Rodin, P.; Ebert, U.M.; Minarsky, A.; Grekhov, I.V. Document VersionPublisher's PDF, also known as Version of Record (includes final page, issue and volume numbers)Please check the document version of this publication:• A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. We present an analytical theory for impact ionization fronts in reversely biased p + -n-n + structures. The front propagates into a depleted n base with a velocity that exceeds the saturated drift velocity. The front passage generates a dense electron-hole plasma and in this way switches the structure from low to high conductivity. For a planar front we determine the concentration of the generated plasma, the maximum electric field, the front width, and the voltage over the n base as functions of front velocity and doping of the n base. The theory takes into account that drift velocities and impact ionization coefficients differ between electrons and holes, and it makes quantitative predictions for any semiconductor material possible.

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 ...

We discuss a new mode of ionization front passage in semiconductor structures. The front of avalanche ionization propagates into an intrinsic semiconductor with a constant electric field Em in presence of a small concentration of free nonequilibrium carriers -so called preionization. We show that if the profile of these initial carriers decays in the direction of the front propagation with a characteristic exponent λ, the front velocity is determined by v f ≈ 2βm/λ, where βm ≡ β(Em) is the corresponding ionization frequency. By a proper choice of the preionization profile one can achieve front velocities v f that exceed the saturated drift velocity vs by several orders of magnitude even in moderate electric fields. Our propagation mechanism differs from the one for well-known TRAPATT fronts. Finally, we discuss physical reasons for the appearance of preionization profiles with slow spatial decay.PACS numbers: 72.20.Ht, Propagation of impact ionization fronts in semiconductor structures represents a spectacular nonlinear effect 1,2,3,4 which has important applications in pulse power electronics. 5 In reverse-biased p + -n-n + diode structures ionizing fronts propagate faster than the saturated drift velocity v s . 1,2,3,4 Such superfast propagation is possible due to the presence of small concentrations n 0 ,p 0 of free electrons and holes in the depleted region. These free carriers which initiate an avalanche multiplication are often coined as "pre-ionization" of the medium. 1,6 According to the conventional concept of ionization fronts in TRAPATT (TRAped Plasma Avalanche Triggered Transit) diodes 7,8 the avalanche multiplication occurs within the ionization zone of length ℓ f = εε 0 (E m − E b )/qN d where electric field exceeds the effective threshold of impact ionization E b (Fig. 1, curve 1). This length is finite due to the slope of the electric field in the n base dE/dx = qN d /εε 0 which depends on the doping level N d (note that n 0 , p 0 ≪ N d ). The finiteness of the ionization zone ℓ f prevents a uniform avalanche multiplication in the whole n base and thus ensures the existence of the traveling front mode of avalanche breakdown. However, this concept is not applicable to p-i-n structures with intrinsic (N d = 0) base ( Fig.

We present numerical evidence of a novel propagation mode for superfast impact ionization fronts in high-voltage Si p + -n-n + structures. In nonlinear dynamics terms, this mode corresponds to a pulled front propagating into an unstable state in the regime of nonlocalized initial conditions. Before the front starts to travel, field-ehanced emission of electrons from deep-level impurities preionizes initially depleted n base creating spatially nonuniform free carriers profile. Impact ionization takes place in the whole high-field region. We find two ionizing fronts that propagate in opposite directions with velocities up to 10 times higher than the saturated drift velocity.

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