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Voltage Current

New Current Source and Measurement Techniques

Currents up to 100A may be required in a wide variety of high-power device characterization applications. High test currents could be needed for devices such as insulated gate transistors (IGBTs), MOSFETs, RF power transistors, high-brightness LEDs, solar cell arrays, and power management devices. There are two problems associated with this type of testing: (1) finding a single DC power supply that can deliver the required current, and (2) avoiding excessive device temperatures when applying such high currents. The latter is usually accomplished by applying high currents as relatively short pulses. This means the power source must be capable of pulse mode operation up to the peak current needed for the test. Finding a DC power supply with these specifications may not be easy.

A pulsed source is often essential for testing a power device because high DC current would skew the resistance value of the device under test (DUT) due to Joule heating. DC current sources typically don't let you pulse their outputs. Although high-power pulse generators are available, they have no built-in measurement capabilities, so they require synchronizing the operation of a separate ammeter with the pulsed test signal. Their cost and complexities in the test set-up tend to make pulse testing expensive. Still, you can create an economical pulsed DC current source yourself with the appropriate source-measure unit (SMU), even if its maximum specified output current doesn't quite reach the level needed.

Pulsed sweeps for higher power. With the right SMU features, you can substitute a pulsed sweep for a DC sweep to obtain higher power I‑V cure with little detriment to your device characterization results. However, you must recognize that testing some DUTs (such as capacitors) with pulsed sweeps may not correlate adequately with DC sweeps. This is due to large displacement currents that can be generated at the sharp edges of the voltage pulse, which may change these devices' electrical properties. On the other hand, pulsed I‑V testing is essential for other device types, such as RF power amplifiers or even low-power nanoscale devices, to obtain optimal results.

During high-power continuous wave DC testing, semiconductor material in the DUT will start to dissipate applied power as heat. As the DUT heats up, conduction current decreases because the semiconductor charge carriers have more collisions with the vibrating lattice (i.e., phonon scattering). Therefore, the measured current will be erroneously low due to self-heating effects. Given that these types of devices typically run in pulsed mode (intermittently rather than continuously), the erroneously low DC current measurements won't accurately reflect their normal performance. In these circumstances, pulsed testing must be used.

You must take two factors into account when changing from a DC sweep to a pulsed sweep. The pulse must be wide enough to allow sufficient time for transient conditions within the DUT, cabling, and other interfacing circuitry to settle out. This allows measurement instruments to take stable, repeatable readings. At the same time, however, the pulse cannot be so wide that it exceeds the test instrument's maximum pulse width and duty cycle limits, which would violate the instrument's allowed power duty cycle. Pulses that are too wide can also create the same device self-heating problems that can occur with DC sweeps.

Combining multiple SMU channels to achieve higher DC current. Using a dual-channel SMU (or two separate SMUs) you may be able to get the test current needed by combining the outputs from two channels. The most common way of doing this is to connect the current sources (channels) in parallel across the DUT. This test setup takes advantage of a well-known electrical principle (Kirchhoff's current law), which states that two current sources connected to the same circuit node in parallel will have their currents added together. In this case, both SMU channels source current to the DUT and measure the resulting voltage across it. All of the LO impedance terminals (FORCE and SENSE) of both SMUs are tied to earth ground. This test situation is described as follows:

IDUT = ISMU1 + ISMU2

VDUT = VSMU1 = VSMU2

IMAX = IMAX(SMU1) + IMAX(SMU2)

VMAX = smaller of the two SMUs' maximum voltage capabilities

In such a configuration, you should set the output currents for SMU1 and SMU2 to the same polarity to obtain maximum output. Whenever possible, one SMU should be in a fixed source configuration and the other SMU performs the sweep. This is preferable to having both sweeping simultaneously. If both SMUs are sweeping, their output impedances are naturally changing, for example as the meter autoranges up and down. The DUT's output impedance may also be changing significantly, such as from a high-resistance off-state to a low-resistance on-state. With so many of the impedance elements in the circuit changing, this could increase overall circuit settling time at each bias point. Although this is a transient effect that damps out, fixing one SMU's source and sweeping the other usually results in more stable and faster-settling transient measurements, for higher test throughput.

