Page 145 - Photonics Essentials an introduction with experiments

Light-Emitting Diodes

Light-Emitting Diodes 139

Problems
6.1 In Fig. 6.4 we showed the emission spectrum of a red LED. We
also measured the intensity of visible light emission as a func-
tion of the applied voltage across the LED. This result is shown
in the figure below.

a) Calculate the photon energy corresponding to the emission
peak.
b) Note from the experimental result obtained in the figure that
the first visible emission is seen when the forward voltage
across the diode is 1.4 V. Given that the light is produced by
the recombination of a single electron that can gain at most
1.4 eV from the applied voltage, explain how it is possible to
obtain emission of photons with the energy measured in (a).
c) What would you expect to see if you repeated the voltage–
light curve measurement at a lower temperature, say 77 K?
6.2 What are the two most important reasons why efficient light
emission from p-n junctions occurs only in forward bias. Note
that the answer is not current. This is a result of light emission.
6.3 Right on time, you receive an expected shipment of 500,000 red
LEDs for your company’s bar-code reader production. You have
specified a quantum efficiency greater than 0.001 (= 0.1%). The
cost of the shipment is $75,000. It is your job to inspect and ap-
prove the shipment. You assign the task to a new recruit who

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