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I'm just wondering sometimes - is there any meaning behind the transistor symbols? I can kinda see that an arrow in BJT points to direction of conventional current flow in BJT and a capacitor on mosfet's gate, is it really so or am I halucinating?

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BJT:
the arrow represents the base-emitter junction, which acts as a common diode (remember the 0.7 V drop). So the arrow is like the arrow in the diode symbol.

MOSFET:
This dates back from the JFET days: in that case there's a gate-channel P-N junction, and the arrow points from the P to N doped silicon.
The three short lines indicate that it's an enhancement FET: they symbolize that the channel doesn't conduct when the FET isn't activated. In a depletion FET that will be one uninterrupted line, indicating that the channel does conduct when not activated; the FET is switched off by applying a gate voltage.
The line at the gate side symbolizes that the gate lays over the channel, forming a capacitor (a FET's operation is based on capacitance) with it.

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  • \$\begingroup\$ This answer is more clear , miceuz, follow this answer. \$\endgroup\$ – Standard Sandun Oct 11 '12 at 3:54
  • \$\begingroup\$ No joking , But I think gravitation thing have a point. We draw diagrams +ve to -ve top to bottom as a good practice. \$\endgroup\$ – Standard Sandun Oct 11 '12 at 4:01
  • \$\begingroup\$ Thank you for making it easier for me to memorize and saving my time that I lose each time I go and look it up in Wikipedia. Yeah, I'm a noob :) \$\endgroup\$ – abdullah kahraman Oct 12 '12 at 18:02
  • \$\begingroup\$ @abdullah - And you're fresh out of college!! You didn't skip class, did you? :-) Note that there's also an NFET symbol where the arrow points in the other direction. Talk about confusing. I have no idea where that came from, but it's also used by CircuitLab. \$\endgroup\$ – stevenvh Oct 12 '12 at 18:08
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    \$\begingroup\$ @abdullah - NFET = N-channel. May be either JFET or MOSFET, but nowadays usually MOSFET is implied. I haven't seen a JFET in 20 years! But "N-MOSFET" would be more accurate. "NMOS" is a name for an older IC technology, which is now superseded by CMOS. CMOS has complementary pairs of an N-MOSFET and a P-MOSFET (or "P-channel MOSFET"). NMOS was used in older microcontrollers. Instead of the P-MOSFET it had another N-MOSFET which acted as a pull-up resistor. \$\endgroup\$ – stevenvh Oct 12 '12 at 18:29
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In your MOSFET symbol you'll notice that the channel is broken, which means it is an enhancement type of device (meaning off until gate is active - thus the segmented channel). The arrow of the MOSFET indicated the channel/bulk orientation. In a PMOS the channel when active is P-type and the bulk is the Nwell which is N-type. In an NMOS the channel is n-type and the bulk/well is P-type. In diodes the anode is p-type and is the arrow, whereas the cathode is n-type and is a bar. So the channel bulk connection is a junction with the arrow indicating the junction direction.

As an aside, CMOS processes only deal with enhancement devices (with a few exceptions - like native transistors) so in chip design we don't use the broken channel symbol. It's just cleaner.

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