In school, I was taught about PMOS and NMOS transistors, and about enhancement- and depletion-mode transistors. Here's the short version of what I understand:
Enhancement means that the channel is normally-closed. Depletion means the channel is normally-open.
NMOS means the channel is made of free electrons. PMOS means the channel is made of free holes.
Enhancement NMOS: positive gate voltage attracts electrons, opening the channel.
Enhancement PMOS: negative gate voltage attracts holes, opening the channel.
Depletion NMOS: negative gate voltage repels electrons, closing the channel.
Depletion PMOS: positive gate voltage repels holes, closing the channel.
It's been six years since I started doing design work for a living, and on at least one occasion I've wanted (or at least thought I wanted) a depletion PMOS transistor. It seemed like a good idea for a bootstrap circuit for a power supply, for example. Yet no such devices seem to exist.
Why are there no depletion PMOS transistors? Is my understanding of them flawed? Are they useless? Impossible to build? So expensive to build that a cheaper combination of other transistors is preferred? Or are they out there and I just don't know where to look?