# Diode as a Voltage Source.

Thread Starter

#### SamR

Joined Mar 19, 2019
2,259
The question in the book was: Because of its barrier potential, can a diode be used as a voltage source? Explain.

And my answer would be: Of course! There is a 0.7V loss overcoming the Barrier Potential, VB, but they are used to rectify AC in power supplies all the time to supply voltage. Of course, they can be voltage sources.

But the answer was: A diode cannot be used as a voltage source. The potential opposes current and produces an equilibrium condition, not a source of energy.

Well, it sure isn't a battery and like any device needs protection from excess current. The Barrier Potential Voltage must be overcome for current to flow, but why would it not be a source of voltage? Did I miss something here? Or, am I overthinking this?

I think this is what I don't get: produces an equilibrium condition

What equilibrium?

#### BobTPH

Joined Jun 5, 2013
2,431
The voltage has to come from somewhere else, so it is not a source.

Bob

#### crutschow

Joined Mar 14, 2008
25,119
It is semantics.
A diode with current flowing through it is not a "source" of voltage any more than a resistor would be.
But it can be generate a reference voltage (albeit not a particularly accurate or stable one).

Thread Starter

#### SamR

Joined Mar 19, 2019
2,259
K, I figured I was overthinking it. No, a diode in and of itself does not provide voltage. Thx guys. Weird question...

#### WBahn

Joined Mar 31, 2012
25,914
In electronics, a "source" is a device that can supply power to a circuit.

What they are getting at is can you hook up a resistor across a diode and use it to heat something because the 0.7 V barrier potential results in 0.7 V across the resistor and, hence by way of Ohm's Law, current flowing in the resistor and, thus, heat being dissipated by it?

#### MrAl

Joined Jun 17, 2014
7,666
K, I figured I was overthinking it. No, a diode in and of itself does not provide voltage. Thx guys. Weird question...
Actually it depends on the conditions of the diode whether or not it becomes an actual voltage source which can power something small.

They could mean a voltage "reference" and that is a diode biased with a resistor and some DC voltage to power it. It's not a great reference but in some cases it will work. I can give a very interesting example.

But if the conditions are right, silicon diodes produce voltage and thus become a voltage source. The glass encased ones work best for this. Shine a flashlight on the diode and measure the voltage and watch it rise.
It's basically like a solar cell but many diodes are encased in dark material or metal so they dont become light sensitive. Of course a solar cell usually has a larger die area so it works better for producing voltage and current, but a diode can produce a little.

Thread Starter

#### SamR

Joined Mar 19, 2019
2,259
Yes indeed. I played with a test circuit using 3 LEDs as light inputs to generate the current necessary to switch a transistor. This was interesting as I have used LEDs as visual indicators for many years and not known this was possible other than IR receiver diodes, LDRs, and phototransistors. In this case, based on the introductory nature of the chapter, I was overthinking the question as to whether it was a V source. The chapter is solely an introduction to semiconductors and diodes and hasn't even gotten to the LED yet. So...

#### WBahn

Joined Mar 31, 2012
25,914
Actually it depends on the conditions of the diode whether or not it becomes an actual voltage source which can power something small.

They could mean a voltage "reference" and that is a diode biased with a resistor and some DC voltage to power it. It's not a great reference but in some cases it will work. I can give a very interesting example.

But if the conditions are right, silicon diodes produce voltage and thus become a voltage source. The glass encased ones work best for this. Shine a flashlight on the diode and measure the voltage and watch it rise.
It's basically like a solar cell but many diodes are encased in dark material or metal so they dont become light sensitive. Of course a solar cell usually has a larger die area so it works better for producing voltage and current, but a diode can produce a little.
But a photodiode converting photon energy into electric energy isn't doing so "because of the barrier potential", which is what the question specifies. At least that's not the case to my understanding.

Thread Starter

#### SamR

Joined Mar 19, 2019
2,259
No, they were merely referring to the 0.7V (for silicon) potential voltage across the diffusion layer that has to be overcome before current can flow across the barrier. That 0.7V of potential is not a source of energy and is merely the equilibrium state of the potential of the ions across the barrier.

#### sparky 1

Joined Nov 3, 2018
260
In the depletion layer resulting from forming a PN junction is congested by opposite charge carriers.
A small potential is required called the forward bias to conduct current. When activated it is possible to have a small field outside the diode which is known to be a parasitic type having only novel energy significance, however the field does exist just as static electricity exists.

#### MrAl

Joined Jun 17, 2014
7,666
In the depletion layer resulting from forming a PN junction is congested by opposite charge carriers.
A small potential is required called the forward bias to conduct current. When activated it is possible to have a small field outside the diode which is known to be a parasitic type having only novel energy significance, however the field does exist just as static electricity exists.
Hi,

What do you mean by "when activated".

The way i understand it is that the barrier potential is made up of static charge but no free charge that can move on its own. Thus if you connected a volt meter with infinite input impedance you still would not measure a voltage solely because of the barrier 'potential'.
It makes me think of moving a weight from a position closer to sea level then moving it to a higher level. Even though at a higher level nothing is moving. It takes some other action to see the difference in potential energy.
The application of light creates free charge and that moves which can cause a current flow.

#### MrAl

Joined Jun 17, 2014
7,666
But a photodiode converting photon energy into electric energy isn't doing so "because of the barrier potential", which is what the question specifies. At least that's not the case to my understanding.
Yeah not really. The barrier potential is something different.
It is interesting that light can cause current flow though.

#### sparky 1

Joined Nov 3, 2018
260
The words congested and activated are simplified terms that convey image without going into space charge.

The observer in 1839 could see
photoconductive effect now better understood that an increase in frequency of light on contact made an improvement.

The space charge shape is being studied. We know that improvement was made by carefully doping. We know the crystal structure matters. But we don't understand everything.
We can say that a converter is not a source. We can show that by isolating a diode or photo diode and measuring the output.

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