4 ms·

Any ideas on why they'd use NMOS [1] instead of CMOS if they're aiming for low power? Too many layers? My (amateur, outdated, and probably wrong) recollection i

Any ideas on why they'd use NMOS [1] instead of CMOS if they're aiming for low power? Too many layers? My (amateur, outdated, and probably wrong) recollection is that CMOS dissipates very little power except when switching, but not so for NMOS. [1] https://www.pragmaticsemi.com/app/uploads/2023/07/Pragmatic-Pocket-Guide-to-Designing-for-FlexICs.pdf https://www.pragmaticsemi.com/app/uploads/2023/07/Pragmatic-...

dragontamer 2y ago
CMOS requires very tight tolerances for the PMOS + NMOS transistors to cancel each other out exactly. Otherwise it doesn't work at all. In particular, you need to ensure that the PMOS and NMOS turn on at the same voltage, otherwise you risk just shorting Vcc to Vss (aka: Power to Ground). NMOS was common in the 1970s before CMOS on silicon was figured out. I'm surprised to hear that this circuit is old-school NMOS, but I probably shouldn't be, as the CMOS step took a lot of research and effort back then.... If we're still at NMOS stage of production on this process, then its probably more relevant to think of analog-based designs. CMOS seems necessary if anyone is to achieve low-power modern-like designs. NMOS was still core to a lot of older chips though, so digital logic still can work on that. But the power consumption will be necessarily huge in comparison to CMOS.
adrian_b 2y ago
Only in few semiconductor materials it is possible to make good transistors with both polarities, silicon, germanium and the silicon-germanium alloy being the main examples. In most semiconductor materials it is possible to make good transistors only with a single polarity, either N or P, depending on the material. So CMOS logic cannot be implemented in most semiconductor materials. This is the main reason why silicon has remained the principal material for complex logic circuits, even if there are a lot of materials with much better properties, except for allowing both polarities for transistors. In the first decades of the semiconductor industry, the main advantage of silicon had been that it was possible to create a high-quality insulator layer on its surface by oxidation. However there are many years since this advantage is no longer relevant for high-density logic circuits, because all their MOS transistors use now insulators with high dielectric constant, e.g. based on hafnia, which are deposited on the surface of silicon in the same way they would be deposited on any other semiconductor.

dragontamer 2y ago

CMOS requires very tight tolerances for the PMOS + NMOS transistors to cancel each other out exactly. Otherwise it doesn't work at all. In particular, you need to ensure that the PMOS and NMOS turn on at the same voltage, otherwise you risk just shorting Vcc to Vss (aka: Power to Ground). NMOS was common in the 1970s before CMOS on silicon was figured out. I'm surprised to hear that this circuit is old-school NMOS, but I probably shouldn't be, as the CMOS step took a lot of research and effort back then.... If we're still at NMOS stage of production on this process, then its probably more relevant to think of analog-based designs. CMOS seems necessary if anyone is to achieve low-power modern-like designs. NMOS was still core to a lot of older chips though, so digital logic still can work on that. But the power consumption will be necessarily huge in comparison to CMOS.

adrian_b 2y ago

Only in few semiconductor materials it is possible to make good transistors with both polarities, silicon, germanium and the silicon-germanium alloy being the main examples. In most semiconductor materials it is possible to make good transistors only with a single polarity, either N or P, depending on the material. So CMOS logic cannot be implemented in most semiconductor materials. This is the main reason why silicon has remained the principal material for complex logic circuits, even if there are a lot of materials with much better properties, except for allowing both polarities for transistors. In the first decades of the semiconductor industry, the main advantage of silicon had been that it was possible to create a high-quality insulator layer on its surface by oxidation. However there are many years since this advantage is no longer relevant for high-density logic circuits, because all their MOS transistors use now insulators with high dielectric constant, e.g. based on hafnia, which are deposited on the surface of silicon in the same way they would be deposited on any other semiconductor.