Q1Analog Electronics

Depletion-type MOSFET is called Normally ON because a conducting channel is physically implanted (doped) during fabrication. At $V_{GS}=0$, drain current $I_D=I_{DSS}$ flows through this existing channel. The channel can be depleted (reduced) by applying a negative $V_{GS}$ (for N-channel) or enhanced by positive $V_{GS}$.

Enhancement-type MOSFET is called Normally OFF because no channel exists at $V_{GS}=0$. Drain current is zero until $V_{GS}$ exceeds a threshold voltage $V_{TH}$. Only then is a channel induced (enhanced) by the electric field of the gate. For an N-channel device, a positive $V_{GS}>V_{TH}$ is required.