[solved] JFETs - Determining which is Source and which Drain purely electrically

[madires]

It sounds like you're describing some existing, un-named, transistor tester.

You seem to have missed the very first sentence of this topic: "I'm at the 'back of an envelope' stage in designing an automated transistor curve tracer."

It's a reply to RandallMcRee's post and the link he has given.

[Cerebus]

It sounds like you're describing some existing, un-named, transistor tester.

You seem to have missed the very first sentence of this topic: "I'm at the 'back of an envelope' stage in designing an automated transistor curve tracer."

It's a reply to RandallMcRee's post and the link he has given.

Weird, that post wasn't, to the best of my knowledge, there when I read yours and I'm pretty certain because I deliberately scanned back to see what you were talking about. Anybody know if this is a 'feature' of the forum software, that is, that it's possible for some readers to see out-of-order updates? Or am I just getting prematurely senile?

[Cerebus]

One caution on broad based testing. Some devices with incredibly small junctions
do not want their gate junction forward biased. Causes damage to the junction.
I recall this, out of my faulty memory, was a JFET problem. Damage was not necessarily
catastrophic, but changed device characteristics.

Yeah, it's worth saying that when testing any semiconductor device it's advisable to have a very low current limit on the signal you apply to any pin until you're certain what you're dealing with and certain that the pin in question will take substantial current safely - think of a limit on the close order of 1 uA.  A 'typical' small signal JFET will have a maximum forward bias current on the gate of around 10 mA but as you say, there are some that will tolerate much less.

Ditto you should heavily limit voltages between pins until you know what you've got - 0.6V would be safe (and of course the magic figure for identifying a Silicon PN junction), 5V would not be - I've seen MOSFETs with gate breakdown voltages on the close order of 5V. A PN junction will usually happily survive reverse breakdown if you keep the current really low but if you don't there may be trouble; ranging from h_FE changes in a bipolar transistor if you reverse breakdown a base-emitter diode with several mA to the release of magic smoke if you use 10's of mA. Needless to say exceeding the gate breakdown voltage on a MOSFET is a catastrophic mistake, as is reverse biasing an LED with any substantive current.

If one wants to be able to characterise both delicate RF MOSFETs and LEDs then one probably needs 'cautious' and 'no-so-cautious' modes to handle the differing current and voltage limits that will need to be used.

[T3sl4co1l]

MOS is obvious, for three-terminal devices: source is common to substrate, so you get a diode from S to D.

Four terminal MOSFETs have gate vs. "back gate" (substrate) ambiguity, but that is easily resolved (substrate acts like a JFET gate, since after all, it's a diode junction, not MOS).  Such devices are very old and (as far as I know) exclusive to RF, so may have symmetrical D/S construction.

A lot of the RF MOSFETs are designed for closely controlled impedances (think striplines) so probably are highly symmetrical. Not my territory really, I rarely go anywhere even remotely near the black arts of RF.

Why would that encourage symmetry?  As Vgs is what matters, there's probably priority to having lower ESR and ESL on that pin.

Most RF parts are designed for, well, whatever they fit into -- after all, a 50V 10A transistor isn't going to drive a 50 ohm transmission line very optimally!  Indeed, terminal impedances tend to be extremely low, a consequence of it being easier to make a transistor "wider" (more cells / surface area / perimeter in parallel) than "taller" (higher voltage, lighter doping).  This is reflected on package and circuit design, where wide flat terminals are used to keep the impedances low.  A matching network solves the rest, give or take how much bandwidth the application needs (narrow band: LC or microstrip; wideband: transformers or esoteric structures).

MMICs, and transistors made for similar purposes, tend to be easily matched (or prematched, or simply made for) 50 ohm (or near that) loads, though.

So, not that you've needed RF, nor probably ever will, but now you know...

Tim

Tim

[Cerebus]

MOS is obvious, for three-terminal devices: source is common to substrate, so you get a diode from S to D.

Four terminal MOSFETs have gate vs. "back gate" (substrate) ambiguity, but that is easily resolved (substrate acts like a JFET gate, since after all, it's a diode junction, not MOS).  Such devices are very old and (as far as I know) exclusive to RF, so may have symmetrical D/S construction.

A lot of the RF MOSFETs are designed for closely controlled impedances (think striplines) so probably are highly symmetrical. Not my territory really, I rarely go anywhere even remotely near the black arts of RF.

Why would that encourage symmetry?  As Vgs is what matters, there's probably priority to having lower ESR and ESL on that pin.

Most RF parts are designed for, well, whatever they fit into -- after all, a 50V 10A transistor isn't going to drive a 50 ohm transmission line very optimally!  Indeed, terminal impedances tend to be extremely low, a consequence of it being easier to make a transistor "wider" (more cells / surface area / perimeter in parallel) than "taller" (higher voltage, lighter doping).  This is reflected on package and circuit design, where wide flat terminals are used to keep the impedances low.  A matching network solves the rest, give or take how much bandwidth the application needs (narrow band: LC or microstrip; wideband: transformers or esoteric structures).

MMICs, and transistors made for similar purposes, tend to be easily matched (or prematched, or simply made for) 50 ohm (or near that) loads, though.

So, not that you've needed RF, nor probably ever will, but now you know...

Tim

Tim

Like I say, black art, mumbling men in robes, goats sacrificed - I generally try to keep away for fear of becoming the sacrifice. When someone shows me a milled aluminium block and claims it's tuned filter I don't know whether to be sceptical or awed.

But yeah, it was the strange four legged devices claiming to be normal semiconductors with 50 ohm matching that I was thinking of.

[T3sl4co1l]

Like I say, black art, mumbling men in robes, goats sacrificed - I generally try to keep away for fear of becoming the sacrifice. When someone shows me a milled aluminium block and claims it's tuned filter I don't know whether to be sceptical or awed.

But yeah, it was the strange four legged devices claiming to be normal semiconductors with 50 ohm matching that I was thinking of.

Goats?  Four legs?

Mr. Burns: Who's that goat-legged fellow, Smithers? I like the cut of his jib.
Smithers: Prince of Darkness, sir. He's your 11 o'clock.

Tim