Help me understand Avalanche Transistors - Page 1
Forum section: electronics / beginners

author: PbFoot
author_rank: Contributor
timestamp: December 20, 2012, 02:31:57 am
content: Hi All,I've been trying to understand exactly what is happening in simple terms, but the math is well beyond my level. I'd be grateful of someone could give me the basics of what goes on, so I might get a grasp on it.I intend to build a pulsed driver for a high powered laser diode. Of course, it can only tolerate very short pulses (not more than 50ns) at high currents.Thanks!- PbFoot

author: IanB
author_rank: Super Contributor
timestamp: December 20, 2012, 02:50:46 am
content: I'm a complete non-expert on the subject, but I imagine that understanding what is happening in simple terms on the one hand, and designing a working circuit on the other hand, are two entirely separate things.However, in simple terms you have avalanche breakdown in the conduction path. A tiny impulse can produce a huge response, just like a tiny disturbance on a mountainside can cause millions of tons of ice and snow to fall.In the semiconductor case, higher voltage differences give electrons more energy. At some point, the electrons have enough energy to knock other electrons free of the atoms holding on to them. These newly freed electrons immediately get accelerated by the electric field and smash into other atoms, knocking yet more electrons free of their grasp. Very quickly an avalanche of free electrons forms resulting in a huge current forming very fast.So in principle, you have an unstable system, just waiting to break down. It only takes a tiny impulse directed in the right way to precipitate the breakdown and produce an avalanche current.The mathematics behind what happens evidently is complex, so knowing the above simplified explanation will in no way help to design a good circuit using this principle. If you want to build such a circuit, make sure that someone else has designed and tested it for you first.

author: PbFoot
author_rank: Contributor
timestamp: December 20, 2012, 02:54:48 am
content: Hi,Thanks! I'm going to use one of the tested designs that are already out there on the net. I just wanted to get an idea of what is going on instead of just plugging in parts. I am starting to get the concept I think. I reading what I can, but I am not getting it all.If anyone else has more to contribute, I am looking forward to it!- PbFoot

author: Kremmen
author_rank: Super Contributor
timestamp: December 20, 2012, 07:45:45 am
content: OK, since you ask.The phenomenon of course is an avalanche breakdown as the name of the component implies. This phenomenon exists in all transistors, not only those specifically designated as such. But it is of course most consistent in the special ones.Firstly, a rising Vce will cause a corresponding rise in the electric field intensity of the transistor junction. Free electrons (well, all charge carriers really) are accelerated by this electric field, gaining kinetic energy in the process. After a critical value of Vce is exceeded, the kinetic energy of those electrons exceed the bandgap energy. Now when the electron finally collides with an atom of the substrate lattice, the resulting energy transfer is sufficient to excite one of the target atom's shell electrons to the conduction band. Thus another electron and a hole have been created in what is essentially an Auger effect (http://en.wikipedia.org/wiki/Auger_effect). All of these old and new charge carriers are in turn accelerated by the junction field, giving raise to further excitation events and constituting an avalanche chain reaction resulting in uncontrolled increase in the junction current. The avalanche proceeds rapidly, on the order of tens or hundreds of picoseconds making an avalanche transistor a very fast switch.

author: mazurov
author_rank: Frequent Contributor
timestamp: December 20, 2012, 08:16:52 pm
content: This paper is useful too (scroll to page 3 for explanation of pulse generation by avalanche transistor) ->http://globaltek.us.com/wp-content/uploads/2012/01/AN-191.pdf