Keysight Technologies B2980A Series Installation guide

Keysight Technologies
Photodiode Test Using the Keysight B2980A Series
B2981A/83A Femto/Picoammeter
B2985A/87A Electrometer/High Resistance Meter
Technical Overview

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 ber optic tele-
communication and laser rangenders). 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 condently
measure down to 0.01 fA and up to 10 PΩ (1016 Ω). These capabilities give them the ability to evaluate
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.

03 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Photodiode Characterization Example
This section explains how to perform PD dark current measurements using the B2985A/87A.
Test System
Figure 1 shows the circuit diagram to evaluate PD characteristics using the B2985A/87A
Electrometer/High Resistance Meter. The B2985A/87A’s voltage source high terminal is
connected to the cathode of the PD to supply a reverse bias voltage, and its ammeter is
connected to the anode of the PD to measure the PD current. The voltage source’s low
terminal and the ammeter common are connected together internally.
Since the B2985A/87A’s voltage source has the capability to sweep voltage while its
ammeter synchronously measures current, the PD’s current – voltage (IV) characteristics
can easily be obtained. In addition, the B2985A/87A’s ability to display the results as an
X-Y graph permits quick examination of the results.
The following example will show how to make a silicon photodiode dark current
measurement. The key measurement parameters are summarized in Table 1. The PD current
is measured by the B2985A/87A’s ammeter as the B2985A/87A’s voltage source applies a
reverse-bias voltage. The voltage is swept from 10 mV to 5 V in 10 mV steps. A xed range
of 20 pA range is used to measure the PD current since the maximum expected current is
several picoamps, and using the 20 pA range provides better accuracy. The measurement
trigger delay time is set at 1 s to give the measured current sufcient time to stabilize.
Of course, since the dark current measurement has to be made without light the PD is
tested in an enclosed test xture.
Table 1. Photodiode characterization key measurement conditions
Figure 1. Circuit diagram to evaluate photodiode characteristics
High
A
+
-
B2985A/87A
PD
Ammeter
Input
B2985A/87A
Parameters Values
PD Voltage Swept Start 10 mV
Stop 5 V
Points 500
PD Current Range 20 pA Fixed
Aperture Time (SPEED) Normal
Trigger Source AUTO
Count 500
Measure Delay 1 s
Period 1.2 s

04 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
The following instructions describe how to set up the B2985A/87A to perform PD dark
current measurements on the instrument front panel.
A le containing the setup used in the example can be downloaded at the following link:
www.keysight.com/nd/SensitiveMeasurement
A sample VBA program using SCPI commands to make this measurement is also avail-
able at the above link.
Setting up the B2985A/87A from its front panel
1. Set “Dual Measure Result Display” off to make the instrument measure only current.
a. Open the Display Preference dialog by pressing the , , ,
function keys.
b. Press to edit “Dual Measure Result Display”. After that, the eld pointer is
highlighted in green (EDIT).
c. Press , and then press to apply the settings.

05 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Setting up the B2985A/87A from its front panel (continued)
2. Select the current measurement range operation and set the current measurement
range.
a. Press , and then press to show the Range Parameters.
b. Press and press to set the current measurement range operation to
“FIXED”.
c. Press repeatedly until is displayed to set the current
measurement range to 20 pA range.

06 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Setting up the B2985A/87A from its front panel (continued)
3. Set the Sweep Parameters to make the instrument perform a voltage sweep.
a. Press , and then press to show Sweep Parameters.
b. Press and press to turn on Single Linear Sweep Source Mode.
c. Rotate to select Sweep Parameters and ll in the values as shown below.
Use the arrow keys to move to the digit you want to edit.
(Start: 10 mV, Stop: 5 V, Points: 500)

07 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Setting up the B2985A/87A from its front panel (continued)
4. Set the Trigger Parameters to congure the Measurement Delay Time.
a. Press , , and then press to show the Trigger Parameters.
b. Press and press to set the Trigger Parameters.
c. Rotate to select the Trigger Parameters and ll in the values as shown below.
(Count: 500, Measure Delay: 1 s, Period: 1.2 s)

08 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Setting up the B2985A/87A from its front panel (continued)
5. Select the Graph View and congure it to show results graphically.
a. Press to show the function keys for View Modes, and then press
to show the Graph View.
b. Rotate and press to show Y-axis graph scaling, and then press to set it
to LOG scale.
c. Rotate and press to show X-axis graph scaling, and then press to set it
to LOG scale.
The Auto Scale function is available both during and after measurement in Graph
View.

