Fundamentals-of-Microelectronics-Behzad-Razavi.pdf

The breakdown phenomenon in junctions occurs by one

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The breakdown phenomenon in junctions occurs by one of two possible mechanisms: “Zener effect” and “avalanche effect.” 2.3.1 Zener Breakdown The depletion region in a junction contains atoms that have lost an electron or a hole and, therefore, provide no loosely-connected carriers. However, a high electric field in this region may impart enough energy to the remaining covalent electrons to tear them from their bonds [Fig. 2.36(a)]. Once freed, the electrons are accelerated by the field and swept to the side of the junction. This effect occurs at a field strength of about V/cm (1 V/ m). In order to create such high fields with reasonable voltages, a narrow depletion region is required, which from Eq. (2.76) translates to high doping levels on both sides of the junction (why?). Called the “Zener effect,” this type of breakdown appears for reverse bias voltages on the order of 3-8 V. This section can be skipped in a first reading.
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 54 (1) 54 Chap. 2 Basic Physics of Semiconductors Si Si Si Si Si e n p E e V R Si Si Si Si Si e e n p E V R e e e e e e e e e (a) (b) Figure 2.36 (a) Release of electrons due to high electric field, (b) avalanche effect. 2.3.2 Avalanche Breakdown Junctions with moderate or low doping levels ( cm ) generally exhibit no Zener break- down. But, as the reverse bias voltage across such devices increases, an avalanche effect takes place. Even though the leakage current is very small, each carrier entering the depletion region experiences a very high electric field and hence a large acceleration, thus gaining enough energy to break the electrons from their covalent bonds. Called “impact ionization,” this phenomenon can lead to avalanche: each electron freed by the impact may itself speed up so much in the field as to collide with another atom with sufficient energy, thereby freeing one more covalent-bond electron. Now, these two electrons may again acquire energy and cause more ionizing collisions, rapidly raising the number of free carriers. An interesting contrast between Zener and avalanche phenomena is that they display opposite temperature coefficients (TCs): has a negative TC for Zener effect and positive TC for avalanche effect. The two TCs cancel each other for V. For this reason, Zener diodes with 3.5-V rating find application in some voltage regulators. The Zener and avalanche breakdown effects do not damage the diodes if the resulting cur- rent remains below a certain limit given by the doping levels and the geometry of the junction. Both the breakdown voltage and the maximum allowable reverse current are specified by diode manufacturers. 2.4 Chapter Summary Silicon contains four atoms in its last orbital. It also contains a small number of free elec- trons at room temperature.
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