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Introduction
A photodiode (PD) is a semiconductor device that converts light into current. When the PD is
reverse-biased, a current is generated as the PD absorbs photons. The PD also produces a small
amount of current when no light is present; this is known as dark current. Since the output current is
directly proportional to the light intensity applied to the PD junction, it can be used to detect small
quantities of light. This makes PDs useful in a wide range of applications such as optical communica-
tion, light detection, and consumer electronics.
When the PD is reverse-biased, the output current consists of dark current (no light) and photocurrent
(caused by light). Dark current is a small leakage current induced by applied voltage. Photocurrent is
typically constant as a function of applied voltage. The dark current is caused by thermal leakage in
the PN junction, so it is frequently characterized as a function of temperature. The photocurrent and
dark current measurements should be made with an instrument that can sweep voltage and measure
current over a wide range (such as an electrometer or source measure unit).
Avalanche photodiodes (APDs) are high-speed photodiodes with high sensitivity, and they exhibit
internal current gain when a large reverse bias is applied. By varying the magnitude of the reverse bias
voltage, the gain of the APD can be controlled. In general, a larger reverse bias voltage results in higher
gain. However, APDs can require high voltages (many hundreds of volts) to fully characterize them.
APDs are used in a variety of applications requiring high sensitivity to light (such as fiber optic tele-
communication and laser rangefinders). Common APD electrical parameters include responsivity,
breakdown voltage, and reverse bias current. The current rating for a typical APD under reverse bias
is 100 µA to 10 mA, while the dark current can be as low as 1 pA or less. The maximum reverse bias
voltage varies with the material from which the APD is fabricated, and for Si devices it can be as high
as 500 V.
The Keysight Technologies, Inc. B2980A Series Femto/Picoammeters and Electrometer/High Resis-
tance Meters are the world's only graphical picoammeters and electrometers that can confidently
measure down to 0.01 fA and up to 10 PΩ (10
Ω). These capabilities give them the ability to evaluate
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a variety of PDs. The B2980A Series' ammeter provides 0.01 fA current resolution and multiple current
measurement ranges (from 2 pA to 20 mA), which can meet all existing and future PD low-current
measurement needs. The voltage source of the B2980A Series' electrometers has 1,000 V voltage
sourcing capability that can support PD evaluation requiring high voltage (such as APDs). Unlike
conventional picoammeters and electrometers, the B2980A series possesses a 4.3" color LCD-based
graphical user interface (GUI) that provides multiple options for viewing data. In addition to numer-
ic format, data can also be viewed as a graph, as a histogram and as a trend chart. These unique
front-panel capabilities facilitate the capture of transient behavior and provide the ability to make
quick statistical analyses without the need for a PC. The B2980A Series also has external trigger in
and out terminals that allow it to receive and send trigger signals from and to external instruments.
This makes it easy to synchronize the B2980A Series with other instruments to
make light-current-voltage (L-I-V) sweep tests to determine the operating
characteristics of a laser diode (LD).
This technical overview shows how to use the B2985A/87A to evalu-
ate PD characteristics and explains how to synchronize the B2980A
Series with other instruments using the example of an L-I-V sweep
test on a LD.
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