 3.2 eV) do not absorb light in the visible spectrum. A material’s band gap plays a major role in determining its electrical conductivity. The gap size can affect the material’s properties and behavior in a transistor. Small band gaps, sometimes called “narrow” band gaps, are found in semiconductors, whereas large band gaps, sometimes called “wide” band gaps, are generally found in insulators.
Direct Band Gap
Direct band gap semiconductors are materials where the minimum of the conduction band aligns directly with the maximum of the valence band in momentum space. This alignment allows electrons to transition between these energy bands by emitting or absorbing photons without a change in momentum.
Indirect Band Gap
Indirect band gap semiconductors are materials where the conduction band’s minimum does not align with the valence band’s maximum. As a result, electrons require a change in momentum (assisted by phonons) to transition between the bands.
Tuning
Bandgap properties may change depending on the material’s formation process. In grown materials, band gap and emission characteristics can be adjusted by controlling the strain in the deposited film using the substrate’s thermal expansion coefficient. Many structural and electrical properties of silicon, germanium, and conventional III-V and II-VI semiconductors are similar. Therefore, without significantly altering the overall integration scheme, materials scientists can adjust the semiconductor composition to tune the bandgap, lattice parameters, and other properties. On the other hand, two-dimensional semiconductors originate from distinct regions of the periodic table. They differ in their chemical composition, lattice structures, and electrical characteristics.
Understanding the differences between direct and indirect band gap semiconductors is essential for selecting the right materials for various electronic and optoelectronic applications.
