With higher voltages (more than about 5V), microcontrollers may be damaged. Is this because the voltage actually physically damages them, or because it allows excessive current to flow? - and thus increases power dissipation beyond safe limits. How does this apply for other devices?
P,I,V are well known, but rarely designers and users pay attention to dV/dt.
In power electronics the damage is caused by dV/dt, say about 5000V/microsecond. At this speed the multiple layers of semiconductors (which very often have parasitic thyristor somewhere) open wide and cause avalanche of destructive events.
So it is possible to damage 1000V 200A device with momentary combination of much lesser current and voltage, because energy will dissipate in parts/places of structure different to normally expected.
Reverse biased PN junctions can only take so much voltage before they start to conduct. Sometimes they are designed for this, like zener diodes, but more often they're not. When multiple transistors are fab'd into an integrated circuit, reverse biased junctions can be used to isolate them. If you get one of these normally reverse biased junctions conducting, for example by exceeding the peak reverse voltage it can take, all sorts of unintended conduction paths can be opened, a cook the IC.
The most common form of electrical damage to things is overheating caused by total power dissipation. In many cases, one can safely get by either limiting voltage to a very low level and not worrying about current, or limiting current to a very low level and not worrying about voltage. There are some exceptions, though:
- It is possible for excessive voltage to cause a sudden current flow, or for excessive current to cause a sudden voltage drop, and these currents or voltage drops may be sufficiently localized that damage can occur with very small total power dissipation. In some cases, particularly with overvoltage, it may possible for localized capacitance to hold enough energy to damage the device even if current is externally limited.
- As others have noted, excessive voltage or current applied to a pin of a device which is powered may cause the device to enter a mode (such as latch-up) that converts a lot of supply power into heat. Even if the power into the over-voltage or over-current pin is limited, the supply may feed enough power to totally destroy the device.
- Overvoltage and overcurrent conditions may accelerate physical or chemical changes in a device sufficiently to cause it to fail prematurely or go out of spec; an electrolytic capacitor which is charged beyond its rated voltage, for example, may have its dielectric gradually get thicker as a result, reducing its capacitance. Note that such effects can cause damage even if power dissipation is slight and cooling is sufficient to prevent overheating.
Answer is, it depends on the device and how the voltage/current is applied.
If you put too much voltage on a CMOS transistors gate, then it'll punch through damaging it. Maybe enough that the circuit doesn't work, or maybe not. It's classic problem with analog IC's, they get zapped, then they get noisy. Same thing can happen to bi-polar transistors.
Classic failure in CMOS IC's is latch up, where a current spike flips on parasitic SCR's associated with CMOS transistors. Current then flows from VCC to ground, potentially overheating the device. And also high currents frying the protection diodes on inputs, causing them to leak.
And as the man said, dV/dt tends to kill power devices. Often because it causes them to partially turn on in localized areas, which then overheat and punch through. Which is why rolling your own motor controller usually results in big smoke.