Active region

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

In order to remove tlie unwanted electrical activity associated witli deep-level impurities or defects, one can eitlier physically displace tlie defect away from tlie active region of tlie device (gettering) or force it to react witli anotlier impurity to remove (or at least change) its energy eigenvalues and tlierefore its electrical activity passivation). 

A logical consequence of this trend is a quantum w ell laser in which tire active region is reduced furtlier, to less tlian 10 nm. The 2D carrier confinement in tire wells (fonned by tire CB and VB discontinuities) changes many basic semiconductor parameters, in particular tire density of states in tire CB and VB, which is greatly reduced in quantum well lasers. This makes it easier to achieve population inversion and results in a significant reduction in tire tlireshold carrier density. Indeed, quantum well lasers are characterized by tlireshold current densities lower tlian 100 A cm . ... 

Because there are two changes ia material composition near the active region, this represents a double heterojunction. Also shown ia Figure 12 is a stripe geometry that confines the current ia the direction parallel to the length of the junction. This further reduces the power threshold and makes the diffraction-limited spreading of the beam more symmetric. The stripe is often defined by implantation of protons, which reduces the electrical conductivity ia the implanted regions. Many different stmctures for semiconductor diode lasers have been developed. 

Fig. 2. Schematic diagram of active layer stmctures employed in LEDs under forward bias showing the conduction band (CB) and valence band (VB). The simplest devices employ (a) a homostmcture active layer wherein the bandgap is constant throughout the device. More advanced stmctures consist of (b) single and (c) double heterostmctures. Heterostmctures faciUtate the confinement and injection of carriers in the active region where the carriers may...

Epitaxial crystal growth methods such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) have advanced to the point that active regions of essentially arbitrary thicknesses can be prepared (see Thin films, film deposition techniques). Most semiconductors used for lasers are cubic crystals where the lattice constant, the dimension of the cube, is equal to two atomic plane distances. When the thickness of this layer is reduced to dimensions on the order of 0.01 )J.m, between 20 and 30 atomic plane distances, quantum mechanics is needed for an accurate description of the confined carrier energies (11). Such layers are called quantum wells and the lasers containing such layers in their active regions are known as quantum well lasers (12). 

Fig. 10. Cross-sectional drawing of a vertical cavity surface emitting laser (VCSEL). Proton implantation is used to channel the current through a small active region. Light is emitted in the direction perpendicular to the plane of the wafer. This makes preparation of two-dimensional arrays quite easy.

Fig. 12. Cut-out drawing of a distributed feedback (DFB) laser showing the active region and a diffraction grating, under the active layer, which produces...

Eig. 10. The n—p—n transistor biased ia its active region, where 7 = current, (------) indicate depletion regions at the p—n junctions, and S is the electric field ... 

Fig. 11. Schematic of edge-emitting laser diodes where the arrows represent the direction of laser emission and U represents the active region (a) standard stmcture with cleaved facets for mirrors and (b) distributed feedback (DFB) laser that employs coherent reflection from a grating to generate optical...

Fig. 12. Schematic of surface-emitting laser diodes where U represents the active region (a) planar cavity surface-emitting laser diode (PCSEL) with 45° etched reflectors and (b) vertical cavity surface-emitting laser diode (VCSEL) with semiconductor-based multilayer mirror stacks grown into the stmcture.

Note also that a galvanic couple can be established between passive regions and active regions of the same stainless steel component. For... 

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