Interaction of traveling current filaments and its relation to a nontrivial thermal breakdown scenario in avalanching bipolar transistor

Abstract

Traveling multiple current filaments (CFs) are investigated by transient interferometric mapping method in avalanching bipolar n-p-n transistors. The number of CFs can vary for identical current pulses and their averaged number increases with the total current. The CF movement is driven by a temperature gradient in it, caused by the self-heating effect. For pulses of 500 ns duration, the existence of two CFs appears dangerous as it causes a nontrivial premature thermal breakdown (TB), which does not occur when only one CF exists at the same current level. TB occurs due to redistribution of current between the two CFs. The current components flowing through each CF depend on CF temperature and are globally coupled by a fixed device current. When a first CF reaches the device end, it heats up and disappears due to vanishing impact ionization rate in it. When a second traveling CF, taking consequently the whole current, reaches the already preheated device end, a TB event occurs. The transition from two to one filament mode is accompanied by a voltage step of 1 V, which is related to a finite resistance of CF access region.

Trap‐Related Breakdown and Filamentary Conduction in Carbon Doped GaN

“…A CF with size larger than the maximum size becomes unstable and therefore splits to smaller CFs, which is also observed in our experiments (Figures and ). Within the stability interval, CFs with different numbers and sizes can appear at the same bias, which is also observed in Figure .…”

“…We think this is due to the fact that the series resistance R com is much larger than R pas (see Figure e). As a result, we think the voltage steps related to I–V branches with different number of CFs are very small and cannot be distinguished from noise in our experiments.…”

“…n‐GaN/TL/Si‐substrate) system. Previously it has been shown that I – V curves related to different numbers of CFs exhibit a zig‐zag behavior related to potential drop on R pas due to different current flowing through a CF . A hypothetical filamentary I – V curve for a device with a maximum number of CFs of four (NF = 4) and for is shown in Figure c.…”

“…The corresponding I – V part is denoted here by the “real homogeneous on‐state”; the naming will be obvious later. However, in case when the active region is in series with a passive Ohmic layer (here it is the n‐GaN layer), formation of multiple filaments, rather than formation of one large CF, is more favorable . Figure a shows schematics of current flow lines in the GaN:C/n‐GaN system aligned to current density distribution (b), conduction band minimum (c), and assumed charge density (d) distribution in GaN:C for such a situation.…”