Merging pulse sweeps with combined SMU channels. New SMU architectures are simplifying the merger of pulse sweep power measurements with multiple SMU channels that are operated in parallel. With certain precautions, you may even be able to use more than two SMUs to achieve even higher test currents. For example, some dual-channel SMUs allow increasing the number of operating SMU channels from two to four. Using pulse sweep and multi-channel capabilities in tandem allows sourcing far higher currents than using a single SMU with DC sweeps.

Obviously, implementing this test method demands the exercise of extraordinary caution to ensure personnel safety. For safety, it is critical to insulate or install barriers to prevent user contact with live circuits. Additional protection techniques are needed to prevent damage to the test setup or the DUT. The multiple pulses must be tightly synchronized (with nanosecond precision) so that one piece of equipment is not applying power and damaging units that are not yet turned on.

The author tested this concept by first using a single SMU to generate a 10A pulse with a width of 300µs, and observing the resulting voltage pulse across the DUT were on an oscilloscope. A high power precision resistor (0.01W, ±0.25%, KRL R-3274) was used as the test DUT. The oscilloscope showed a nearly square waveform of 0.1V (10A × 0.01 ohm) in amplitude and 300 microsecond width. Combining four SMUs in parallel to pulse 40A across the same DUT resulted in a waveform of 0.4V magnitude with excellent synchronization (low jitter) between the channels. Pulse consistency was verified using the same test setup and pulse waveform.

With the pulse performance verified, the test set-up was configured for a pulse sweep that combined the outputs of four SMUs and took measurements to generate an I‑V curve for a P-N diode as the DUT. There was excellent correlation as one-SMU conducted DC sweeps up to 3A, and another was used for one-SMU pulse sweeps up to 10A. Then, the I‑V curve was extended on up to 40A using four SMUs for pulse sweeps, each outputting a 10A pulse. There was smooth continuity in the curve all the way up to 40A.

This experiment verifies the validity of combining four SMU channels and pulsing to achieve 40A on two-terminal devices (resistor and diode). With certain modifications, this technique is equally valid when applied to testing a three-terminal device, such as a high-power MOSFET.

Implementation of multi-SMU pulsed sweeps. Several factors are critical to maximizing device characterization accuracy and precision when using this multi-SMU pulsed sweep approach. In addition, precautions must be taken to prevent damage to an SMU due to inappropriate connections or accidental disconnection of the DUT during a test. These factors are detailed below:

Using source readback: An SMU has both source and measure functions built into the same unit, so it's capable of reading back the actual value of the applied voltage using its measurement circuitry. The programmed value for the source voltage may not be the same as the voltage actually applied to the DUT; with multiple SMUs in parallel, the source offsets may add up to be quite significant, so using source readback provides a clearer picture of the level of voltage actually being sourced, not just the voltage that's been programmed.
Making four-wire measurements: Four-wire (Kelvin) measurements are necessary when doing high current testing because this technique bypasses the voltage drop in the test leads by bringing two very high-impedance voltage sense leads out to the DUT. With very little current flowing into the SENSE leads, the voltage seen by the SENSE terminals is virtually the same as the voltage developed across the unknown resistance. At 40A levels, even a small resistance, such as 10milliohms in the test cable, can generate a voltage drop of 0.4V. So if the SMU is forcing 1V at 40A current and the cable resistance is 10milliohms and there are two test leads, the DUT might only receive a voltage of 0.2V, with 0.8V dropped across the test cables.

Unlike source readback, which primarily impacts just the source values, making four-wire measurements will result in significantly better accuracy on both the sourced and measured values. The reason is that Kelvin connections eliminate the voltage drop in the current-carrying wires that would otherwise affect the measurement.

Putting no more than one voltage source at each DUT node: It is common in many test sequences to perform voltage sweeps, i.e., force voltage and measure current (FVMI). In the case where more than one SMU is connected in parallel to a single terminal of the DUT, the obvious implementation would be to have all of the SMUs in voltage-source mode and measure current. However, three factors must be considered:

- SMUs when sourcing voltage are in a very low-impedance state.

- DUTs can have impedances higher than an SMU that's in voltage-source mode. The DUT's impedance can be static or dynamic, changing during the test sequence.