09 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Setting up the B2985A/87A from its front panel (continued)
6. Enable the Voltage Source and Ammeter.
a. Press Voltage Source to enable the Voltage Source.
b. Press Ammeter to enable the Ammeter.
Controlling the B2985A/87A using SCPI commands
If you prefer to control the B2985A/87A remotely using SCPI commands, then the follow-
ing material explains how to do this.
The series of commands shown in group “A” congure measurement parameters such
as measurement auto-ranging (turned off), measurement range, and aperture time, and
they should be sent rst. Next the series of commands shown in group “B” should be
sent to set up the voltage source. Finally, the series of the commands shown in group “C”
should be sent to set up the B2985A/87A’s trigger parameters.
Why is a measurement delay time required?
In general, measurement paths have stray impedance that can cause leakage cur-
rents and dielectric absorption when a voltage is applied. To obtain accurate results
it is necessary to wait for these transients to die away before starting measurements
(especially in the case of ultra-low current measurements). The required wait time
for a given measurement depends on the magnitude of the applied voltage step.
Larger voltage steps require longer wait times.

10 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Performing photodiode dark current measurements
After conguring the instrument, perform the following procedures to execute PD dark
current measurements.
If you are using the front panel, trigger the B2985A/87A to start sweeping the reverse-bias
voltage to the PD by pressing .
If you are controlling the instrument remotely using SCPI commands, then send the
“:INIT (@1)” command to the instrument to start the dark current measurement.
The measurement results can be displayed on the B2985A/87A’s graphical user interface
as shown in Figure 2. The Graph View function allows you examine the measurement
results quickly.
The APD dark current measurements are performed by the same procedures as shown
in Figure 3.
Figure 2. Photodiode dark current measurement results
Figure 3. Avalanche photodiode dark current measurement results

11 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
L-I-V Test Example
This section explains how to synchronize the B2980A Series with other instruments using
the example of a laser diode (LD) L-I-V sweep test using a photodiode (PD).
Test System
Figure 4 shows a system diagram example to evaluate the L-I-V characteristics of a LD
using the B2900 Precision Instrument Family. The Keysight B2911A Precision Source/
Measure Unit (SMU) is used to apply drive current to the LD and to measure the LD’s
voltage. The B2911A can cover currents from 10 fA to 3 A (DC)/10.5 A (pulsed) and
voltages from 100 nV to 210 V. The SMU has the capability to source and measure both
positive and negative voltages and currents, so it can easily characterize the LD’s DC
parameters. Since the currents supplied to the LD can be quite large, a 4-wire connec-
tion (remote sensing) conguration is commonly used.
The B2985A/87A’s ammeter input terminal is connected to the PD’s anode and the
B2985A/87A’s voltage source high terminal is connected to the PD’s cathode. Since the
ammeter’s and voltage source’s low terminals are connected internally to the circuit
common, the PD current can be measured by applying voltage to the PD from its voltage
source.
In order to synchronize the B2985A/87A with the B2911A during the LD current sweep
operation, a trigger signal is sent from the B2911A to the B2985A/87A. The N1294A-031
GPIO-BNC Trigger Adapter converts the B2911A’s digital I/O output to BNC outputs.
The N1294A-031 allows you to synchronize the triggering of the two units using an
inexpensive coaxial cable.
Figure 4. System conguration to evaluate the L-I-V Characteristics of a laser diode

12 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Test System (continued)
A distributed feedback (DFB) laser diode is used in this example. The key measurement
conditions are summarized in Table 2. The PD current is measured by the B2985A/87A
as the drive current to the LD is swept from 0 A to 50 mA in 250 µA steps by the B2911A.
In order to step currents to the LD at regular intervals, timed triggering is selected, the
trigger period is set to 500 µsec and xed measurement ranging is used. Since laser
diode characteristics can be affected by self-heating, for accurate results it is essential
to carefully control the LD measurement time and make it as short as possible. In this
example a 500 µs trigger period and 200 µs aperture time are used.
Table 2. Summary of measurement conditions for L-I-V Testing
A trigger signal is sent from the B2911A’s digital I/O port 9 to the B2985A/87A’s BNC
trigger-in terminal immediately after each B2911A current step completes as shown in
Figure 5.
Figure 5. Triggering the B2985A/87A from the B2911A
The following example describes how to set up the B2985A/87A and the B2911A to
perform LIV testing on a LD.
The setup used in the example can be downloaded from the following link:
www.keysight.com/nd/SensitiveMeasurement
A VBA sample program using SCPI commands is also available at the link above.
Parameters
V
alues
LD Current
Swept
Start
0 m
A
Stop
50 m
A
Points
201
LD
V
oltage
Limit
2.5 V
0.01 PLC
T
rigger
T
ype
TIMER
Count
201
Measurement Delay
100
sec
Period
500
sec
Output
Digital I/O Port 9
B2911A
Aperture
Time (SPEED)
Parameters
V
alues
PD
Voltage Constant
0.1 V
LD Current Measurement Range
2 m
A Fixed
Aperture
Time (SPEED)
0.01 PLC
T
rigger
T
ype
Manual
Count
201
Measurement Delay
100
sec
Source
TRIGGER IN
B2985A/87A

