How Does a Semiconductor Laser Work?

By Ibtisam AbbasiReviewed by Lexie CornerFeb 20 2025

Since the discovery of semiconductor lasers in 1962, this technology has been integral to advancements across various fields.¹ Semiconductor lasers are widely used in optical communications, biomedical applications, integrated optics, and materials science.² But how do they work? Understanding their structure, key properties, and operating principles is essential to exploring their applications and performance.

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Electrical Properties of Semiconductor Materials

Semiconductors are widely used due to their long lifespan, compact size, low power requirements, and compatibility with modern technology.³ Their electrical properties can be adjusted to either enhance or restrict electron flow, making them essential in electronic and laser applications. Their conductivity falls between that of metals and insulators, allowing controlled electrical behavior.⁴

Electrical conductivity is determined by the movement of free electrons. Electrons occupy different energy levels, with the least tightly bound ones in the valence band. Above this is the conduction band, where electrons must transition to enable electrical flow. The energy difference between these bands, known as the band gap, dictates a material’s conductivity.

Semiconductors have a narrower band gap than insulators, enabling controlled electron movement. This property is key to their use in modern electronics and optoelectronic devices, including semiconductor lasers.⁵

The P-N Junction and Its Role in Semiconductor Lasers

N-Type Semiconductor Doping

Doping is a fundamental process in semiconductor technology, essential for forming P-N junctions. A semiconductor material like silicon, for example, has four valence electrons in its pure state. When doped with an element like phosphorus, which has five valence electrons, four of them form bonds with the silicon atoms, leaving one free electron. This free electron moves easily at room temperature, imparting a negative charge to the material and creating an n-type semiconductor.

P-Type Semiconductors

Similarly, doping a semiconductor with an element that has fewer valence electrons, such as boron (which has three), results in an electron deficiency or hole. Since fewer electrons are available to form bonds, electrons from neighboring silicon atoms move to fill these holes, effectively creating a flow of positive charge. This process forms a p-type semiconductor, where holes act as charge carriers.

Formation of the P-N Junction and Built-in Electric Field

A p-n junction forms when a p-type semiconductor (e.g., boron-doped silicon) is placed in contact with an n-type semiconductor (e.g., phosphorus-doped silicon). Free electrons from the n-type region diffuse into the p-type region, where they recombine with holes. This movement creates a positively charged region near the junction in the n-type material and a negatively charged region in the p-type material.

As charge carriers continue to diffuse, a built-in electric field develops at the junction, preventing further movement of electrons and holes. This leads to the formation of the depletion region, where immobile positive and negative ions accumulate. Once equilibrium is reached, the net electron flow across the depletion region becomes zero.

Depletion Region and Forward Biasing

The depletion region and built-in electric field can be controlled using an external voltage. In forward bias, where the positive terminal of a battery is connected to the p-type material and the negative terminal to the n-type material, the applied electric field opposes the built-in electric field. This reduces the depletion region’s thickness, lowering resistance and allowing current to flow.

When an electron gains enough energy to transition from the valence band to the conduction band, it produces mobile charge carriers, which facilitate electron and hole movement. This process is key to modern semiconductor devices, including semiconductor lasers.⁶

Recombination and Photon Emission

Recombination of electrons and holes is a fundamental process in semiconductors and light-emitting diodes (LEDs). When an external electric field is applied, electrons in the conduction band and holes in the valence band move toward each other. As recombination occurs, the energy difference between these bands is released in the form of a photon, with its energy equal to the band gap of the material.⁷

Another way to describe this process is that an electron loses energy as it transitions from the conduction band to an available hole in the valence band. The lost energy is emitted as a photon, with a wavelength determined by the band gap.

Light Amplification: Stimulated Emission

Semiconductor diodes are widely studied, with advanced materials such as quantum well structures being integrated to enhance optical gain. The relationship between gain and spontaneous emission in semiconductor materials is a key factor in performance evaluation.⁸

LEDs operate based on spontaneous emission, whereas laser diodes rely on stimulated emission to amplify light. In semiconductors, an electron can absorb energy and transition from a lower energy state, such as the valence band, to a higher energy state in the conduction band. The electron then loses energy and returns to a lower state, emitting a photon in the process. This emission can occur spontaneously or be stimulated by an incoming photon of matching energy.

Stimulated Emission in Laser Diodes

In laser diodes, stimulated emission occurs when an incident photon of a specific energy interacts with an excited electron, causing it to transition to a lower energy state while releasing a second photon. The emitted photon is in phase with the original photon, meaning both have the same energy, direction, and wavelength. This process enables the gain medium to amplify light, a fundamental principle in laser operation.

Population Inversion and Light Gain

In contrast to stimulated emission, spontaneous emission results in photons being emitted randomly. For light amplification to occur, a condition known as population inversion must be achieved, where more than 50 % of atoms exist in an excited energy state.⁹ This ensures that stimulated emission dominates over spontaneous emission, allowing continuous optical gain in the laser system.

Optical Cavities and Their Role in Light Amplification

Stimulated emission in a population-inverted laser diode enables light amplification, a fundamental process in high-powered lasers. When an electron decays from a higher to a lower energy state, it emits an in-phase photon. This photon can then stimulate other excited electrons to undergo the same transition, emitting additional in-phase photons. This cascading effect accelerates the stimulated emission process, increasing the intensity of emitted light.

Encasing this system within an optical cavity—formed by mirrors at both ends of the diode—enhances the amplification process. The mirrors reflect in-phase photons back and forth, reinforcing stimulated emission and further increasing photon generation. This repeated reflection sustains continuous light amplification, a key mechanism for generating high-energy laser beams.

