An apparatus was constructed in the lab to measure charge carrier lifetime.
- An LED illuminates an n-typed germanium semi-conductor with a square function with a constant frequency (the period of the illumination is such that the charged carrier density reaches saturation).
- A constant current flows through the semiconductor so that the voltage on it is measured time dependent using a scope.
Using the described apparatus, an exponential decay of the voltage is measured each time the LED is turned off, and its life time is measured.
When the current that flows through the semiconductor is increased, the lifetime is changed (the negative current part can be ignored for this question)
My question is how can one explain this phenomenon? I might be able to describe the decrease in life time, by knowing that the temperature increases with current, hence electron density increases which lowers the lifetime (more electrons for the holes to perform recombination).
But the part i really can't understand is the increase in lifetime at the low current values. Any insight will be valuable, thank you.
*EDIT - it may be important to mention that the experiment was reproduced on 3 different samples.