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*Subject*: Re: [Rollei] selenium/silicon cells*From*: "Gene Johnson" <genej2 >*Date*: Wed, 6 Nov 2002 07:14:01 -0800*References*: <200211061014.gA6AED308480

Wow, Emmanuel, I bow humbly in your general direction. I will save this message for that day when I can comprehend it more fully! Gene - ----- Original Message ----- From: <bigler To: <rollei Sent: Wednesday, November 06, 2002 2:14 AM Subject: Re: [Rollei] selenium/silicon cells > From Siu Fai: > > >..my limited knowledge in solid state physics does not explain the > >logarithmic relationship.characteristics > > No need of a complex solid ste physics course to understand this if > you admit the equivalent electrical model. > > A good tutorial : > > http://usa.hamamatsu.com/cmp-detectors/photodiodes/diodes.htm#Equivalent Circuit > > > Well I would not be surprised if a selenium cell has a current-voltage > characteristics under various light fluxes similar to a silicon > photodiode. After all a selenium cell is in fact a metal-semiconductor > junction. > > For precise photometric measurements, silicon photodiodes are usually > reverse-polarized, then you get an extremely linear relationship in a > range of 1:10,000 beween the reverse current and the light flux. > Reverse polarized, the cell has an extremely high impedance. Used > un-polarized, as a generator, you get a solar cell with a non linear > current vs light response except for very small series resistors. I do > not know for selenium cells, but the simplest model for the silicon > photodiode is simply an exponential curve for which the zero asymptote > is shifted downward linearly with respect to the incident light flux. > > I = Is*(exp(eV/kT) -1) - Iflux > > where Iflux is proportional to the incident light flux : > Iflux ~ k*flux, where k is a sensitivity constant that varies with the > incident wavelength. > > Reverse polarized V<0 you get very quickly, even for small values of V: > > I = -Is -Iflux, but since Is is extremely small (few nA), eventually > this reduces to I ~ -Iflux, hence the extremily good linearity of the > reverse current vs the incident flux. > > Used as a generator V>0 you get the non linear behavior. With a null > series resistor, V=0, you get I = -Iflux and a linear behavior. Well > impossible to actually get V=0 with a classical am-meter readout, but > connected to an op-amp, you can do it "virtually" with a so-called > current-to-voltage converter. > > For a non-zero series resistor, you can solve graphically the current > vs photon flux if you assume that Iflux = k*flux. For an infinite > series resistor or connected to an op-amp mouned as a "voltage > follower", I = 0, you get the logarithmic response : > > V = (kT/e) Log (1+Iflux/Is). > > So for any series resistor you get a behavior which is anywhere > in-between a linear and a logarithmic response : this makes a lot a > various behaviors possible !! With the classical am-meter readout, you > are always non linear since there is always an equivalent resistor for > the coil (not speaking of the "critical resistor" required for proper > damping of the needle). > > This explains very simply that, at least for the silicon photodiode, > whether you add a small or a large series resistor you get either a > linear or log response in a V-I diagram. > > I would assume that a selenium cell behaves quite similarly although > the apparent "universality" of the function eV/kT makes difficult to > believe that the same expression would be true for all kinds of > junctions whether they are PN silicon, PN germanium of > semiconductor-metal. > > -- > Emmanuel BIGLER > <bigler ------------------------------

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