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MonMotha:

--- Quote from: SavannahLion on January 05, 2011, 08:28:53 pm ---Holy crap in a hand basket monmotha. I'm still trying to digest what you said. However something escapes me. Why aren't discrete FETs used more often as opposed to your typical PNP/NPN (These are the BJTs right?) transistors?
--- End quote ---

Yes, a BJT = "Bipolar Junction Transistor" aka your bog standard NPN/PNP stuff.

Discrete MOSFETs are actually used fairly often as load switches, these days, and it's very rare to see a SMPS design using a discrete switch that uses anything other than a MOSFET.  Small FETs are much more available in surface mount packages than traditional through-hole type packages, so you might not be noticing them in hobbyist designs as they can be inconvenient from that POV.

There also seem to be fewer "standard" MOSFET part numbers (compare to your 2N3904 type BJT parts that are relatively exactly crossed by everyone in the industry), which can make cross refs difficult.  This also may lead to less usage in hobbyist type projects.  BJTs also tend to be more available at retail (e.g. Radio Shack), which could again lead to hobbyist projects skewing towards bipolar devices.

BJTs also have some robustness advantages over MOSFETs at the expense of efficiency.  You can have a fairly small SOT-23 MOSFET switch a load that would require like a TO-220 bipolar transistor to switch due to the lower on-state losses and total lack of base current on the MOSFET, but the MOSFET can be ESD zapped fairly easily, and relatively minor failures such as floating the gate may (literally) blow the device up, depending on the load.  This is much less likely to happen with the giant bipolar transistor.  Hence, big bipolar transistors are still sometimes used in applications where size is not a factor and robustness is (such as lab bench supplies) whereas the MOSFET may be used in a similar power application where size and efficiency are a major concern, and the environment can be more carefully controlled to prevent failure (such as a mobile phone).

I think there may also still be a number of (mostly older) engineers doing designs that are very comfortable with bipolar transistors but less comfortable designing with FETs.  This may contribute to some "odd" decisions on commercial product designs.

SavannahLion:
Googling for "floating gate" gets me FGMOS. I take it that's not what your talking about. So when a gate floats, that means its "disconnected"? As in an input that's not tied to high or low?

MonMotha:
Ah yes, "floating gate" is also a term used in IC fabrication for something different.  Yes, when I said "floating the gate", I meant not driving it with anything i.e. leaving it disconnected.  This can happen inadvertently.  For example, many microcontrollers leave their IO pins in a floating (high impedance) state during reset since the software has not yet run to configure them as inputs, outputs, or whatever.

If the gate of a MOSFET is allowed to float, the gate may rise to a voltage sufficient to partially turn the MOSFET on but not enough to take it into its low on-resistance state.  If this happens on a MOSFET used as a switch for a large load, the MOSFET will conduct, but it will have much higher than intended drain to source voltage.  This may result in a rather large (several dozen or even hundreds of watts, in some cases) amount of power dissipated in the MOSFET, which can literally blow them up.  And people wonder why I wear safety glasses while troubleshooting SMPSes :)

(There is a solution to this, btw: put a pull up or down resistor on the gate of the MOSFET)

lilshawn:
I think it's just a cost or availability issue. Although superior in many ways, the FET is often more expensive than a transistor. In circuits where speed or current carrying ability are low, or just simpLe switching is needed, its more cost effective just to use a transistor.  A few pennies here and few there over 10's of components, over thousands of units adds up really quick!

Besides, the transistor has always been a transistor, the fet has gone through several incarnations from the old tube style to what you see now (grid, source, drain? Yes inside the vacuum tube was an actual GRID of metal that ions of electricity would emanate from)

From its crude beginnings...

MonMotha:

--- Quote from: lilshawn on January 06, 2011, 01:34:52 am ---I think it's just a cost or availability issue. Although superior in many ways, the FET is often more expensive than a transistor. In circuits where speed or current carrying ability are low, or just simpLe switching is needed, its more cost effective just to use a transistor.  A few pennies here and few there over 10's of components, over thousands of units adds up really quick!

Besides, the transistor has always been a transistor, the fet has gone through several incarnations from the old tube style to what you see now (grid, source, drain? Yes inside the vacuum tube was an actual GRID of metal that ions of electricity would emanate from)

From its crude beginnings...

--- End quote ---

I'm not sure about cost.  Discrete MOSFETs are CHEAP, now, possibly cheaper than many conventional bipolar devices, and they're waaaaay better in many applications than a BJT.  I use both: it depends on what I need to do.  Likely availability is more an issue than cost due to the aforementioned difficulty in cross referencing them should one supplier run out (which is happening a LOT these days).

I'm not aware of anyone ever calling the "gate" of a FET the "grid", though it may have happened and it is true that FETs are possibly the closest semiconductor equivalent to an amplifying tube (with the conventional 3-terminal FET being close to a triode; you can also get FETs with several gates just like you could get tubes with several grids aka tetrodes, pentodes, etc., but they're not overly common).  The internal physics aren't the same, but they're analogous.

It is true that there are several kinds of FETs, but MOSFETs are far and away the most commonly used and have been for years.  Just like BJTs and tubes, you can get them with varying spec tradeoffs.

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