What do the arrows in transistor symbols point to (and from)?

Question

Transistor symbols are often drawn with arrows pointing in one direction or the other, depending on type, like in the following symbols:

But what is the arrow actually pointing at? And from where are they pointing? Is it the same principle behind it in each symbol and if so, why do they sometimes point FROM the transistor and sometimes TO it?

And why do the arrows point from different origins (sometimes base, emitter, gate, source etc) in different types?

Is there any general principle behind how the arrows are directed?

Answer 1

For BJT's there is a PN junction between the base and emitter. The arrow indicates the order of the junction (base to emitter or emitter to base). An NPN has stacked N, P, and N doped channels. The PN junction (between base and emitter) goes from the center out. PNP likewise is the opposite.

Observations, not necessarily fact:

In a MOSFET, the body is often connected to the source. For an N-channel MOSFET, the source is N-doped and the body is P-doped, so the arrow points from the source to the body. Likewise, a P-channel MOSFET has the reverse condition. Interestingly, Wikipedia has symbols for "MOSFETS with no bulk/body" which have opposite arrow directions. I have no good explanation for why these are this way, though I suspect it might follow a similar pattern and the semiconductor topology is different from "traditional" MOSFET topologies.

Your symbols for b (FET) are JFET symbols. Here, the PN junction is between the gate and "body" (semiconductor section connecting the drain and source; I don't know what the correct for this part of a JFET is so I just called it the body because it takes the bulk volume of the JFET). For an N-channel, the gate is P-doped and the body is N-doped, so the arrow points from the gate in. The P-channel JFET is the opposite so the arrow points out of the gate.

I've never used unijunction transistors (case d), but looking at the wikipedia page shows a similar doping structure as the JFET, the only difference the lack of an insulated gate (names also have changed, apparently it follows the "BJT" type naming of base and emitter). I would not be surprised if the arrow direction convention follows the order of the PN junction (wasn't immediately obvious to me which type the example structure on Wikipedia was for).

Additional info:

Bipolar Junction Transistors

MOSFET

JFET

unijunction transistors

Answer 2

In short, the arrows show the current direction of a PN junction when forward biased.

• In BJTs the PN junction is the base-emitter one.
• In JFETs it is the one between gate and channel.
• In MOSFETs it is the one which exists between the channel and the substrate (the terminal where the arrow is placed in the symbols you posted), which is not available externally. Note that when the substrate terminal is not shown in the symbol, the arrow is placed on the source and will point in the opposite direction, since the source has the same semiconductor polarity of the channel, which is always opposite to the substrate polarity: N-channel devices have P-type substrate and viceversa.
• In UJTs (case d) it is the one between the emitter and the bulk semiconductor which connects the two base terminals.

Note that this convention is the same used for semiconductor diodes: the anode is connected to the part of the symbol which is a big arrowhead, and the anode is the terminal where the current enters the device when forward biased.

This doesn't mean that those junctions must be operated in forward bias conditions. This depends on the specific device type and operating conditions. E.g. JFETs are always operated with their gate junction reverse biased, whereas BJTs may work in different operating regions depending on whether their BE and BC junctions are forward or reverse biased.

Answer 3

The arrows in the bipolar transistors (emitter) show the direction of flow of conventional current. That is, current flow from positive to negative. Diode and LED symbols are the same.

EDIT: clarified, the emitter arrows.

Answer 4

About the bipolar transistor symbol.

First transistors were made with needles tucked into a germanium crystal, and were derived from point contact germanium diodes.

Galena diodes (for radio) used also nails set over the crystal surface, not soldered connections.

The symbol is litterally that. A specific junction transistor symbol once existed, but the point-contact symbol sticked.