Numerical simulation of spatially nonuniform switching in silicon avalanche sharpening diodes

Abstract

535Using the phenomenon of delayed avalanche breakdown in high voltage p + -n-n + diode structures, which takes place due to a fast buildup of the reverse voltage, it is possible to ensure device switching from the blocking to conducting state for a time below 100 ps [1][2][3]. Silicon avalanche sharpening (SAS) diodes and dynistors operating on this principle can provide subnanosecond switching of voltages with amplitudes up to 10 kV per device and constitute a basis of modern pulse power electronics [3][4][5][6].Despite numerous attempts at numerical simula tion of the subnanosecond switching process [6][7][8][9][10], no quantitative agreement between the results of mod eling and experiment has been achieved so far. This circumstance seems to be related primarily to insuffi cient knowledge of the physical mechanisms of switching. In particular, the mechanism of delayed triggering of the avalanche ionization, which takes place at a voltage that significantly exceeds the steady state breakdown value, has still not been unambigu ously established. The degree of uniformity of the switching process over the device area is also unclear. With respect to the mechanism of conductivity modu lation, all previous simulations of the switching pro cess [6-10] assumed that this mechanism was based on the plane ionization front propagation analogous to that in the TRAPATT diode.Recently, we have numerically simulated delayed impact ionization breakdown in a p + -n-n + diode structure [11] with allowance for the field enhanced electron emission from deep thermal defects in the n base. This approach provided for the first time a good agreement with experiment in respect of the switching voltage, thus confirming the hypothesis formulated previously [12] according to which the deep process induced thermal defects found in the base [13] can act as a source of free carriers that trigger delayed ava lanche breakdown. However, the calculated switching time [11] was several times as large as that observed in experiment. In addition, the voltage drop during switching of a model device exhibited a nonmono tonic character that was not observed in experiment. These discrepancies were probably related to the adopted assumption [11] of spatial uniformity of switching. In real devices, switching can take place over only part of the device area-either because of a transverse instability of the ionization front [14] and/or because of an initially nonuniform start of the front. Local switching in GaAs based structures has been experimentally observed by Vainshtein et al. [15].In the present work, the process of delayed ava lanche breakdown has been numerically simulated under the assumption that it takes place over only part of the p + -n-n + diode structure area. The aim was to study the dependence of the rate and character of switching on the area of the active part of the device structure involved in the switching process. It is estab lished that (i) a decrease in the active area sharply decreases the switching time and (ii) the mechanism of ...

Picosecond‐Range Avalanche Switching Initiated by a Steep High‐Voltage Pulse: Si Bulk Samples Versus Layered pn Junction Structures

“…The situation with p + nn + is more complicated since one‐dimensional simulations systematically overestimate the switching time: 150–200 ps versus less than 100 ps in experiments (see Figure ). The simulated switching time becomes shorter if it is assumed that only part of the device undergoes avalanche transient . This suggests that the avalanche switching in diodes may not develop uniformly in the transverse direction.…”

Picosecond-Range Avalanche Switching of High-Voltage Diodes: Si Versus GaAs Structures

“…The concept of travelling ionizing front that passes across the base faster than an individual carrier and generates a dense electronhole plasma behind is widely accepted (see [7] and the references therein). However, the switching time predicted by the ionizing front model is in quantitative disagreement with experiments [8]; thus, the mechanism of superfast conductivity modulation in SAS diodes still remains under discussion. Last but not least, it is not clear whether the switching is uniform across the device cross section or has filamentary character.…”

Joint effect of temperature and voltage rise rate on the switching process of Si thyristors triggered in impact ionization wave mode

“…Secondly, it has not been established how homogeneous the process of propagation of the impact-ionization front is over the area of the structure [16,22]. The only possible assumption to explain a number of experimental data and to obtain agreement between the experimental and calculated waveforms at the stage of voltage drop across the structure is that the front extends over a part of the area of the structure called an active area [12,15,[23][24][25].…”