Page 254 - Photonics Essentials an introduction with experiments

Experimental Photonics: Device Characterization in the Laboratory

248 Characterizing Photonic Devices in the Laboratory

orientation looks like the one on the right, simply remove the diode
from the socket and reinsert it with the lead positions exchanged. In
any case, mark the p-side lead with red nail polish for future refer-
ence.
Take some I–V characteristics of the p-n diode. In reverse bias, you
can usually apply several volts before the current increases beyond 1
microamp. It is good practice to keep the reverse current below this
level. In forward bias, the diode can handle several milliamps, usually
at a forward bias of less than 2 volts. Because of the very different
conditions between forward bias and reverse bias, you will want to
measure them separately. In order to keep light from affecting the
measurements, place a cover, such as a cardboard box, over the diode.
Measure the forward current–voltage relationship starting from the
minimum detectable current over as many decades of current as pos-
sible until 1 milliamp is reached.
In reverse bias, you may have difficulty measuring any current in
the range from 0 to –10 volts, particularly for the case of the Si photo-
diode. In this case, you can only place an upper limit on the reverse
current dictated by the sensitivity of your instrument.
Remove the cover from the photodiode and record the change in the
reverse current. This is photocurrent resulting from the absorption of
the photons that make up the room light. If you have a desk lamp or
flashlight nearby, use this to change the light intensity on the diode.
Note the results.
Reduce the bias voltage to 0 volts and place the cover over the
diode. Adjust the curve tracer so the current voltage spot is centered
in the viewing screen. Change the voltage sensitivity to 100 millivolts
per division.
Remove the cover from the photodiode and notice how the spot
moves along both the current and voltage axes. The effect of the light
is to put a bias voltage on the diode. Use another light source to vary
this bias voltage.
Using the voltage control on the curve tracer, slowly increase the
bias voltage in both the positive and negative directions until the cur-
rent in the positive direction crosses the zero-current axis. Note the
voltage intercept. This voltage is the open-circuit photovoltage.

Analysis
Plot the forward I–V characteristic as log current versus voltage. De-
termine the ideality factor of the diode using Eq. 3.16. Then deter-
mine the converted electrical power and the maximum value of the
photovoltage.
Answer these questions in your write-up:

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