5 ms·

Author here for all your voltage reference questions :-)

• monocasa 5y ago
  Is there any chance you can expand on the reasoning behind the circular/coaxial designed transistors? The normal sorta logic style transistors are straight forward, and the serpentine nature of the high current transistors makes sense for the surface area, but I never understood the reasoning for the round transistors.
  • kens 5y ago
    Theoretically you could make a PNP transistor by reversing the doping of an NPN transistor. The main problem is that boron diffuses rapidly, making it hard to fabricate a buried P-layer. Boron also has less solubility than phosphorus, making it hard to dope the emitter. Also, holes have only 1/3 the mobility of electrons, so PNP and NPN aren't symmetrical. To deal with these issues, PNP transistors are usually built with lateral construction (i.e. horizontally). The ring structure ensures that almost all of the carriers injected by the emitter are intercepted by the collector. (This is based on The Art of Analog Layout, p280. I don't know all this doping stuff myself.)
    • exmadscientist 5y ago
      It's worth noting that all of this mess was solvable, and made economical. Good (or at least fast) PNP and NPN transistors are now available together with modern complementary bipolar processes. These processes are in mass production and, these days, barely more expensive than the traditional ones even if they often involve fun things like SiGe.
  • adrian_b 5y ago
    For this case of lateral PNP transistors, the reason is as Ken has said. Because both the emitter and the collector are on the surface, making them both circular ensures that the distance between them, which is the same as the width of the base, is constant. The properties of the bipolar transistors vary very strongly with the width of the base. If the width is not constant, then the current becomes crowded in only a part of the base and many characteristics become worse. Unlike in lateral transistors, in vertical transistors the width of the base is not determined by geometry, but by doping doses and diffusion times, so the form of the emitter is less important. Nonetheless, in early planar transistors the emitter was also circular. The reason is that in bipolar transistors with very narrow bases, the resistance of the narrow base layer becomes large and in the center of the base under the emitter, the base-emitter voltage drops to a lower value than at the terminals of the transistor, which makes the central part of the emitter and base non-functional (i.e. only a very small fraction of the current passes through there). So in vertical transistors, only the periphery of the emitter matters. When it is circular, the symmetry guarantees that the current is uniformly distributed on the periphery, for maximum current capability. Unfortunately, increasing the density of current per peripheral length of the emitter over a threshold triggers a positive feedback that will destroy the transistor if the current is not limited externally. This is usually the main factor that determines the specification of a maximum current for a bipolar transistor. If the current is non-uniform over the periphery, the threshold will be reached at a much lower current than computed by multiplying the threshold density with emitter perimeter. Because there is a limit for amperes per millimeter of emitter periphery, to increase the maximum current in a given area, the form of the emitter must be changed from a circle to a form with a longer perimeter, without increasing the occupied area. Early power transistors had various fancy forms for the emitters, e.g. Christmas tree, snow flake and so on. However, it was quite difficult to ensure that the current is distributed uniformly on the periphery of such complex forms. Later, after the photolithography had improved and smaller dimensions were no longer problem, instead of having an emitter with a complex sinuous boundary, 2 simpler solutions have been adopted to increase the perimeter of the emitter. Either the transistor had a large number of small emitters connected in parallel, or it had one large emitter, but with a large number of small holes in the emitter (mesh emitter).
• cushychicken 5y ago
  Got any good references for how to identify silicon structures as certain components?
  • kens 5y ago
    I got an old, cheap copy of "The Art of Analog Layout" (Hastings), which describes these structures in detail. For the most part, the structures are fairly easy to recognize after you've seen them once or twice. But then there are the bizarre mystery circuits that require some puzzle-solving. For instance, where they combine a couple of transistors to save a bit of space.
    • dasudasu 5y ago
      Seeing that the latest edition of The Art of Analog Layout came out of 2005, is there any other book you'd recommend on analog layouts specifically?
      • DiabloD3 5y ago
        I'd hazard a guess that the 20 to 50 year old tech described in that book hasn't changed much; anything that is novel is probably still under patent and will someday make it into a newer edition.
      • kens 5y ago
        I mostly look at chips from the 1970s, since modern chips have features that are too small for my microscope. So I don't have any recommendations for a "modern" book.
• colejohnson66 5y ago
  How do you tell if a chip’s substrate is N or P?
  • kens 5y ago
    There are electrical ways to determine it, but I don't have an easy way to tell.
• grishka 5y ago
  Why am I getting "access denied" when trying to open your website? I had to use a VPN to get past this.
  • kens 5y ago
    Are you trying to access from Russia? Russia is currently blocked.
    • grishka 5y ago
      Yes. Why?
      • kens 5y ago
        Because I consider Russia's behavior in Ukraine to be unacceptable.
        • blacklion 5y ago
          I think, you mostly banish people who are against war. Politics, bandits, marauders don't read about in-depth silicone analysis.
        • kimmik 5y ago
        • grishka 5y ago
          There's the government, and there's the people. These are very separate entities. You can't combine them like that. You're effectively punishing Russian people for something they have no say in. There is no public opinion in this country and the government lacks any meaningful feedback mechanisms.
          • chipsa 5y ago
            Sanctions largely punish the people, not the government, and we implement them anyways. There's ways to feedback, even if they aren't petitioning the government for redress of grievances. The US infamously has a saying about four boxes of liberty: "Soap box, ballot box, jury box, ..." Given the first three are non-functional, there remains the fourth.