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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.

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    \$\begingroup\$ read the datasheet before you ask \$\endgroup\$ – jsotola Nov 12 '18 at 20:28
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    \$\begingroup\$ 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. \$\endgroup\$ – Spehro Pefhany Nov 12 '18 at 20:37
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The physics are the same for NPN or PNP.

Incident light creates hole-electron pairs in the Base-Emitter PN junction. These are driven by the external field to provide a base current that biases the transistor. This base current will be amplified by the beta of the transistor to generate a collector-emitter current.

The only difference is in the direction of the current, i.e., how to externally bias the transistors.

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In a bipolar junction transistor, current flows between collector and emitter when there's carriers in the base that can flow across the reverse-biased collector-base junction.

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).

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