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RE: Question on Base/Collector Junction

lee, I worked MAKING semiconductors….both integrated circuits and 'discrete' for over 30 years as a process tech.
I taught the basics to new people in some technology classes covering the basic process divisions of Photo, Etch, Metalization, Thin Films and Implant.
Suffice it to say that a 'transistor' is made from a piece of Very Pure silicon. Mostly 'grown' in an autoclave at very high temps and with fairly precisely controlled amount of dopant to produce either 'N' type or 'P' type….using Boron, Phosphorus or Arsenic as the main dopants. The substrate is what you might call 'single crystal' and all the atoms are nicely line up…….with some exceptions generally called 'dislocations'….which I won't go into.

When you have a piece of pure silicon….MOST LIKELY in the form of a disc from 150mm on UP….and the larger sizes prevail today for yield (how many good vs total devices on a wafer) and processing ease. The small stuff today….the 6"=150mm is LARGE by historical standards. When I started, it was in a 3" facility which 'upgraded' to 4" then an entire NEW facility was built to accommodate 6" material. Today? I'm out of touch but don't forget that doubling the diameter increased the area by 4X.

You start with a given type of silicon in a given crystal orientation. You than, thru various process steps selectively DOPE parts as 'n' and other parts as 'p', ending up with a NPN or PNP device ON A SINGLE PIECE OF SILICON. To do so requires a precise set of steps….each well controlled….and in a particular order under Very highly controlled conditions. For example, a typical furnace operation will be controlled to +-0.5C at temps from say…..900C to nearly 1200C for anything from 30 minutes at temperature to 4 or 5 hours….or even longer. You will 'cook' the wafer in various atmospheres…….Nitrogen is 'intert' while Oxygen will 'grow' an oxide, which is an insulating layer. ADD Hydrogen to the Oxygen? You get STEAM and the oxide grows very rapidly and potentially very thick. Sometimes as thick as 1.5 microns. Various photo steps coupled with etch will remove areas for OTHER treatments, like implant or diffusion of dopants into the silicon.

My weakest area is 'final test' where the complete devices are tested. The test equipment is highly automated and each defective part is marked so it goes NO FURTHER. The tests are for a dozen or more parameters. Various resistences are measured. Leakage current. Voltage thresholds. And so forth. Like I said, I'm weak in this area.

Please look at the link provided for a cross section of a transistor. You can see how it is built in layers. The link is for MOSFET devices, but other transistors are fabricated using similar schemes….some quite novel. The power devices I made for years had the drain on the Bottom of the device, not the top…..so the device was bonded inot the package with a conductive material which was connected to the drain. Gate and Source were on top. Look up 'HexFet' for a clearer description.
Too much is never enough


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  • RE: Question on Base/Collector Junction - pictureguy 17:36:26 02/13/14 (0)

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