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.
(See 1st figure)
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)
(See 2nd figure)
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).
The part i really can't understand is the increase in lifetime at the low current values.
Any insight will be valuable, thank you.
My assumption is ( I can be wrong) without any current flow the energy separation between majority carrier and minority carrier is minimum. So, most of the recombination process can accomplish in one step. But if we put some current which in another word putting energy in the system increases this energy difference which can lead to multi-step recombination process which requires more time hence increased life time. As, we increase the current thermal generation of majority carrier starts to influence as you mentioned.
Dear Dor,
I hope, that I understood well your experiment.
After my knowledge, the carrier lifetime in semiconductors is very short - in the range of nanoseconds. Me seems, what you exhibit, is the charge and discharge of the diode capacity. The capacity of a diode depends on the applied voltage. So lower the voltage, so higher the capacity.
Regards
R. Mitdank
Greetings.
View the following work by prof. Goryunova V.A. and his colleagues, conclusions and literature for it, perhaps its methodology and emphasis placed will tell you the direction of searching for an adequate model of the phenomenon and its explanation, namely: the presence (at least) of two essential components of the carrier recombination mechanism that determine the lifetime (for different values current through p-n junction).
https://cyberleninka.ru/article/v/ob-izmenenii-vremen-zhizni-nositeley-zaryada-pri-impulsnom-fotovozbuzhdenii-v-kremnii-s-glubokimi-primesnymi-tsentrami
It would be nice to give the " n-typed germanium semi-conductor" device/sample structure and related information; to measure the device I-V and C-V, work out the differential resistance and RC constant (depend on current); if the RC constant was comparable to the measured time constant need analyse/calibration; the LED illuminates switch on/off transient time related to the plus power and LED features; some long lifetime related to defects (traps, interface, .... etc.).
Increasing the current by increasing the electric field in the specimen may affect the speed of recombination of the generated electron hole pairs.
What do you observe in your experiment is the collected current from the terminal of a photoconductor illuminated by light pulses by the LED.
The dynamics can follow the the lifetime under certain conditions.
A similar experiment is described in the link: Method Measuring the semiconductor parameters
For the analysis of the photoconductors dynamic please refer to S M Sze book the Semiconductor devices physics
Best wishes
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