Jump to content

Mosfets, Jfets...?


Pott

Recommended Posts

  • Members

In short (without going too technical...) what are they..? Especially the difference between the two...

Also, if I understand right... a JFET has 3 prongs, one for bias, one output, one input?

Link to comment
Share on other sites

  • Members

In general the jFET is closer in requirements to a vacuum tube.

 

If you look at the voltages on the terminals for the jFET in a circuit, you see that the gate voltage for the jFET has to be at a voltage at or below the source voltage (these are assuming N MOSFET and N-channel jFET) since there is a diode between the gate and the channel. If you get the gate to postive, the diode starts to conduct and you lose that high input impedance, and usually the channel is fully on by this point. To turn off a jFET device, you need some negative voltage (hard to say exactly as it varies with the device and the randomness of the processing).

 

For most MOSFET devices, you have to have a positive voltage between the gate and the source for it to turn on. This voltage depends on a lot of parameters but in general you have to have between .7 and 2 1/2 volts gate-source for it to turn on. Often you can slam the gate with a lot more voltage and get the channel on much better.

 

Both of these devices are voltage controlled current sources, with the relationship between gate voltage and drain current controlled by a square law equation (sometimes the square gets smaller for some IC processes but that is getting ahead of the question).

Link to comment
Share on other sites

  • Members

 

Originally posted by Sir H C

In general the jFET is closer in requirements to a vacuum tube.



For most MOSFET devices, you have to have a positive voltage between the gate and the source for it to turn on. This voltage depends on a lot of parameters but in general you have to have between .7 and 2 1/2 volts gate-source for it to turn on. Often you can slam the gate with a lot more voltage and get the channel on much better.


 

 

What about Depletion mode MOSFETS? From what I remember, your description matches Enhancement mode MOSFETS, but you can also get sufficiently doped depletion mode MOSFETS that work like JFETS (without the junction issues). (Not 100% on the terminology but I don't have a semiconductor book at work)

 

Of course this is just was I remember from electronics class. Is it actually possible to buy depletion mode MOSFETs or are they just super rare?

Link to comment
Share on other sites

  • Members

 

Originally posted by hoerni



What about Depletion mode MOSFETS? From what I remember, your description matches Enhancement mode MOSFETS, but you can also get sufficiently doped depletion mode MOSFETS that work like JFETS (without the junction issues). (Not 100% on the terminology but I don't have a semiconductor book at work)


Of course this is just was I remember from electronics class. Is it actually possible to buy depletion mode MOSFETs or are they just super rare?

 

 

Yeah, didn't want to get too much into the whole depletion/enhancement MOSFET issues. I know of few depletion mode MOSFETs. I guess GaAs devices are there, but it is very sketchy to me as they also have the diode like connection of the gate to the channel. The closest to a depletion mode device would be native devices in modern IC processes. No one likes to have to add a rail to turn off a transistor.

Link to comment
Share on other sites

  • Members

 

Originally posted by Sir H C



Yeah, didn't want to get too much into the whole depletion/enhancement MOSFET issues. I know of few depletion mode MOSFETs. I guess GaAs devices are there, but it is very sketchy to me as they also have the diode like connection of the gate to the channel. The closest to a depletion mode device would be native devices in modern IC processes. No one likes to have to add a rail to turn off a transistor.

 

 

Okay, so I guess what you're saying is don't plan on running into these in audio circuits. I'm pretty sure I've seen dep. mode MOSFETS in the NTE charts; I wonder what they are being used for.

 

Didn't know GaAs was depletion mode. Cool.

Link to comment
Share on other sites

  • Members

Originally posted by Pott

... yes...


ok




Er


Mhmm


So... what do they do?
;)

If I understand, like tubes they make your signal stronger and help in clipping it. Right?

 

A FET is a voltage controlled current source. A nice analogy would be a garden hose where you step on it to control the flow. The hose is the drain-source path where the water flow = current, and your foot is the gate that controls this flow.

 

All active devices (FET, BJT, IC, Tube) are used to increase the signal either in current and/or in voltage.

Link to comment
Share on other sites

  • Members

 

Originally posted by hoerni



Okay, so I guess what you're saying is don't plan on running into these in audio circuits. I'm pretty sure I've seen dep. mode MOSFETS in the NTE charts; I wonder what they are being used for.


Didn't know GaAs was depletion mode. Cool.

 

 

Not sure. I think they are relatively old technology. I have not had to deal with them at all in the 10+ years I have been doing chips and the like.

Link to comment
Share on other sites

  • Members

 

Originally posted by Pott

Alright, thanks a lot!


Oh another quickie... Opamps are FETs too?

 

 

Op-amps can be bipolar, jFET/Bipolar, BI-CMOS, CMOS, or a host of other technologies. It gets pretty confusing in the end.

 

Hoerni-

 

I am wondering if you are seeing DMOS and not depletion devices. I forget what that D stands for, but these are usually vertical devices with the gate somehow working in conjunction with what is happening in the silicon. These are always asymetrical devices, the body (also called back gate for these) is tied to the source unlike it being a separate terminal in the standard CMOS devices.

Link to comment
Share on other sites

  • Members

 

Originally posted by Sir H C



Hoerni-


I am wondering if you are seeing DMOS and not depletion devices. I forget what that D stands for, but these are usually vertical devices with the gate somehow working in conjunction with what is happening in the silicon. These are always asymetrical devices, the body (also called back gate for these) is tied to the source unlike it being a separate terminal in the standard CMOS devices.

 

 

Not sure what it was I remembered. I just looked throuhg NTE's catalog and couldn't find any depletion mode devices or DMOS. Oh well, guess it's not important.

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...