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My textbook has a brief explanation of how semiconductors are used in modern computer chips because they can be made to conduct (by applying enough voltage to excite electrons across the band gap) or to insulate (by dropping the voltage to a lower level or shutting it off entirely) and this conditional, binary state can be used as an 'on/off' logic gate.

In modern computer systems, which state is "on" and which is "off"? In other words, is it the case that when we increase the voltage to cause conductance in the semiconductor that we have a "1" value and that when we drop the voltage down, we have a "0" value, or is it the other way around?

Or is it more appropriate to say "read" vs "write"?

Does the on/off or read/write state vary from one manufacturer to another? Or from one type of memory to another? I.e. does DDR SDRAM use conducting as "on" and insulating as "off" while other forms do it the other way around?

P.S. If comp sci isn't the right SE, please could you advise a better SE for this question?

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In order to understand how physical background of the computers works, you should consider higher-level abstractions of the semiconductors. Otherwise, you will get stuck in a mess of semiconductors. Without basic knowledge of transistors and how these transistors form a logic gate, it will not be easy to comprehend the memory structures and read/write cycles.

To visualize, in CMOS technology, there are two complementary logic structures where one of them is connected to the ground and the other one is connnected to the high voltage source. In rest, these structures are "on" and "off" complementarily. When your output changes, on-off states of them are inverted, thus there are both 'on' and 'off' logic in every case. The scheme is completely different in another technology, let's say PTL.

In the memory case, 'HIGH' and 'LOW' voltages represent immediate memory state rather than individual 'conducting' and 'insulating' states of transistors. If a transistor between low voltage source and your memory output is 'conducting' while a transistor between high voltage source and your memory node is 'insulating', then you have a 'logic 0'. Still, all of them are technology dependent.

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In general it is conventional to use high voltage for true/one and low voltage for false/zero. In fact, a notation with the signal name overscored ($\overline{SIGNAL}$) is used to indicate active-low (alternatively SIGNAL#).

This convention is somewhat arbitrary, like programming languages associating true/false with 1/0. In addition, information can be encoded in less conventional ways (e.g., Gray code, memory scrambling, error correcting codes) and more than two voltage levels can be used.

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