Ok I’m still not satisfied with the answers. I believe it’s purposefully evading the direct question due to the duality of electron and light theories. Electrons are actually ejected in the photoelectric effect. There is an electron flow out of the base and creating hole current in the system.

 

I believe it’s purposefully evading the direct question

What "direct question"?

There is an electron flow out of the base and creating hole current in the system.

And the point is....?

 

Hey @crutschow, Im not sure why these phototransistors are so confusing I’m re-reading the examples... when I get a direct question I’ll ask. I’m mostly just frustrated that this one keeps eluding me.

 

Ok I’m still not satisfied with the answers. I believe it’s purposefully evading the direct question due to the duality of electron and light theories. Electrons are actually ejected in the photoelectric effect. There is an electron flow out of the base and creating hole current in the system.

Yes, but here we are talking about the photovoltaic effect, here, we don't get unbound electrons ejected into space. While similar in theory there is a difference between the two in physical operation.
https://en.wikipedia.org/wiki/Photovoltaic_effect

The main distinction is that the term photoelectric effect is now usually used when the electron is ejected out of the material (usually into a vacuum) and photovoltaic effect used when the excited charge carrier is still contained within the material. In either case, an electric potential (or voltage) is produced by the separation of charges, and the light has to have a sufficient energy to overcome the potential barrier for excitation. The physical essence of the difference is usually that photoelectric emission separates the charges by ballistic conduction and photovoltaic emission separates them by diffusion, but some "hot carrier" photovoltaic device concepts blur this distinction.

 

Photons knock the minority carriers out in the lightly P doped base causing additional electron hole pairs. The free electrons migrate towards the collector while the holes attract electron flow from the emitter region. In a phototransistor the band gap energy is provided by photons. Since there is no actual base connection, there is no current flow going out of the base like standard transistors. Thinking through the electron flow helps. So the current through the emitter and collector is the same, the base being activated by photon energy reduces the depletion region causing electron flow from the emitter to the collector proportionate to the beta.

 

@nsaspook that is exactly what I was mistaking. Thanks. I think I understand now.

 

@nsaspook that is exactly what I was mistaking. Thanks. I think I understand now.

I can read minds and predict the future.

 

That photoelectric bit I ran into brought up the possibility there may be electron flow out of the base but I see that it doesn’t make sense in our situation. This is all very elusive but fascinating subject. And this stuff won’t even be on the test but I want to understand.

 

That photoelectric bit I ran into brought up the possibility there may be electron flow out of the base but I see that it doesn’t make sense in our situation. This is all very elusive but fascinating subject. And this stuff won’t even be on the test but I want to understand.

You're not the first person to have questions about this. There is a reason why Einstein won a Nobel on this subject.

Always think about the energy required for X to happen first. In semiconductors electrons are the gears that direct EM energy, they are rarely the actual KE carrier of that EM energy. I work with lots of high energy stuff under vacuum and see "fill in the blank'electric effects of particles flying (as actual carriers of KE transformed from the EM energy source) across the vacuum all the time while controlling heavy ion emission. We have suppression systems to keep them away from critical work functions.

 

That’s pretty cool, My buddy was showing me examples of electron welding last night. Just a quick look at the concept reminded me of the klystron amplifiers with the magnetic focusing and guides. Although there’s an actual current flow in this situation this is completely different as the welded object becomes a part of the circuit and much easier to understand.

 

That’s pretty cool, My buddy was showing me examples of electron welding last night. Just a quick look at the concept reminded me of the klystron amplifiers with the magnetic focusing and guides. Although there’s an actual current flow in this situation this is completely different as the welded object becomes a part of the circuit and much easier to understand.

A good demonstration of what happens when electrons have KE, things get hot. For most circuits that transform or transport energy we want to reduce the KE of electrons to the lowest point possible in the shortest possible time to increase efficiency. In tubes the KE of the actual current flow is wasted as heat usually while slamming into the anode unless that KE is transformed into another type of energy first.

 

to understand a phototransistor start with a photodiode
http://www.vishay.com/docs/81521/bpw34.pdf
the reverse lights *currents* varies with the irradiance, check the charts in the specs

next you'd need to understand the BJT transistor
https://en.wikipedia.org/wiki/Bipolar_junction_transistor
the transistor amplifies the *currents*

so a phototransistor is a photo-diode fabricated so that it is part of the transistor
https://en.wikipedia.org/wiki/Photodiode#Other_modes_of_operation:
A phototransistor is a light-sensitive transistor. A common type of phototransistor, called a photobipolar transistor, is in essence a bipolar transistor encased in a transparent case so that light can reach the base–collector junction. It was invented by Dr. John N. Shive (more famous for his wave machine) at Bell Labs in 1948,[5]:205 but it was not announced until 1950.[6] The electrons that are generated by photons in the base–collector junction are injected into the base, and this photodiode current is amplified by the transistor's current gain β (or hfe).

i think the equivalent circuit of a phototransistor is such:

        Vcc --- + ------------- +      > ------------| gnd
                |                 \ _ /
                + ------|< ---------|
                         ^ this is your photodiode, reverse biased

the reverse currents flows when light shines on it, amplified by the transistor
hence, photo transistor