An observation about the types of logic that can be etched into silicon.

One of the questions which I had blogged about before, was that, of whether the MOSFET transistor-type inherently has 3 pins or 4. This question has a practical aspect, which I did not mention in that posting, but which is eventually interesting.

When the very high-end Electrical Engineers design chips – ICs – and specifically, when they designed earlier-generation CMOS circuitry, they were not only limited by what the fundamental properties of a MOSFET were, but also, by how many layers the machines at the time could deposit onto the chip, each of which needed to be etched and treated in a separate, very precise stage of the manufacturing process. This is why I find it helpful that the WiKiPedia article I just linked to, displays a CMOS circuit, and how it was originally implemented, as their explanation of the subject.

What the reader may take note of, is the example of the P-channel MOSFET, which consists of a Source, a Drain, a Gate, a Bulk Electrode, and a Substrate. The role of this Bulk Electrode needs to be given some special attention. Because of the way these transistors were in fact etched, additional, unintentional, “parasitical” transistors could form, for example, a ‘hidden’ PNP, Bipolar Transistor, between the Drain, the N-doped well, and the P-doped substrate. In theory, if the N-doped well became negative enough, with respect to either the potential of the substrate or that of the P-doped Drain, then this parasitical transistor could become forward-biased, and start to conduct and amplify its own currents, with the Drain acting as Emitter, with the N-doped well acting as Base, and with the Substrate acting as Collector. The same thing could theoretically also happen, with the P-doped Source acting as Emitter instead.

The way this behaviour was prevented, was by connecting the N-doped well to the positive supply voltage, and always keeping the P-doped substrate connected to the negative supply voltage. This formed a so-called ‘isolation diode’, and prevented the parasitical transistor from becoming active.

Well in the circuit which I had clicked together using the NG-SPICE software, each MOSFET was a 4-pin component, and my main point of attention was on how to get my software to acknowledge whatever circuits I had entered. If the circuit in question needed to be etched, using the original technology, then the bulk electrode of each MOSFET would also need to be connected either to the negative supply voltage in the case of an N-channel MOSFET, or to the positive supply voltage in the case of a P-channel MOSFET. Hence, the existence of 3-pin MOSFETs was not due to whether the transistor-type was inherently so, but just due, to how certain forms of the technology were being manufactured, as consisting of a minimal number of layers. And this also forced the inclusion of the transistors as having 3 electrodes into certain schematics, just because their implementors could not implement the 4-electrode variety in certain cases – and in fact, often.

If the schematic was ever to be etched into silicon, as I drew it in my earlier posting, and as NG-SPICE was simulating it, then at the very least, a much-more recent form of Integrated Circuit would need to be used as architecture. And then one problem which would next follow would be, that this more-recent architecture also makes the transistors much smaller, such as 40 Nanometre or 10 Nanometre technology (?), which would result in individual transistors that cannot handle the amounts of current which discrete circuits require, so that the complications of driving output pins would become more pronounced.

One reason for which I did not elaborate this fact in my past posting was the realization that I’d have to link to yet-another WiKi-page in order to do so, and that WiKiPedia articles get edited from time to time. I did not know that the WiKi would keep the traditional layout of the CMOS layers a part of their article for so long.

Also, such use of isolation diodes within the chip to this day, pose a particular risk in the case of household appliances, into which the user is required to insert batteries. Some plausible risk exists, that the user could insert the battery in a reversed direction – let’s say because that user was distracted at the moment. And then, if the battery-potentials were simply applied to the chip as supply voltages, theirĀ  reversal could cause an unhealthy, high amount of current to flow, and could fry the chip. All the isolation diodes would become forward-biased.

I could well imagine that modern circuits make up for this, by putting some sort of protection circuit for such an eventuality. But whether a protection circuit against this scenario is in fact included in low-budget appliances is really up to chance, and users might not want to gamble with the health of such appliances. And so this presents yet-another good reason, always to insert batteries with the correct polarity. More could be at stake, than whether the appliance finally works or not, when its power is switched on.

Dirk

 

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