Transistor symbol confusion - Page 1
Forum section: electronics / beginners

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: July 07, 2020, 05:55:41 pm
content: Hi,I've had an agonizing time figuring out how transistors work, and the puzzle is finally starting to come together. But one major source of confusion that has befuddled me the past few days is how to understand the logic behind the symbols of BJT and MOSFET transistors.NPN: POSITIVE base causes current to flow from collector to emitter. Symbol is arrow pointing OUTWARDS, WITH current flowPNP: NEGATIVE base causes current to flow from emitter to collector. Symbol is arrow pointing INWARDS, WITH current flowNice and simple. Now, suddenly, here's MOSFETs:N: POSITIVE gate causes current to flow from source to drain. Symbol is arrow pointing INWARDS, AGAINST current flowP: NEGATIVE gate causes current to flow from drain to source. Symbol is arrow pointing OUTWARDS, AGAINST current flowSo not only have the N's and P's "swapped meaning", but the symbols also don't seem to follow the same logic. I'm sure those symbols had a long way through several committees before being accepted, so what's the reasoning here?Or maybe I've confused them once again..

author: exe
author_rank: Unknown
timestamp: July 07, 2020, 06:12:20 pm
content: I agree with you, it's confusing. There is an explanation why it's put this way, but it doesn't help me. That's why I strongly suggest use only mosfet symbol with body diode shown. This way it's clear which way it blocks current. If you take a close look, you'll find that the arrow is actually electrically disconnected, so it doesn't really show the current flow. I'd say it shows how gate should be biased to open the fet. So, for n-fet the gate should be above the source.

author: TimFox
author_rank: Super Contributor
timestamp: July 07, 2020, 06:32:35 pm
content: Similarly, for a JFET, the arrow indicates the polarity of the input diode between the gate and channel. Therefore, an N-channel JFET has the arrow pointing from the gate to the channel so that it forward-biases with positive voltage on the gate with respect to the source. The P-channel JFET has the arrow point away from the channel, so it forward-biases with negative voltage on the gate with respect to the source. Ordinarily, that junction is reverse-biased for normal operation (and usually can only sustain a small current without damage to the device).

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 07, 2020, 08:29:22 pm
content: You are not alone. I am still confused with FET's simbols too.

author: magic
author_rank: Super Contributor
timestamp: July 07, 2020, 08:38:21 pm
content: First off, MOSFET is a four terminal device: drain, body, source and gate. You can see the three layers on the symbol and the gate is separately on the left.Body is the central part, doped oppositely to source and drain to isolate them from each other. It is usually shorted to the source, as seen on the symbol.When appropriate voltage is applied to the gate, part of the body near the gate turns into a "channel" of the same polarity as source and drain.The arrow is always drawn on the body and I suppose it represents the "PN diode" between the inverted channel near the gate and the reminder of the body. Arrow pointing towards the gate means "N channel", arrow pointing outwards is "P channel".This has nothing to do with normal conduction through the FET. Current flows between source and drain and there are no arrows on those terminals.Is it confusing? Maybe

author: Jwillis
author_rank: Super Contributor
timestamp: July 08, 2020, 06:01:44 am
content: It goes back to the idea that electrons moved from the positive to negative , which we now know don't . Before solid state transistors were invented , the point contact diode was invented with the same convention that electrons moved positive to negative. The symbol of a diode is a depiction of the point contact diode . Anyway , changing it now would just lead to more confusion.

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: July 08, 2020, 01:24:47 pm
content: Thanks for the feedback, I'm always amazed at how many knowledgeable people there are here. Good to know I'm not the only one finding stuff somewhat.. non-pedagogic. Anyway, I'm kind of starting to get used to it. Dreading looking at JFETs or others though.. I assume more confusion awaits

author: TimFox
author_rank: Super Contributor
timestamp: July 08, 2020, 02:29:54 pm
content: Quote from: Jwillis on July 08, 2020, 06:01:44 amThe convention (in "conventional current") is thatcurrentflows from positive to negative. Electrons havenegative charge, and therefore flow in the opposite direction. The arrows in semiconductor symbols follow that convention. In some educational systems, including WW II military electronics training, electron current is used instead of conventional current, which seemed more appropriate for vacuum tubes, where the charge carriers were electrons. In solid state devices, charge carriers can be electrons or (positive) holes.