Electronic – the difference between CPU transistors and generic ones

cpumosfetpower electronicssemiconductors

Recently I had to build a h-bridge driver for my project, wireless power transmission. I learnt that I needed to have dead-time in my PWM signal given to IR2110, which doesn't have any dead-time control function. As my model only requires below 200kHZ, I had to code the arduino uno which I used to generate the pwm signal, to have dead-time. My brain got stuck in a question, what is the difference between the generic mosfets and the mosfets/transistors used inside the processor? As modern CPU clock is nearly 5GHz while a normal CPU can have a clock of around 3Ghz. So how does they do it?

How do they reduce the turn-off delay in CPU? How do the mosfets inside a CPU switches so fast while a generic mosfet like IRFZ44N have a turn off delay time of 44ns according to the datasheet?

Thanks.

Best Answer

MOSFET load drivers are much, much larger than the FETs used on a CPU chip: they have to be to deal with the voltage and current they’re designed to drive. As a consequence of their size they have large gate capacitance and so take longer to change their gate voltage from ‘on’ to ‘off’ state. Also, to achieve low Rds(on) appropriate for a driver, they need higher Vgs(on) than a CPU FET. This adds even more to the charge burden, and thus the switching time.

In contrast, CPU transistors are physically small, work at low voltage, and are driving relatively tiny on-chip loads. They have small gate capacitance and a low Vgs(on) so they can switch very quickly. They’re also sometimes allowed to have an extra-low gate threshold and even a bit of leakage, so the gate voltage swing needed to switch them is reduced even further.