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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?

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  • \$\begingroup\$ They are just less performant than enhancement transistors, and CMOS is an extablished technology now \$\endgroup\$ – clabacchio May 24 '12 at 20:30
  • \$\begingroup\$ With GaAs transistors, depletion mode has been and still seems to be more common than enhancement mode. Don't ask me why. \$\endgroup\$ – Telaclavo May 24 '12 at 23:05
  • \$\begingroup\$ Yes even I have wondered about that! Some additional info in case anyone stumbles on to this: Supertex makes some nice n-channel depletion mode mosfets, have use them: supertex.com/pdf/misc/d_mode_mosfets_SG_device.pdf They have application notes too! \$\endgroup\$ – user9957 May 25 '12 at 9:12
  • \$\begingroup\$ Yes, we know about NMOS, the question is specificly about PMOS depletion! \$\endgroup\$ – Federico Russo May 25 '12 at 9:15
  • \$\begingroup\$ "Operation without power: Passing analog video" discusses one particular application where depletion-mode transistors might come in handy. \$\endgroup\$ – davidcary May 31 '12 at 5:51
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Wiki says...

In a depletion-mode MOSFET, the device is normally ON at zero gate–source voltage. Such devices are used as load "resistors" in logic circuits (in depletion-load NMOS logic, for example). For N-type depletion-load devices, the threshold voltage might be about –3 V, so it could be turned off by pulling the gate 3 V negative (the drain, by comparison, is more positive than the source in NMOS). In PMOS, the polarities are reversed.

So for a depletion-mode PMOS it is normally ON at Zero volts but you need 3V or more on the gate higher than the supply voltage to turn OFF. Where do you get that voltage? I think , that's why it is uncommon.

In practise now we call them High Side Switches or Low Side switches for power MOSFETs. They prefer not to combine enhancement and depletion mode in the same chip as the processing costs are almost double. This patent defines some innovation and better physical desc. than I can remember. http://www.google.com/patents/US20100044796

It is possible though what you are suggesting and performance are key issues. However when it comes down to low ESR, MOSFETS are like voltage controlled switches with ESR changing over a wide range of DC voltages unlike bipolar transistors which are 0.6 to < 2V for max peak in some case. Also for MOSFETs it is constructive to think of them as having an impedance gain of 50 to 100 when looking at loads and ESR of source. So consider you need a 100 ohm source to drive 1 ohm MOSFET and 10 ohm source to drive a 10mΩ MOSFET if you use 100:1, Conservative is 50:1. This is ONLY important during the transition period of the switch, not the steady state gate current.

Whereas bipolar hFE drops dramatically so you consider hFe of 10 to 20 good when saturated for a power switch.

Also consider that MOSFETS as charge-controlled switches during transition, so you want to have a big charge available to drive the gate capacitance and load reflected into gate with a low ESR gate drive, if you to make a fast transition and avoid commutation ringing or bridge cross-over shorts. But that depends on design needs.

Hope that isn't too much info and the patent explains how it works for all modes of P N type depletion and enhancement in terms of device physics.

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  • 2
    \$\begingroup\$ One use I can imagine for a depletion-mode PFET with a ~2V switch-off point would be draining the storage caps for a device that can be switched on and off. Some devices (assemblies) will, when power is removed, sit for a long time with VDD around 0.6-0.8 volts. If such a device is connected to a processor input and outputs what it thinks is a "high", that may increase the processor's current consumption by dozens of microamps. Adding a depletion-mode PFET that would draw nothing when its device was "powered up", but draw a few hundred microamps as it powered down could help. \$\endgroup\$ – supercat May 25 '12 at 17:03
  • \$\begingroup\$ I think my key confusion was about what point the gate voltage is referenced to. Gate is always referenced to source, but source is the "positive" terminal in a PMOS, where it's the "negative" terminal in an NMOS. If the depletion PMOS was across your system's 5V rail, you'd need 6V (relative to system common) on the gate to hold it off. So like you said, where do you get that voltage? It should still work for my bootstrap circuit, I think, where I was using a resistor/zener to generate 15V to run a switcher whose output was 24V. Depletion PMOS would cut off the zener once 24V was established. \$\endgroup\$ – Stephen Collings May 29 '12 at 13:14
  • \$\begingroup\$ Upon further consideration, it still wouldn't work the way I intended. \$\endgroup\$ – Stephen Collings May 29 '12 at 13:20
  • \$\begingroup\$ So, Tony, your answer (excuse) is that this transistor category is simply not needed by industry. Right? \$\endgroup\$ – Ale..chenski Jan 12 '17 at 20:55
  • \$\begingroup\$ Yes and No, Yes for new design.. D-K still stocks about 80 N Depletion mode types but 0 P types and 6,105 Enhancement mode types (excuse me, give or take) \$\endgroup\$ – Sunnyskyguy EE75 Jan 12 '17 at 21:17

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