The arrow indicates the diode. The channel is a common strip of same-doped material: treat it as a resistor between terminals; preferably, short D+S together.
It's worth repeating for those readers in the back: when you see a diode in a [traditional, grumble*] semiconductor symbol, it indicates a PN junction.
A diagonal line to a substrate also does this; hence C and E in the NPN transistor, where the emitter is a PN with the N going to the terminal (arrow pointing out), and C is implied by symmetry to be the same way.
Square lines (like the JFET D/S) indicate ohmic contacts to a substrate.
The UJT thus is drawn essentially identically to a JFET, and really isn't much different in construction either: the catch is, in the JFET, the channel is made thin enough to pinch off under reverse bias, while the UJT uses a wide and long channel that can't be pinched off (or at least, not by very much), but where conductivity modulation due to charge injection (forward-biasing the "gate") is exaggerated, hence its negative-resistance behavior. THey chose to draw it with a tilted "gate" to differentiate it from a JFET.
In the MOSFET, the channel is indicated with a continuous strip (depletion mode) or three segments (enhancement mode), the substrate with a PN diode pointing at the middle of the channel. Normally, substrate is tied with source, making the familiar three-terminal device.
*Grumble grumble: the regrettably-popular modern symbol is to draw the MOSFET in analogy to the BJT, where voltages are referenced common to the be-arrowed terminal, and the arrow points outward for the N-type part. The N-MOS thus becomes a "square legged" NPN. Which as you can see from the above, is... utterly meaningless, you can't have an ohmic AND rectifying contact at the same time, that's a contradiction. What's worse, I've seen dozens of examples where this symbol is confused with the traditional symbol, bringing back the substrate line segment, swapping around which one has the triangle, which direction the triangle is pointing, etc. Major manufacturers like TI can't even get the symbology correct; it's a huge festering mess.
Also, note that MOSFETs are often drawn as a MOSFET in parallel with a body diode, but, as you can tell from the above description: the body diode is already there, that's what the triangle is pointing out to you! I suspect, newbies missed this for a long time, so manufacturers started showing the parallel diode to emphasize that's what's in there. But still to this day, you see schematics on the regular where newbies have connected external diodes (even with woefully inadequate ratings compared to actual body diode ratings, i.e. they'll never carry significant load current anyway) across symbols that show two diodes already. The solution was never -- and will never be -- increasingly ornate symbology; newbies just don't know how to parse those symbols at all, they're just "[shape] means [part]", and I mean what else do you have to go on if no one ever discusses the meaning of these things -- and I have never seen an appnote mentioning it, and I doubt many curricula discuss symbology in common use either. So, I have made it a point of adding this lecture whenever semiconductor symbols come up -- because they are, in fact, quite descriptive and physically relevant, and you can make sense of them, given a little knowledge of the bits used to draw them!
Tim
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