Junction diodes

The emission linewidth of LEDs is typically 10 nm, which is quite broad in comparison to that achievable with laser diodes. Narrower linewidths down to ca. 0.9 nm can be obtained using resonant cavity LEDs.(61) Superluminescent LEDs have been produced with low spectral ripple (less than 10% at wavelengths down to 670 nm) by suppressing the optical feedback in laser diode junctions.(62)... 

A ZnS coating 46 is used to encapsulate the detectors. A dielectric filler is deposited in the channels between the detector elements to provide a supporting surface for a common electrode and to provide lateral mechanical support for the detector elements. Next, diode junctions 54 of the detectors are created by ion-implantation of boron ions. Indium contact pads 56 are formed in holes formed in the coating 46, and a common indium electrode 58 is formed on top of the dielectric material 50. 

Variable capacitors are available with values up to a few hundred picofarads. These are commonly formed from sets of interleaved plates, one fixed and the other attached to a shaft. Rotation changes the effective area and thereby the capacitance. Arrangements with sliding cylinders are also used and dielectrics include air, mica, and ceramic. Varacter diodes, junction diodes in which the capacitance is determined by the reverse bias voltage, are now finding increasing use in circuits, because their capacitance can be actively controlled by other electronic circuit elements. 

Fig. 15. Schematic diagram used to derive an approximate expression for the ac capacitance of an amorphous semiconductor diode junction as described in the text.

Wafer processing is often called planar processing or planar technology because small and thin planar structures are built on thin wafers 500 pm thick) of ultra pure silicon or germanium or any other suitable semi-conductor material. The thin wafers are cut from a rod of pure material, which is a single crystal, and then polished. The structures built on the surface of the wafer are electrical components such as resistors, capacitors, diodes, junction transistors, MOSFET transistors, etc. Each wafer contains 200 to 500 chips, with each chip identical to the others. 

Figure 5.25 Effect of diode junction temperature on LED lifetime.

Gallium-aluminum (GaAlAs) lasers are, as are all semiconducting diode lasers, excited by electrical current which creates excited hole-electron pairs in the vicinity of the diode junction. Those carrier pairs are the lasing species which emit spontaneously and with photon stimulation. The beam emerges parallel... 

Lasing species Nd3t in YAG lattice Hole-electron pairs at diode junction... 

For the first time, there was a mathematical expression for the impedance dispersion corresponding to the circular arc found experimentally. The equation introduced a new parameter the somewhat enigmatic constant a. He interpreted a as a measure of molecular interactions, with no interactions a = 1 (ideal capacitor). Comparison was made with the impedance of a semiconductor diode junction (selenium barrier layer photocell). 

Figure 9.41 shows a diode junction for generating the inverted population combined with a Fabry-Perot resonant cavity. When the junction is forward biased, one can obtain laser emission from such a structure. To get some of fhe Hght out of the cavity at least one, and often both, of the mirrors are only partially reflecting. It turns out to be easy to fabricate such mirrors in a single crystal material such as a semiconductor... 

FIGURE 9.51 Plots of diode laser optical power vs. diode current with diode junction temperature as a parameter for (a) a representative laser, (b) a laser designed for a low-temperature coefficient (Kazarinov, 1995). 

It is interesting to note here that if there is no battery, but just an ammeter in the circuit, and light shines on the diode junction from some external source, then a small electric current will flow, but in the "reverse" direction, with electrons coming out of the N-type material and going through the ammeter into the P-type half of the diode. This is the principle of the "solar cell," and an experiment on the phenomenon will be described in Chapter 23. 

The terms anti-barrier and non-directing junction are used in the further text both for isotype semiconductor homo- or heterojunctions and for the corresponding Schottky junctions, while the terms barrier and directing junction are used to denote p-n junctions (both homo- and heterojunctions) and Schottky diode junctions. 

The various dynamic resistances all depend on the area of the diode junction - the larger the area, the lower the resistance. To a good approximation, the product of R and the junction area Aj is a constant. Instead of reporting the dynamic resistance R, it is conventional to report and compare the RA products -RqAj, R Aj, and R Aj. That introduces one more possible source of miscommunication - we often don t know the junction area, and often people will use some other convenient area (the nominal, or pitch-based area, or the optical area). That is not a serious problem if we are clear on the convention used. 

From a brief consideration of the capacitance, C=sA/d, and the fact that the depletion region is the most resistive portion of the diode, it should not be surprising to learn that diode junctions have a significant capacitance. The effective separation of the capacitor plates in the junction is the depletion width, W. The resulting capacitance is voltage dependent as can be seen from Equation 3.20 and depends primarily on the concentration of dopant on the lightly-doped side of the junction as... 

This relationship is used in the capacitance-voltage technique for profiling carrier concentrations near diode junctions and is usable for any type of diode in which one side of the junction is much more heavily doped than the other. It can also be used in a transient mode to detect and analyze point defects as they charge and discharge with bias voltage changes. 

Not all diode junctions involve contacts between different doping types of the same semiconductor ( homojunctions ). The remainder, junctions between dissimilar materials or heterojunctions , are generally divided into metal/semiconductor and semiconductor/semiconductor junctions. We consider metal/semiconductor heterojunctions first. A sufficiently degenerate semiconductor behaves essentially as a metal and produces results nearly identical to the metal/semiconductor behavior. Metal/semiconductor junctions turn out to have either linear (ohmic) or diode-like current voltage characteristics and are called ohmic contacts or Schottky diodes, respectively. 

Dangling bonds in dislocation cores may affect doping and free carrier concentration to a large radius around the core and may act as shrmt paths in diode junctions. 

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