Making Low Current Pulse I-V
Measurements with the 4225-PMU
Pulse Measure Unit and 4225-RPM
Remote/Preamplifier Switch Modules
––
APPLICATION NOTE
Making Low Current Pulse I-V Measurements with the
4225-PMU Pulse Measure Unit and 4225-RPM Remote/Preamplifier Switch Modules
APPLICATION NOTE
Introduction
The ability to source high-speed voltage pulses and
simultaneously measure low current simultaneously is
important to many semiconductor device applications. Using
pulsed I-V rather than DC signals to characterize devices
allows eliminating or studying the effects of self-heating or
to minimize current drifting in measurements due to trapped
charge. Transient I-V measurements allow an engineer or
scientist to capture a current or voltage waveform in the time
domain or study a dynamic test circuit.
The 4200A-SCS Parameter Analyzer with the 4225-PMU
Pulse Measure Unit option supports making ultra-fast I-V
measurements. Adding 4225-RPM Remote Preamplifier/
Switch Modules (Figure
1)
expands the current ranges of the
PMU down to 100 nA full scale to make sensitive pulsed low
current measurements.
Given that semiconductor device measurements often require
measuring low currents, using the PMU with two RPMs can
provide these capabilities:
• Two independent or synchronized channels of pulsed I-V
source and measure
• A wide current measurement range, from ± 800 mA down
to ± 100 nA full scale
• Autoranging over all current ranges – ideal for I-V tests
on devices with a wide range of current response,
such as diodes
Figure 1. 4225-RPM Remote Preamplifier/Switch Modules.
Figure 2. Offset current for the 100 nA range at 0 V (average 10
discreet pulses)
• The ability to switch between DC I-V, C-V, and pulsed I-V
• Built-in interactive software for easy test configuration
and execution
This application note defines ultra-fast I-V, explains the
fundamental limits of current measurements as a function of
time and measure window, and describes the techniques for
making ultra-fast I-V low current measurements.
The graph in Figure
2
illustrates the low current capability
of the PMU with RPMs. In this graph the open circuit
offset current is measured as a function of time at 0 V. The
measurements were made on the 100 nA range with a pulse
width of 800 µs and averaging 10 pulses.
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Making Low Current Pulse I-V Measurements with the
4225-PMU Pulse Measure Unit and 4225-RPM Remote/Preamplifier Switch Modules
APPLICATION NOTE
What is Ultra-Fast I-V?
The 4225-PMU can perform three types of ultra-fast I-V tests: pulsed I-V, transient I-V, and pulsed sourcing. These three modes
are illustrated in
Figure 3.
Samples
Sequence A
Voltage
Voltage
V2
V3
5
V4
1
V1
2
3
4
Time
Measure Window
Time
Sequence A De nition
Segment Start V Stop V Duration
1
V1
V2
T1
2
V2
V2
T2
3
V2
V3
T3
4
V3
V3
T4
5
V3
V4
T5
Pulsed I-V
Pulse/Measure with DC-like results
Train, Sweep, Step modes
General device characterization
Transient I-V
Time-based I and V measurements
Waveform capture
Dynamic device testing
Pulsed Sourcing
Multi-level pulsing
Arbitrary waveform generator
AC stress testing with measure
Figure 3. The 4225-PMU Ultra-Fast I-V Module’s three modes of operation.
Pulsed I-V
refers to any test with a pulsed source and
a corresponding high speed, time-based measurement
that provides DC-like results. The current and/or voltage
measurement is an average of readings taken in a
measurement window. This average of readings is called the
“spot mean.”
Transient I-V,
or waveform capture, is a time-based current
and/or voltage measurement that is typically the capture of
a pulsed waveform. A transient test is typically a single pulse
waveform that is used to study time-varying parameters.
Pulsed Sourcing
involves outputting user-defined two-level
or multi-level pulses using the built-in Segment ARB
®
function
or outputting an arbitrarily defined waveform.
