Shockley-Read-Hall (SRH) recombination is a fundamental process in semiconductor physics. It describes how charge carriers, electrons and holes, disappear within a material, often converting their energy into heat. Understanding this mechanism is important for designing and optimizing electronic devices, from solar cells to light-emitting diodes, enhancing their efficiency and reliability.
Basic Semiconductor Principles
Semiconductors are materials with electrical conductivity between that of a conductor and an insulator. Their unique properties arise from their atomic structure, where electrons exist in distinct energy bands.
The valence band is the highest energy band where electrons are tightly bound to atoms. Above it lies the conduction band, where electrons are free to move and conduct electricity. A forbidden energy region, the band gap, separates these two bands. An electron must gain sufficient energy to cross this band gap to move from the valence band to the conduction band.
When an electron absorbs enough energy, often from heat or light, it can jump from the valence band to the conduction band, leaving a vacant spot. This vacancy, a “hole,” behaves as a positively charged mobile carrier. The electron in the conduction band and the hole in the valence band form an electron-hole pair. Both can move through the material and contribute to electrical current, with continuous generation occurring even at room temperature due to thermal energy.
Understanding Carrier Recombination
Charge carrier recombination is the reverse process of electron-hole pair generation. A free electron in the conduction band returns to the valence band and combines with a hole, effectively annihilating both charge carriers. Recombination occurs as the semiconductor system seeks to return to equilibrium, where the number of free electrons and holes is stable. This process releases the energy initially absorbed to create the electron-hole pair.
The energy released during recombination can manifest in different forms. In radiative recombination, the energy is emitted as a photon of light, which is the principle behind light-emitting diodes. However, in non-radiative processes, the energy is released as heat. This non-radiative recombination is often undesirable in devices designed to produce light or electrical power, as it represents an energy loss.
The Shockley-Read-Hall Mechanism
The Shockley-Read-Hall (SRH) mechanism is a form of non-radiative recombination involving specific energy states within the semiconductor’s band gap. These “trap levels” or “recombination centers” arise from imperfections in the crystal lattice, such as impurities or structural defects. These traps exist at energy levels between the valence and conduction bands.
The SRH process occurs in two steps. First, a free electron from the conduction band is captured by a trap level within the band gap, temporarily residing at this intermediate energy state. Second, a hole from the valence band moves to the same trap level and recombines with the trapped electron. This sequence allows the electron and hole to combine without directly crossing the full band gap, avoiding light emission.
The energy released during this two-step process is converted into heat, vibrating the atoms within the semiconductor lattice. The efficiency of SRH recombination depends on the density of these trap levels and their energy positions within the band gap. Materials with more defects or impurities tend to exhibit higher rates of SRH recombination. This theory was independently developed by William Shockley and William Read in 1952, and by George Hall in 1951, explaining recombination involving defects.
SRH Recombination in Modern Devices
SRH recombination impacts the performance of modern semiconductor devices by reducing their efficiency. In devices relying on charge carrier collection, such as solar cells, SRH recombination leads to a loss of generated electron-hole pairs before they can be collected as electrical current. When electrons and holes recombine via SRH, their energy dissipates as heat rather than converting into useful electrical power, diminishing the cell’s power conversion efficiency and resulting in lower output.
For light-emitting diodes (LEDs), SRH recombination reduces light output. In an ideal LED, electrons and holes combine radiatively, producing photons. However, when SRH recombination occurs, energy releases as heat instead of light, leading to lower brightness and reduced luminous efficiency. Minimizing SRH recombination is a primary goal in semiconductor manufacturing.
Engineers employ various strategies to mitigate SRH recombination, including using high-purity materials and advanced fabrication techniques to reduce defect densities. Controlling material quality and processing steps helps limit the number of trap levels available for non-radiative recombination, improving performance and reliability in electronic and optoelectronic applications.