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If I understood JFET diagram correctly, the p-n junction is only used to isolate gate. Using same principle, would it be possible to construct a MOSFET with no junction at all, i. e. a single semiconductor type transistor?

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The answer to your question is yes a single N or P type depletion mode mosfets can be built. If you do a search on TFT transistors you will find how they are use in LCD displays. This is from a my solid state devices text book, which is very old;

TFT single semiconductor type

Usually depletion mode mosfets are built on a substrate of the opposite type of material. That is because it is easy to do with current fabrication methods. The substrate is not needed for the opporation of the device.

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  • \$\begingroup\$ Can you name your devices textbook, btw? \$\endgroup\$ – ThreePhaseEel Nov 13 '16 at 16:55
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    \$\begingroup\$ The textbook is "Solid State Electronic Devices" by Streetman published by Prentice Hall 1972. \$\endgroup\$ – owg60 Nov 13 '16 at 17:00
  • \$\begingroup\$ So, to be sure, can a MOSFET (as per the question) be built with no junction at all and is there any having data sheets? \$\endgroup\$ – Andy aka Nov 13 '16 at 18:55
  • \$\begingroup\$ Yes, a mosfet can be built with no junction at all. That is what the picture is showing. A semiconductor is deposited on a glass insulator. One end is the drain. The other is the source. A gate insulated from the channel is added to control current flow. I do know of any devices like this for sale with a datasheet. I also think that most TFTs used in LCD displays are probably run in enhancement mode. \$\endgroup\$ – owg60 Nov 13 '16 at 20:52
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would it be possible to construct a MOSFET with no junction at all, i. e. a single semiconductor type transistor?

enter image description here

The above is for a P channel JFET as linked in the question.

The gate source region needs to form a depletion layer so that the P channel can be progressively cut-off thus restricting current flow from source to drain. Without the N type material at the gate, a depletion region cannot be formed therefore you cannot control the drain current other than by brute force; taking current away from the source (via the gate) that would otherwise get to the drain.

That's not a transistor any more as far as I see it.

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  • \$\begingroup\$ So, a depletion region wouldn't form without an opposite type semiconductor (by just applying electric field)? \$\endgroup\$ – valplo Nov 13 '16 at 11:47
  • \$\begingroup\$ @valplo you got to ask yourself: why do PN diodes work in forward direction? What would happen if I took away the N, or the P part? You'll find that you'd have built something that doesn't continously allow current to flow. So, no, without the N zone, you can't build any kind of transistor. \$\endgroup\$ – Marcus Müller Nov 13 '16 at 11:53
  • \$\begingroup\$ Both P and N material are not needed to create a depletion region. Take for example the Schottky diode. en.wikipedia.org/wiki/Schottky_diode \$\endgroup\$ – owg60 Nov 13 '16 at 16:53
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would it be possible to construct a MOSFET with no junction at all

No because it is the reverse biased PN junction which provides the isolation needed to turn the device off, meaning no current can flow.

A single bar of N or P type semiconductor in general behaves as a resistor. Certain types of on-chip resistors are made this way.

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