Electronics > Projects, Designs, and Technical Stuff
Why isn't a PNP transistor architecture the opposite of NPN on IC's?
Kleinstein:
N+ stands for higher N doped material. There is not just the question of n or p doping, but also the parameter doping level.
The every PN junction acts like a diode. As all the transistors share a single piece of silicon there are usually also diodes where one does not need them. A common case are diode junctions towards the substrate. For this reason chips usually don't like an input voltage outside the supply range (at least at one side).
iMo:
The N+ (ie the above picture) is usually used under the metallic contacts, to avoid creation of a schottky diode..
IanB:
To add another dimension to the question, I recall that the early transistors were germanium based and tended to be PNP types. Would that be because germanium processes favored the fabrication of PNP transistors over NPN transistors?
magic:
--- Quote from: Beamin on November 08, 2019, 10:01:51 pm ---How is the parasitic diode formed? Is that undesirable?
--- End quote ---
What they didn't mark on the diagram is that the substrate and the isolation diffusions are P-doped and connected to ground / negative supply.
That's how collectors of NPNs and bases of PNPs are isolated from one another. There is no real dielectric isolation between them, only a reverse biased PN junction.
As a result, any of those nodes cannot be brought more than one diode drop below the negative supply.
T3sl4co1l:
--- Quote from: IanB on November 08, 2019, 11:09:50 pm ---To add another dimension to the question, I recall that the early transistors were germanium based and tended to be PNP types. Would that be because germanium processes favored the fabrication of PNP transistors over NPN transistors?
--- End quote ---
Not in ICs. That was a matter of materials, alloy junction transistors were made by melting indium into the crystal.
Or, I forget if there was reason to continue using PNP into the planar era?
Tim
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