# Do NPN and PNP phototransistor switch complementarily?

I am trying to understand whether there is a difference in the switching characteristics of NPN and PNP phototransistors.

My understanding is: NPN: current flows when light is incident, no current without light

PNP Option 1: current flows when light is incident, no current flows without light

PNP Option 2: current flown when NO light is incident, no current flows with light

Is option 1 or option 2 correct?

Many thanks.

• read the datasheet before you ask – jsotola Nov 12 '18 at 20:28
• They act similarly- light = conducting. If you are not using the base you can flip E and C on one and approximate the other. PNP phototransistors are scarce as hen's teeth. – Spehro Pefhany Nov 12 '18 at 20:37

In the usual presentation, those carriers are injected into the base via the emitter and the base connection, because the base-emitter junction has been forward-biased by increasing $$\V_{be}\$$ above zero (in an NPN) or decreasing it below zero (in a PNP). However, those carriers can also be left over when you suddenly turn off voltage to the base, which is why transistors have a reverse recovery time. They can also be created by photons smacking into electrons in the base region and shoving them out of position, creating electron-hole pairs.
To a first approximation, a phototransistor is just a regular transistor that forgot to close the bedroom curtains. In a dark room, with no voltage on its base, it acts like a regular transistor with no voltage on its base -- it doesn't conduct. When light shines on its base, it generates carriers, because photons (see above). As a consequence current starts to flow from collector to emitter (and it develops its own $$\V_{be}\$$, as a side effect).