Why a bipolar transistor is not symmetrical/reversible? - Page 1
Forum section: electronics / beginners

author: opablo
author_rank: Regular Contributor
timestamp: July 12, 2013, 01:19:45 pm
content: Everytime we see a model of the physical construction of a bipolar transistor... it looks symmetrical/reversible.What is missing ? What manufacturing process distinguishes the E from the C ?

author: amyk
author_rank: Super Contributor
timestamp: July 12, 2013, 01:36:14 pm
content: Because the actual structure looks more like this:

author: jpb
author_rank: Super Contributor
timestamp: July 12, 2013, 01:39:14 pm
content: Even in your diagram there is asymmetry. The Emitter region is doped n++ whilst the collector region is doped n+.My area of expertise is GaAs FETs rather than BJTs but I suspect that there will be physical dimensional differences as well as doping differences. The devices will need to be designed for a reasonable breakdown voltage, the base-emitter junction is generally forward-biased so will have a narrow depletion region whilst the base collector junction is reverse biased so will have a wide depletion region (and is lower doped) which will help breakdown (the voltage is dropped over a greater length so a lower peak field).If you reversed collector and emitter you would get some transistor action but it would not be a very good transistor and would probably breakdown at a low voltage.

author: opablo
author_rank: Regular Contributor
timestamp: July 12, 2013, 01:58:11 pm
content: I'm trying to understand...so...C = n+ = Phosphorus (a little) doped SiliconB = p = Boron doped SiliconE = n++ = Phosphorus (a lot) doped Silicon(I think that the notation could be better as n p nn instead of n+ p n++ because didn't n meant negative?)and if the depletion region between CB and BE is narrow or wide is not about if it works or not... it's about how to make it perform at it's best.did I got it right ?

author: free_electron
author_rank: Super Contributor
timestamp: July 12, 2013, 01:58:37 pm
content: Go read my post on the subject. It'll all become clear then.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 13, 2013, 02:42:57 am
content: Back in the Dreamtime,when sabre-toothed Wombats roamed the Earth,& such firms as Mullard & STC made transistors,therewasactually such a thing as a BJT which was reversible---It in fact was called a "Bipolar Transistor" to distinguish it from the common or garden varieties of BJTs that existed.Just as an aside,"point contact" transistors were also reversible,if I remember correctly.Yes,Virginia,there was such a thing as a "point contact " transistor!

author: 4to20Milliamps
author_rank: Regular Contributor
timestamp: July 13, 2013, 10:43:35 am
content: https://en.wikipedia.org/wiki/Bipolar_junction_transistorQuote from the article:"A cross section view of a BJT indicates that the collector–base junction has a much larger area than the emitter–base junction."

author: free_electron
author_rank: Super Contributor
timestamp: July 13, 2013, 12:53:09 pm
content: Point contacts were indeed symmetrical. Actually... They werent... Due to manufacturing tolerances they tested them and the orientation with the biggest gain was determined. Then emitter and base were denoted.Today the transistoris assymetrical. A side effectof this is that you have to be careful not to apply voltages above roughlt 5 volts across vbe in reverse.(even current limited ! )The be junction in reverse , above 5 volt, will start an electron migration process. You are essentially trapping electrons permanently in the base. This reduces conductivity of the base region (for an npn) and you slowly destroy the transistor. This process is almost irreversible (baking them above 300 c can remove the electrons, but there is residual damage)

author: vk6zgo
author_rank: Super Contributor
timestamp: July 13, 2013, 02:38:10 pm
content: The transistor which for a while,was called a "Bipolar Transistor" was a special device which for some unfathomable reason,wasspecially designed to be symmetrical & reversible.I'm pretty sure it was a British or European device.The problem was that the term "Bipolar transistor" was already in wide use for normal BJT devices,so it was a bit confusing.This was solved by the device falling out of use,so the possibility of confusion was removed.