Light dependent resistor(LDR) with very low resistance

Why I never find a light dependent resistor having very low resistance (around 20 ohms) when lighting?

Does anyone know? Thanks.

Hi,

Add a couple parts and a Mosfet and it could be milliohms.
Or go to a LDR factory and ask them to design an LDR with your requirements.

I assume the first approach is faster, cheaper and smaller..

Klaus

photoresistors work by conductivity modulation of a
semiconductor material. CdS is one such, has a nice
low bandgap so it will not "ignore" infrared. Such resistors
want both a high dark resistance and a low* illuminated
resistance. Thus resistance has to depend on illuminanation.

You get one additional carrier (e-) per photon in the visible
and IR (above bendgap cutoff). You have only so many
photons per area and a fixed small chip area. So, only so
many excess carriers in the resistor body and only so much
conductivity modulation.

To do better, wants gain. But generally adding a gain stage
or a gain mechanism, takes you away from a 2-terminal
linear element. You'd have to like something like a photo
transconductance amp, with proper behavior in at least
quadrants 1 and 3, but that in itself is a stretch using
normal FETs or BJTs.

If your application is more forgiving then maybe a single
ended transconductance amp, to GND, will do the job.
That's for you to determine. A free-floating general
purpose optically modulated resistor, you're probably
stuck with one electron per photon.