Interactive Tutorials - Spinning Disk Fundamentals

Interactive Tutorials

Microscope Light Sources

Light-Emitting Diode Operation

 

Among the most promising of emerging technologies for illumination in optical microscopy is the light-emitting diode (LED). These versatile semiconductor devices possess all of the desirable features that incandescent (tungsten-halogen) and arc lamps lack, and are now efficient enough to be powered by low-voltage batteries or relatively inexpensive switchable power supplies. The diverse spectral output afforded by LEDs makes it possible to select an individual diode light source to supply the optimum excitation wavelength band for fluorophores spanning the ultraviolet, visible, and near-infrared regions. This article explores how two dissimilar doped semiconductors can produce light when a voltage is applied to the junction region between the materials.

Photon-emitting diode p-n junctions are typically based on a mixture of Group III and Group V elements, such as gallium, arsenic, phosphorous, indium, and aluminum. The relatively recent addition of silicon carbide and gallium nitride to this semiconductor palette has yielded blue-emitting diodes, which can be combined with other colors or secondary phosphors to produce LEDs that emit white light. The fundamental key to manipulating the properties of LEDs is the electronic nature of the p-n junction between two different semiconductor materials. When dissimilar doped semiconductors are fused, the flow of current into the junction and the wavelength characteristics of the emitted light are determined by the electronic character of each material. In general, current will readily flow in one direction across the junction, but not in the other, constituting the basic diode configuration. This type of behavior is best understood in terms of the transition of electrons and holes in the two materials and across the junction. Electrons from the n-type semiconductor move to the positively doped (p-type) semiconductor, which has vacant holes, allowing electrons to "jump" from hole to hole. The result of this migration is that holes appear to move in the opposite direction, or away from the positively charged semiconductor toward the negatively charged semiconductor. Electrons from the n-type region and holes from the p-type region recombine in the vicinity of the junction to form the depletion region, in which no charge carriers remain. Thus, a static charge is established in the depletion region that inhibits current flow unless an external voltage is applied.

In order to configure a diode, electrodes are placed on the opposite ends of a p-n semiconductor device to apply a voltage that is capable of overcoming the effects of the depletion region. Typically, the n-type region is connected to the negative terminal and the p-type region is connected to the positive terminal (known as forward biasing the junction) so that electrons will flow from the n-type material toward the p-type and holes will move in the opposite direction. The net effect is that the depletion zone disappears and electrical charge moves across the diode with electrons driven to the junction from the n-type material, whereas holes are driven to the junction from the p-type material. The combination of holes and electrons flowing into the junction enables a continuous current to be maintained across the diode. Although control of the interaction between electrons and holes at the p-n junction is a fundamental element in the design of all semiconductor diodes, the primary goal of LEDs is the efficient generation of light. The production of visible light due to injection of charge carriers across the p-n junction only takes place in semiconductor diodes having specific material compositions, which has led to the search for new combinations that feature the necessary band gap between the conduction band and orbitals of the valence band. Furthermore, research is ongoing to design LED architectures that minimize absorption of light by the diode materials and are more robust at concentrating light emission in a specific direction.

Contributing Authors

Tony B. Gines and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.