Page 65 - Photonics Essentials an introduction with experiments

Photodiodes

Photodiodes 59

a. What is the responsivity of the diode when
= 1000 nm?
b. What is the quantum efficiency at
= 1000 nm?
c. You would like to use the diode for a free-space optical link
operating at
= 600 nm. You decide on an incident power of 1
watt. What do you expect the photocurrent to be?
d. To clarify the issue, you decide to measure the responsivity as
a function of wavelength. Before measurement, you make a
trial calculation of responsivity based on the above informa-
tion. Please make a graph of your estimate in the range 400
nm <
< 1400 nm.
3.3 In the lab, you measure the I–V characteristic of a Ge photodiode
using a curve tracer. The result is shown in the figure below.

Ge area = 8 × 10 –3 cm 2

A/div

Current = 5 × 10 –7

Voltage 1 V/div

a. Identify and measure the photodiode dark current.
b. Is this dark current larger or smaller than that for other
diodes you have measured?
c. What features of this photodiode contribute to the magnitude
of the measured dark current?
3.4 In the laboratory, you measure the following current–voltage
characteristic of a p-n diode in the forward direction (see figure
at the top of the next page).
a. What does the dashed line imply about the relationship be-
tween current and voltage?
b. Determine the ideality factor of the diode.
3.5 As an engineer, you are designing a free-space optical communi-
cations link using a red LED, the peak wavelength of which is

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