On this page of the online textbook

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-1/from-electric-to-electronic/

there is a illustration of 3 devices. Each has in the bulb e^-1 with an arrow.

What does e^-1 represent here? Where can I find more information about that usage?

 

It is a shorthand notation for an "electron" with 1 unit of negative charge. That unit of electric charge is equal to approximately 1.60217662 × 10^-19 coulombs.

The notation is used to describe the reactions of elementary particles of which the electron is one.

https://www.nuclear-power.net/notation-nuclear-reactions/