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I understand that photodiodes should be reverse-biased, my question is, should I follow the same convention with phototransistors?

Paul Soorenko
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2 Answers2

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Photodiodes are reverse biased to compress the space-charge region and reduce the junction capacitance. This allows higher bandwidth. There's no direct analogy to a phototransistor. Usually, phototransistors will be slow compared to photodiodes due to long minority-carrier recombination times. Light acts as the base current, so in an NPN phototransistor the collector would have a positive voltage applied, maybe through a resistive load, while the emitter could be grounded.

John D
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    Also, you don't always want to reverse bias photodiodes; reverse biasing increases dark current. – helloworld922 Mar 14 '16 at 07:46
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    @helloworld922 And I quote: "The Zero-Bias Heresy. It is a sad fact that almost all the photodetector circuits published in the history of the world have been designed by circuits people who weren’t also optics people. The author has no statistics to back this up, but he feels that it must be so, because almost all show the photodiode being operated at zero bias, often with great care being exerted to make the bias exactly zero. – DKNguyen Nov 22 '19 at 19:32
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    This will reduce the dark current through the photodiode, all right, but that isn’t the problem we need to solve Photodiode dark current is almost never the limiting factor in a visible or near-IR measurement. Fixing this nonproblem costs you a factor of 5–7× in bandwidth (or the same factor in high frequency SNR), as well as destroying the large-signal linearity, which makes it an expensive blunder. Don’t do it ." - Philip Hobbs, "Building Electro-optical Systems, Chapter 3.5.2 Optical Detection – DKNguyen Nov 22 '19 at 19:32
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After experimentation, I find that phototransistors are more sensitive when they are reverse biased. This includes OSRAM SFH-series and TOPS. A quick way to determine this is to use a TV remote control as a source. The phototransistor is attached to a preamplifier with alligator clips (so you can try it forward and reverse biased). The preamp output goes to a simple Radio Shack Miniamp or similar. At the same distance, you will certainly hear a change... which determines which way you might want to install it.

Jim DiGriz
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    I wonder if that is related to the avalanche effect. How much are you reverse biasing it by? I don't know the details but I hear that reverse biasing photodiodes very close to their breakdown voltage greatly increases their sensitivity (and power consumption) due to the avalanche effect. – DKNguyen Nov 22 '19 at 19:36