Avalanche breakdown

At still higher fields carriers can acquke enough energy from motion in an electric field to create electron—hole paks by impact ionization. Eor siUcon the electron ioniza tion rate, which is the number of paks generated per cm of electron travel, depends exponentially on electric field. It is about 2 X 10 cm for a 50 kV/cm field at 300 K. The electric field causes electrons and holes so created to travel in opposite dkections. They may create other electron—hole paks causing positive feedback, which leads to avalanche breakdown at sufficiently high fields. 

Lp Pi 50 pm. and the reverse saturation current would be 17 x 10 = 17 pA for a square centimeter of junction area. Typical reverse saturation currents are about one thousand times greater as a result of generation—recombination currents in the depletion region (9). As the reverse voltage bias increases, the field increases in the depletion region until avalanche breakdown occurs, resulting in the characteristic shown in Figure 7. 

Em is limited by the breakdown field strength Ebd of silicon, which is about 3x 105 V cm4. The figure on the inner front cover shows the width of the SCR as a function of doping density and applied bias, as well as the limitation by avalanche breakdown. 

For n-type electrodes with doping densities below 1018 cm-3, avalanche breakdown in the SCR dominates for an anodization bias in excess of about 10 V. Avalanche breakdown corresponds to a radius of curvature in excess of about 100 nm and is proposed to be the cause of the formation of large etch pits, which will be termed macropits, as discussed in Chapter 8. 

For moderately doped substrates the crossover from tunneling to avalanche breakdown occurs at pore diameters of about 500 nm, corresponding to a bias in excess of 10 V. Above doping densities of 1017 cm-3 breakdown is always dominated by tunneling. Tunneling is therefore expected to dominate all pore formation in the mesoporous regime and extends well into the lower macropore regime, while avalanche breakdown is expected to produce structures of macropor-ous size. 

For n-type doping densities below 1017 cm-3 and an anodization bias above 10 V, avalanche breakdown becomes relevant. The interface morphology generated in this regime is very complex and shows large etch pits, macropores and mesopores. The formation of this structure is not understood in detail. A hypothetical model will be discussed in Section 8.5. 

Etch Pit Formation by Avalanche Breakdown in Low-Doped n-Type Silicon... 

In contrast to p-type electrodes, an n-type electrode is under reverse conditions in the anodic regime. This has several consequences for pore formation. Significant currents in a reverse biased Schottky diode are expected under breakdown conditions or if injected or photogenerated minority carriers can be collected. Breakdown at the pore tip due to tunneling generates mainly mesopores, while avalanche breakdown forms larger etch pits. Both cases are discussed in Chapter 8. Macropore formation by collection of minority carriers is understood in detail and a quantitative description is possible [Le9], which is in contrast to the pore formation mechanisms discussed so far. 

The different pumping methods, such as the commonly used current injection or optical pumping electron beam pumping and avalanche breakdown have been studied in detail (for further refs, see and information has been obtained regarding the excitation probabilities of the different interband transitions. The very short laser pulses (less than 10 sec) obtained enable rapid processes and their time dependence to be studied. 

A problem with the monolithic arrays is that the techniques for building metal-oxide-semiconductor (MOS) devices in silicon cannot be transferred intact to narrow bandgap materials such as mercury cadmium telluride, mainly due to tunneling and avalanche breakdown occuring at very low voltages. A monolithic array, in which read-out electronics is integrated in the same mercury cadmium telluride chip as the infrared detectors, is therefore difficult to achieve. 

Another source of additional holes for anodic etching of n-type semiconductors is the avalanche breakdown. The break-... 

Holway Jr., L.H., Mid Fradin, D.W. (1975) Elecfron Avalanche Breakdown by Laser Radiation in Insulting... 

FIGURE 1.17. Illustration of the breakdown of the potential barrier at the semiconductor/electrolyte interface, (a) Zener breakdown (b) avalanche breakdown (c) interface tunneling. 

The field required for breakdown to occur and the mode of breakdown depend on doping level. As the dopant concentration increases, the width of the space charge layer decreases and the probability of tunneling increases rapidly so that Z mr breakdown becomes more likely than avalanche breakdown. Zener breakdown is, in general, involved in the electrode processes on p and n materials under a reverse bias. 

Figure 1.76 Band models of the various oxide systems. The oxides can be placed into four groups regarding their suitability as ultra-thin dielectric film. Performance degrades from left to right, (a) Ta205, AI2O3 and SiC>2 show an amorphous structure. This is essential for ultra-thin dielectric films, as the continuum oflocalized states in the mobility gap act as electron traps. This prevents electron avalanche breakdown resulting in highest bdv (breakdown voltage).

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