Keysight Technologies B2980A Series Guía de instalación

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

B2985A/87A

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

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