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This is more of an acquiring more knowledge question.

I have used a few ICs, such as the 741 op-amp, AND, OR, NAND gates, etc.

I have also used a few micro-controllers like the atmega32, atmega328, etc, which are made of the ICs.

However when the micro-controller are being manufactured what determines their operational voltage?

For example, an atmega chip requires 5V but why is it 5V, why couldn't it be designed to operate at low-voltages like 3.3V or lower. What component determines the operating voltage when these micro-controllers are being designed, also what determines the operating voltage of that component? Please don't say the datasheet...

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    \$\begingroup\$ Since you're not asking about power consumption, I think a more correct wording for your question is "When an IC is designed, how is the operating voltage selected?" This question seems too broad for a concise answer here. The answer could fill a textbook. A few of the factors are: compatibility with other circuits (other ICs, wiring to remote components, etc.), availability of power sources, switching speed, semiconductor material choice and immunity to noise. \$\endgroup\$
    – Theodore
    Apr 15, 2021 at 14:12

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However when the micro-controller are being manufactured what determines their operational voltage?

All ICs are fabricated in a certain manufacturing process. That process is the recipe to make the transistors and other components.

That process together with the design of the transistors (large or small) also determines what the maximum allowed voltages are that the transistors can handle.

That process also determines the maximum speed at which an IC can operate. It also determines the power consumption of that IC. Note that the design (the circuits in an IC) also plays a big role here, fast and complex circuits generally require more power than slow, simpel circuits (assuming both are made using the same technology).

If you would use the transistors above that maximum voltage the transistors can suffer damage. The lowest usable voltage is also process dependent, for CMOS technology the threshold voltage is an important factor there. That threshold voltage is also process dependent.

For example, an atmega chip requires 5V but why is it 5V, why couldn't it be designed to operate at low-voltages like 3.3V or lower

It can, look at the datasheet of an ATmega328p and see that it can operate down to a supply voltage of 1.8 V! It can go even lower but then you're going below the operating voltage at which the manufacturer guarantees proper operation. So you can use the ATmega328 at 1.5 V but if it shows weird behavior, it is your own fault for using such a low supply voltage.

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While Bimpelrekkie's answer addresses how manufacturing process technology and transistor sizing influence voltage (and speed and power consumption), the business reasons for choosing a higher voltage are also worth considering.

In some cases, microcontrollers are designed to be drop-in (or near-drop-in, e.g., adding some connections with localized board changes) replacements. For the customer this can reduce design, validation, and even certification effort. The maximum operational voltage is then constrained by legacy requirements.

Supporting a higher voltage can also reduce the count and cost of system components outside the microcontroller. Motors, e.g., often use voltages higher than 3.3V; supporting more direct control of motors can reduce cost and increase reliability.

(Resilience under voltage variation can also be a factor in purchase decisions. Process technology, transistor sizing, and other factors influence voltage resilience.)

The choice of process technology is also guided by manufacturing cost. Using older process technology is less expensive per unit area; if pad count constrains area, using an older process is more cost effective (if energy efficiency and such are adequate). Also, for tiny integrated circuits, the wafer area lost from sawing may be significant, reducing the benefit of a smaller die. The availability of persistent memory (such as NOR flash) also influences process technology choice and biases toward the use of older processes.

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