enhancement and depletion mode

MOSFETs come in two flavours that differ in their resting state. An enhancement-mode device is normally off: with zero gate voltage there is no channel, and you must ENHANCE (build up) a channel by applying gate voltage to make it conduct. A depletion-mode device is normally on: a channel already exists at zero gate voltage, and you DEPLETE (squeeze away) that channel with the opposite gate voltage to turn it off. Think of a door that is shut by default versus one that is open by default.

For an N-channel device the difference shows up in the threshold sign. An enhancement N-channel MOSFET needs a positive gate-to-source voltage (say above +2 V) to switch on. A depletion N-channel MOSFET conducts at 0 V and needs a negative gate voltage to switch off. Enhancement-mode is overwhelmingly the common type: nearly every switching MOSFET and every digital-logic MOSFET you meet is enhancement-mode, because normally-off is the safe and convenient default.

Why does it matter? A normally-off (enhancement) switch is fail-safe: if the gate drive is missing or the controller has not started, the load stays off rather than blasting on. Depletion devices are rarer and show up in special roles like constant-current sources or self-biased RF stages. The JFET, by its nature, is always a depletion device. When you read a datasheet, the mode tells you instantly what the part does with no gate drive applied.