13 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Setting up the B2985A/87A from its front panel
1. Set “Dual Measure Result Display” off to make the instrument measure only current
(if this has not already been done).
a. Open the Display Preference dialog by pressing the , , ,
function keys.
b. Press to edit “Dual Measure Result Display”. The eld pointer should then be
highlighted in green (EDIT).
c. Press , and then press to apply the settings.
2. Set the current measurement range operation to FIXED.
a. Press to show the Meter View function keys and then press to set
the current measurement range operation to FIXED.
b. Press repeatedly until is shown on the function key. Note: In
this example the 2 mA range is best because the expected maximum current is
around several hundred micro amps.

14 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Setting up the B2985A/87A from its front panel (continued)
3. Select Manual Aperture Mode and set the Aperture Time
a. Press to select the manual aperture mode.
b. Press to show , where PLC stands for Power Line Cycle. 0.01 PLC
is the desired aperture time in this example, which is short enough to suppress
self-heating affects and long enough to provide the required accuracy.
4. Set “Triggered Voltage Source” to the desired PD voltage.
a. Hide the condensed Roll View by pressing , .
b. Rotate to select “Triggered Voltage Source” and press to edit it.
c. Set the voltage value to the desired PD voltage, which is 0.1 V in this example.
Use the arrow keys to move to the digits that you want to edit.

15 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Setting up the B2985A/87A from its front panel (continued)
5. Set the Trigger Parameters to make the instrument perform measurements
whenever it receives a trigger signal from the B2911A.
a. Press to show Trigger Parameters.
b. Press and press to select the manual trigger setting mode.
c. Rotate to select Trigger Parameters and ll in the values as shown below.
(Measure Count: 201, Measure Delay: 100 µs, Measure Trigger: TIN)
d. Press , , , to open the BNC Conguration dialog,
which denes the trigger signal properties.
e. Make sure that “Input Polarity” is set to “NEGATIVE” and then press .
If not, set it to “NEGATIVE”.

16 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Setting up the B2985A/87A from its front panel (continued)
6. Enable the Voltage Source and Ammeter.
a. Press Voltage Source to enable the Voltage Source.
b. Press Ammeter to enable the Ammeter.
Controlling the B2985A/87A using SCPI commands
If you prefer to control the B2985A/87A remotely using SCPI remote commands, then
the following material explains how to do this.
The series of commands shown in group “A” congure measurement parameters such
as measurement range and aperture time. Next the series of commands shown in group
“B” should be sent to set up the voltage source. Finally, the series of commands shown in
group “C” should be sent to set up the B2985A/87A’s trigger parameters.

17 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Set up the B2911A via front panel operation
1. Set the Source Function to Current
a. Press repeatedly until Single View for Channel 1 is shown.
b. Press and then press to set the Channel 1 Source Function to Current.
c. Press and then set the Channel 1 Source Value to 0 A.
2. Set the Voltage Limit Value
a. Press and then set the Channel 1 Limit Value to 2.5 V as shown.
3. Set the Aperture Time
a. Press , and then press to set the Aperture Time to SHORT
(0.01 PLC).
4. Enable remote sensing
a. Press , , to open the Output Connection dialog.
b. Press to select Sensing Type and then press to enable remote sensing.
c. Then press to apply the changes.

18 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Set up the B2911A via front panel operation (continued)
5. Set up the Sweep Parameters to make the instrument perform a current sweep.
a. Press to show the Sweep Parameters.
b. Press and press to turn on Single Linear Sweep Source Mode.
c. Rotate to select Sweep Parameters and ll in the values as shown below.
(Start: 0 A, Stop: 50 mA, Points: 201)

19 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Set up the B2911A via front panel operation (continued)
6. Set the Trigger Parameters to make the instrument step current at regular intervals.
a. Press to show the Trigger Parameters.
b. Press and press to select Timer Trigger Setting Mode.
c. Rotate to select Trigger Parameters and ll in the values as shown below.
(Measure Count: 201, Source Count: 201, Measure Delay: 100 µs,
Measure Period: 500 µs, Source Count: 500 µs)

20 | Keysight | Photodiode Test Using the Keysight B2980A Series - Technical Overview
Set up the B2911A via front panel operation (continued)
7. Set the Trigger Parameters to make the instrument output a trigger signal each time
the channel steps current.
a. Press , to open the Trigger Conguration dialog.
b. Press to select Layer and then press to select ACTION for Layer.
c. Rotate to select Trigger Output and ll in the entries as shown below.
d. Then press to apply the changes.
e. Press , , and then press to open the DIO
Conguration.
f. Press to edit Pin # and then select to set it to Pin 9.