Coherent Light Emission in Lasers

If one mirror of the optical cavity is slightly transparent, photons can escape, forming a laser beam. The continuous reflection and amplification of in-phase photons increase the beam's intensity. This results in a coherent and monochromatic laser beam, where all photons have the same energy, equal to the band gap of the material. Compared to incoherent, non-monochromatic white light, laser beams exhibit significantly higher energy density.

For sustained light amplification, the emission rate must exceed absorption, a condition achieved through population inversion. Maintaining this state ensures a high stimulated emission rate, which is essential for generating a coherent laser output. To keep a sufficient concentration of excited electrons, the gain medium is continuously energized using techniques such as optical pumping.¹⁰

Key Performance Parameters of Semiconductor Lasers

Several factors influence the performance of a semiconductor laser. One critical parameter is the threshold current, the minimum current required to achieve population inversion and initiate spontaneous emission, ultimately leading to a coherent monochromatic laser beam.

Temperature significantly affects semiconductor laser performance. An experimental study on a laser diode (LD) chip with a threshold current of 11.15 mA demonstrated that as temperature increased, the threshold current also increased, while optical output power decreased. Within a temperature range of 0–30°C, slope efficiency declined from 189 mW to 188 mW, and the threshold current rose to 11.8 mA. These variations result from temperature-dependent loss mechanisms in the active region of the laser band gap, affecting both threshold current and optical power.¹¹

Ongoing research continues to improve semiconductor laser performance. Mode control using parity-time (PT) symmetry has been demonstrated and is rapidly advancing. This approach has enabled simpler yet highly efficient laser designs while refining mode characteristics.¹² Future high-powered lasers are expected to play a key role in quantum computing and solid-state spectral analysis.

For more information on semiconductor lasers and their applications, please visit the following resources:

• What Are Semiconductor Lasers and How Are They Used?
• Exploring Semiconductor Lasers for Telecommunications
• The Advantages and Disadvantages of Semiconductor Lasers

References and Further Reading

1. Welch, D. F. (2000). A brief history of high-power semiconductor lasers. IEEE Journal of selected topics in quantum electronics, 6(6), 1470-1477. Available at: https://doi.org/10.1109/2944.902203
2. Li, X. et al. (2022). Development of a High-Power Surface Grating Tunable Distributed-Feedback Bragg Semiconductor Laser Based on Gain-Coupling Effect. Appl. Sci. 12. 4498. Available at: https://doi.org/10.3390/app12094498
3. Zhang, Q. et. al. (2021). Halide perovskite semiconductor lasers: materials, cavity design, and low threshold. Nano Letters, 21(5), 1903-1914. Available at: https://doi.org/10.1021/acs.nanolett.0c03593
4. Intel. (2024). What are Semiconductors? Tech 101, Manufacturing. [Online]. Available at: https://www.intel.com/content/www/us/en/newsroom/tech101/semiconductors-101-how-chip-is-made.html#gs.jji767 [Accessed on: January 26, 2025].
5. Toshiba Semiconductor. (2025). Energy band diagram. Basics of Schottky Barrier Diodes. Chapter 1-2. [Online]. Available at: https://toshiba.semicon-storage.com/ap-en/semiconductor/knowledge/e-learning/basics-of-schottky-barrier-diodes/chap1/chap1-2.html [Accessed on: January 28, 2025].
6. Dr. Sproul, A. (2021). Understanding the p-n Junction. University of New South Wales. Solar Cells: Resource for the Secondary Science Teacher. [Online]. Available at: https://www.unsw.edu.au/content/dam/pdfs/science/general/resources/2021-10-faculty/2023-08-UNSW_Understanding_the_p-n_Junction.pdf [Accessed on: January 29, 2025].
7. Van Der Holst, J. et. al. (2009). Electron-hole recombination in disordered organic semiconductors: Validity of the Langevin formula. Physical Review B—Condensed Matter and Materials Physics, 80(23), 235202. Available at: https://doi.org/10.1103/PhysRevB.80.235202
8. Girardin, F. et. al. (1997). Characterization of semiconductor lasers by spontaneous emission measurements. IEEE Journal of Selected Topics in Quantum Electronics, 3(2), 461-470. Available at: https://doi.org/10.1109/2944.605694
9. Paschotta, R. (2008). Field Guide to Lasers. (SPIE) The International Society for Optics and Photonics Press. 9780819469618. [Online]. Available at: https://spie.org/publications/spie-publication-resources/optipedia-free-optics-information/fg12_p02_spontaneous_and_stimulated_emission#:~:text=If%20an%20atom%20is%20in,process%20is%20called%20spontaneous%20emission. [Accessed on: January 31, 2025].
10. The School of Physical Sciences, University of California, Irvine. How a Laser Works? [Online]. Available at: https://ps.uci.edu/~cyu/p224/LectureNotes/lecture8/lecture8.pdf [Accessed on: January 31, 2025].
11. Zyoud, S. et. al. (2020). The Impact of Temperature on the Performance of Semiconductor Laser Diode. International Journal of Advanced Science and Technology. 29(6). 1167-1180. Available at: https://www.researchgate.net/publication/342707254_The_Impact_of_Temperature_on_the_Performance_of_Semiconductor_Laser_Diode
12. Sha, H. et al. (2024). Advances in Semiconductor Lasers Based on Parity–Time Symmetry. Nanomaterials. 14(7):571. Available at: https://doi.org/10.3390/nano14070571

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