More from Optical Semiconductor Devices.
Double Heterostructures
A double heterostructure is two pn junctions in a row, where each of the three materials has different band-gap energies. The most widely used structure (at least in optics) is a p-type wide band-gap / p-type narrow band-gap / n-type wide band-gap. This combination creates an impressive barrier that must be overcome for diffusion current to flow. However, diffusion current only occurs in very high excitation conditions. In normal forward bias operation, carriers crowd into the middle narrow band-gap material (the active layer), and spontaniously recombine through the material. The high density of carriers, caused by both the narrow band-gap and the injected carriers, causes a high rate of spontanious recombination. This recombination is the central operation principle behind LEDs.
Quantum Wells
A quantum well is created by a p(wide)/p(narrow)/p(wide) or n(wide)/n(narrow)/n(wide) heterostructure. The first thing to realize is that many carriers will be trapped in the "well" formed by the heterostructure, unable to move to either wide band-gapped material. The key, is that the narrow band-gap material's thickness is comprable to the de Broglie wavelength of the carriers. Since the length is comprable to the de Broglie wavelength of the carriers, the possible energy states of the carriers in this direction become quantized into discrete levels, greatly reducing the number of possible states. This means that the carriers in the well have limited motion in that direction (the direction of the thickness), confining them to a two-dimensional plane.
When multiple quantum wells are put together, and if the wide band-gap materials are also very thin, then a super lattice structure is formed.
Subscribe to:
Post Comments (Atom)
No comments:
Post a Comment