Fundamental Limits of High-Speed
Low Current Measurements
The fundamental limit to a current measurement is Johnson-
Nyquist noise in the source resistance. In any resistance,
thermal energy produces motion of charged particles. This
charge movement results in noise which is also called thermal
noise. The power (P) available from this motion is given by:
P = 4kTB
where:
k
= Boltzmann’s constant (1.38 x 10
–23
J/K)
T
= absolute temperature (K)
B
= noise bandwidth (Hz)
Johnson current noise (I) developed by a resistor (R) is:
I
=
4k
T R B
R
amperes, rms
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Making Low Current Pulse I-V Measurements with the
4225-PMU Pulse Measure Unit and 4225-RPM Remote/Preamplifier Switch Modules
APPLICATION NOTE
All real current sources have internal resistance, therefore, they exhibit Johnson
noise.
Figure 4
shows the current noise generated by various resistances vs.
measurement window in time at room temperature (T = 290K). The measurement
window, in seconds (s), is defined in terms of the bandwidth (B) by:
1
B
=
(2 ×
Measure Window)
High Impedance
Measurement Techniques
To make optimal low current and high
impedance pulsed I-V measurements
you must use the appropriate
measurement techniques which are
outlined in the following sections.
If an application requires a current measurement that is below the line representing
the DUT resistance, it will not be possible to make the measurement due to Johnson
noise in the resistor. Reducing the temperature of the source resistance will help to
reduce the noise levels.
Johnson-Nyquist Noise vs. Measure Window
1E–7
1E–8
Correct for Current
Offsets Using
Connection Compensation
You can easily correct for offset current
errors caused by connections and
cable length between the PMU and the
device under test by using the built-
in connection compensation feature.
When connection compensation is
Current RMS Noise (A)
1E–9
1E–10
1k
1E–11
1E–12
1E–13
1E–14
1E–9
1E–8
1E–7
1E–6
1E–5
1E–4
1E–3
1E–2
10 k
100 k
1M
10 M
100 M
1G
10 G
enabled, the measured offset current
values are factored into each current
measurement. The PMU offset current
correction is a two-part process:
1.
Acquire the current offset on each
current range with an open circuit.
Probes must be up or the device
removed from the test fixture during
the compensation. In Clarius, go
to the Tools menu at the top of the
screen and select PMU Connection
Compensation. Select Measure
Offset and the error current on all
ranges on both channels will be
measured and stored.
2.
Once the offset correction data is
acquired, enable the connection
compensation within a test. From
the Configure view within the
test, go to the Terminal Settings
tab and select Advanced. In
the upper-right corner, select
Offset Current Correction. Stored
compensation measurements will
be automatically subtracted from
subsequent readings.
Measure Window (s)
Figure 4. Current noise vs. measure window at various source resistances.
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Making Low Current Pulse I-V Measurements with the
4225-PMU Pulse Measure Unit and 4225-RPM Remote/Preamplifier Switch Modules
APPLICATION NOTE
Figure 5
illustrates an example of pulsed I-V curves taken
with and without offset correction applied.
When CH1 sources the voltage, some of the current flows to
the device (I
DUT
) and some of the current flows through the
leakage resistance and capacitance of the cable (I
LEAK
). The
measurement includes both the current of the device and the
leakage current of the cable (I
MEAS
= I
DUT
+ I
LEAK
).
CH2 is set to output 0 V so that both the center conductor
and outside shield of the cable are at the same potential,
0 V. As a result, no leakage flows through the cable. The
measured current is the device current (I
MEAS
= I
DUT
). Multiply
the current of CH2 by (-1) using the Formulator because the
current will be the opposite polarity. The current measured by
CH2, not CH1, is used in the high impedance measurement.
This method allows for faster measurements (because there’s
need to wait for CH1 to settle) and avoids errors due to
leakage of the cable and long settling times.
Figure 5. Pulsed I-V sweeps of 1 GΩ resistor generated with and
without offset correction.
I
DUT
5V
0V
Coax Cable
DUT=HiR
I
DUT
0V
0V
For optimum performance, repeat the connection
compensation whenever the connection setup is changed or
disturbed, including changes in temperature and humidity.
Use the “Low Side” Measurement Technique
The “low side” measurement technique is used to make
ultra-fast high impedance measurements. With this method,
illustrated in
Figure 6,
CH1 is used to source voltage only and
CH2 is used to measure current only.
PMU CH1
Source V
I
MEAS
= I
DUT
+ I
LEAK
PMU CH2
Measure I
Set V=0V
I
MEAS
= I
DUT
Figure 6. “Low side” high resistance ultra-fast measurement
technique.
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