Does the arrow in a mosfet has a particular meaning vs internal diodes ? - Page 1
Forum section: electronics / beginners

author: tigrou
author_rank: Regular Contributor
timestamp: August 20, 2024, 12:05:43 pm
content: AFAIK a mosfet (P or N channel) as two internal diodes. One of those is shorted out by connecting thegatebody and the source.The arrow on the mosfet symbol can be used to distinguish between the P or N channel.Is the direction of that arrow just a convention, or does it have a particular meaning regarding those diodes ?

author: PGPG
author_rank: Frequent Contributor
timestamp: August 20, 2024, 12:32:23 pm
content: Quote from: tigrou on August 20, 2024, 12:05:43 pmYou have to know that for N MOSFET you can set its Gate at -10V and at +10V regarding its Source. So there certainly is no diode to be shorted by connecting Gate with Source.See in table at first page of datasheet:https://www.mouser.pl/datasheet/2/196/Infineon_IRF520N_DataSheet_v01_01_EN-3362783.pdfVgs Gate-to-Source Voltage ±20V

author: tigrou
author_rank: Regular Contributor
timestamp: August 20, 2024, 12:34:43 pm
content: Quote from: PGPG on August 20, 2024, 12:32:23 pmSorry I meant "body".

author: wraper
author_rank: Unknown
timestamp: August 20, 2024, 12:44:03 pm
content: While there are two intrinsic/parasitic diodes between the body and drain/source one of them is already shorted by internal connection of body to the source in 99.99% of parts. These parasitic diodes have nothing to do with MOSFET operation otherwise. What you see on the symbol as center plate with an arrow connected to the source is that body. I suppose arrow indicates body type (N or P). If it indicated that parasitic diode it basically would need to be drawn with a short connection in parallel to it and in opposite direction.

author: magic
author_rank: Super Contributor
timestamp: August 20, 2024, 01:44:32 pm
content: The arrow is drawn between the channel and the body, and it indicates which is P and which is N.N-ch: from body to channelP-ch: from channel to body

author: janoc
author_rank: Super Contributor
timestamp: August 20, 2024, 02:46:46 pm
content: Quote from: magic on August 20, 2024, 01:44:32 pmThere is actually another style of the FET symbol that explicitly shows the body diode. I actually prefer that symbol because one doesn't need to remember which direction is the arrow supposed to be facing for the which FET type (and it doesn't help that for BJTs the arrows face the other way - NPN has the arrow going away from the base, N-type FET towards the gate).

author: exmadscientist
author_rank: Frequent Contributor
timestamp: August 20, 2024, 04:00:13 pm
content: Quote from: janoc on August 20, 2024, 02:46:46 pmOf bloody well course it does, because it's not really an "N-type" FET, no matter what we might call it day to day. It's an N-channelFET: the conducting channel under the gate is formed of N-doped silicon. Whereas the base of an NPN BJT is formed of P-doped silicon. So the arrow had better point the opposite direction for these two parts!In properly drawn symbols, it's always a diode arrow: pointing away from P-doped silicon and toward N-doped silicon.

author: janoc
author_rank: Super Contributor
timestamp: August 20, 2024, 04:58:25 pm
content: Quote from: exmadscientist on August 20, 2024, 04:00:13 pmThat's correct but besides the point. For BJTs there is a lovely Czech mnemonic - "NPN - šipka ven!" ("NPN - arrow out!" - in Czech it rhymes, though), for N channel (or N-type, sorry) one has to explicitly remember. I really don't carry in my head which type of silicon which end has (and whether it even is silicon in the first place) - it is not an information actually useful for anything unless one works in semiconductor design.

author: tooki
author_rank: Super Contributor
timestamp: August 20, 2024, 06:05:24 pm
content: In English, for BJTs, I learned "points inwardsproudly".When in school, they explained that the arrow represents a P-N junction, with the arrowhead pointing to N, it made it easier to remember everything else.But the last time I expressed this on the forum, various pedants jumped down my throat, saying it's not true because of Schottky and PIN diodes. But for most practical purposes, it's true.

author: pcprogrammer
author_rank: Super Contributor
timestamp: August 20, 2024, 06:42:41 pm
content: Quote from: tooki on August 20, 2024, 06:05:24 pmIn Dutch I learned "pijl naar plaat" for PNP BJT's, meaning arrow towards plate, and this is forever stuck in my brain.

author: BennoG
author_rank: Frequent Contributor
timestamp: August 20, 2024, 07:13:24 pm
content: Quote from: pcprogrammer on August 20, 2024, 06:42:41 pmLOL that is the same I use that makes FETS for me the wrong way around.Benno

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 23, 2024, 01:27:00 pm
content: Quote from: janoc on August 20, 2024, 02:46:46 pmin fact the symbol is not free to use as we wish, it's tied to fet type, if the channel is implanted or not or...seehttps://onechip.com/quick-reference/fet-cheat-sheet.htmlandhttps://forum.digikey.com/t/fets-field-effect-transistors/13119(when student they learnt us the channel is initially already present or it's induced after, maybe it's not eligible in english...)

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 27, 2024, 05:03:40 pm
content: The arrow / diode drawn towards (or from) the gate is (probably) some kind of left over from J-Fets. J-Fets are mostly a PN junction which is always used with a reverse bias. MOS-Fets have an isolated gate and I'm not sure if the "gate diode" does anything in those, or if it's just a left over / convention.