LED with IV curve showing current driven negative resistance ... Any clue ???

[Enea]

Hi Folks,
I was testing some 650nm GaAs/AlGaAs LEDs when I found a batch showing this very strange IV curve (attached) with current driven negative resistance. Any idea what is going on

?

The affected LEDs have all an unusual higher forward voltage. Even up to 4V for 1mA instead of a regular 1.8/2V for my processed LEDs

The negative resistance curve has a threshold-like behavior. Increasing just few mA brings down the Voltage of roughly 1V (from 5V where the threshold seem to be) and increase current from7/8 mA to 20/25mA

Light Output is bigger before the NDR takes place, suggesting some sort of tunneling but I have no idea where ...

Increasing the current above 40mA rectifies the IV curve, brings back the curve to a textbook graph, decrease the initial higher forward voltage of a couple of Volts, but also decrease of 50% light output at 1mA (I believe simply because I am pumping too much current in my very thin hereto junction)

Any helps or advice would be useful. The structure of the LEDs and metalization has nothing fancy at all, just regular stuff for this kind of red LEDs

Please Ignore the lines at 2V in the picture. Is just an artifact created by the machine!!

 

[Harald Kapp]

Are these truly regular LED's? Maybe these are ones with integrated current limiting which might produce the observed behavior.

 

[Enea]

They are just plain, simple, LEDs ... nothing fancy, just LEDs not electronics attached

 

[Harald Kapp]

not electronics attached

The electronics may be built into the LED chip as is e.g. often the case with LEDs for flickering candles. You may not be able to see the additional electronics with the naked eye. See e.g. here.