Question about FET symbols - Page 1
Forum section: electronics / beginners

author: LEECH666
author_rank: Frequent Contributor
timestamp: October 17, 2012, 07:36:32 pm
content: So yeah I haven't used FETs much so a question I've kept asking me is the meaning of the little line that sometimes connects "the arrow" with the source (see attached picture).Does this indicate the connection of source to the chassis / housing / tab of the device?There is some information here, but I don't really get it. I find it inconclusive.http://www.dnatechindia.com/Tutorial/Transistors/The-MOSFET.htmlSometimes there is some sort of diode connected internally across the source / drain. Should I change my symbols so that the diode is shown?Could someone more knowledgeable shed some light on this topic?Cheers,Florian

author: c4757p
author_rank: Super Contributor
timestamp: October 17, 2012, 08:06:38 pm
content: "The arrow" represents the substrate, or body. (This is distinct from the tab of the device, which is not shown in the symbol and is almost universally connected to the drain). All MOSFETs have a diode. This diode is formed by the interface between the substrate and the channel, so this is already shown by the substrate arrow. The extra diode that some symbols show is redundant. Almost all discrete MOSFETs have the substrate internally linked to the source, though there are some obsolete signal types that have a fourth pin for it.

author: LEECH666
author_rank: Frequent Contributor
timestamp: October 17, 2012, 08:42:46 pm
content: Okay thanks for explaining that to me. I thought Substrate = Tab, but apparently that's not the case. I guess I will alter my symbols now to always have that little line until I encounter one of these special devices that isn't connected like that (probably never).I think I'll omit the diodes then too.Cheers,Florian

author: c4757p
author_rank: Super Contributor
timestamp: October 17, 2012, 09:05:04 pm
content: On any type of transistor, any heat-removing part (the tab on TO-220, the rear heat sink on TO-247, the metal can of TO-18) is almost universally connected to the current-sinking lead (drain or collector). A few small-signal JFETs in metal cans used to have a separate lead for just the can, I assume to ground for shielding, but these have always been rare. (What good shielding the can will do when the leads themselves are exposed is beyond me.)

author: c4757p
author_rank: Super Contributor
timestamp: October 17, 2012, 09:07:36 pm
content: Interestingly, in a vacuum tube, it is also the anode (equivalent to the drain) that overheats if you dissipate too much power. I guess there must be some physics-related reason why heat dissipation occurs in or near the current sink electrode, but I don't understand the internal physics of the devices well enough to know. I'd have thought the heat would be dissipated around the grid (equivalent to the gate), which is where the current flow is actually restricted.

author: Colfaxmingo
author_rank: Contributor
timestamp: October 17, 2012, 09:18:34 pm
content: I suspect that it is the kinetic energy of moving electrons that cause this heating.The electrons being boiled off the cathode and hurled at the anode impart energy to it.

author: c4757p
author_rank: Super Contributor
timestamp: October 17, 2012, 09:27:04 pm
content: Colfaxmingo, well, er...... yeah.... that does seem kinda obvious now. God I feel dumb. Thank you!