- Even when all SMUs in parallel are programmed to output the same voltage, small variations between SMUs related to the instruments' voltage source accuracy mean that one of the SMU channels will be at a slightly lower voltage (millivolt order of magnitude) than the others. If, for example, three SMUs are connected in parallel to one terminal of a DUT, and each SMU is forcing voltage and outputting near-maximum currents, and the DUT is in a high-impedance state, then all current will go to the SMU that is sourcing the slightly lower voltage. It's more than likely this will damage that SMU. Therefore, when connecting SMUs in parallel to a single terminal of a DUT, only one SMU should be sourcing voltage. (Other SMUs can be sourcing current.)

Mitigating excessive energy dissipation due to contact failure: When you connect two or more SMUs with the same output capacity in parallel to a single node in the circuit, one SMU must be able to sink all of the current being output by the other SMU. This scenario can occur, for example, when one of the leads breaks contact with the DUT (i.e., if the lead is accidentally disconnected or a contact isn't made properly). That means there is a short period during which one SMU must sink all the current from the other instrument. However, when there are more than two SMUs connected in parallel at a single circuit node, a single SMU cannot sink all of the current coming from the other units. The SMU that will be forced to sink current if there's a break in contact with the DUT is the SMU at the lowest voltage or lowest impedance (most likely the one sourcing voltage).

In order to protect the signal input of the SMU forcing voltage, a diode such as the 1N5820 can be placed between the voltage source SMU output and the DUT. A diode is preferable because a fuse would react too slowly to provide protection and a resistor will cause too large of a voltage drop across it. A diode offers a much faster response than a fuse and has a much smaller maximum voltage drop across it (typically around 1V) than a resistor. However, to be truly safe when using this method, a diode should be used to protect all the SMUs in the configuration. That's because if the DUT goes into a high-impedance state, the current sources will try to force their current into the voltage-sourcing SMU, but that would not be possible because the voltage-sourcing SMU is protected by a diode. That would cause the current-sourcing SMUs to increase their output voltage until they reached their voltage limit. Once this occurred, the current sources would go into compliance and become voltage sources themselves. That would mean there would be multiple voltage sources in parallel. Even if their voltage limits were set to exactly the same value, their outputs would still likely be very slightly different and they would damage each other.

It's important to be aware that putting a diode on each and every SMU in the configuration has some consequences. First, the inclusion of any diodes in the configuration means this method can only be used to source power but not to sink it because the diodes will not allow current to pass into the SMU. The second consequence is that, in order to obtain maximum output voltage, you will need to use four-wire connections on the current sources around the diode because the voltage drop across diode may cause the current sources to reach compliance prematurely. At these current levels, the typical voltage drop across a diode is about 1V.
Safety issues: Many electrical test systems or instruments are capable of measuring or sourcing hazardous voltage and power levels. If voltages in excess of 40V will be used during the test sequence, the test fixture and SMUs must have the proper interlock installed and be operated in accordance with normal safety procedures. It's also possible, under single fault conditions (e.g., a programming error or an instrument failure), to output hazardous levels even when the system indicates no hazard is present. These high levels make it essential to protect operators from any of these hazards at all times. Protection methods include:

- Verify the operation of the test setup carefully before it is put into service.

- Design test fixtures to prevent operator contact with any hazardous circuit.

- Make sure the device under test is fully enclosed to protect the operator from any flying debris.

- Double insulate all electrical connections that an operator could touch. Double insulation ensures the operator is still protected, even if one insulation layer fails.

- Use high reliability, fail-safe interlock switches to disconnect power sources when a test fixture cover is opened.

- Where possible, use automated handlers so operators do not require access to the inside of the test fixture or have a need to open guards.

- Provide proper training to all users of the system so they understand all potential hazards and know how to protect themselves from injury. It's the responsibility of the test system designers, integrators, and installers to make sure operator and maintenance personnel protection is in place and effective.

Summary. SMUs offer a simple, highly integrated approach to designing cost-effective test and measurement systems for a wide range of electronic devices. For the growing number of test applications that demand the ability to source and/or measure higher currents, the techniques outlined in this article offer useful alternatives to combining separate sources and measurement instruments, which may include expensive high-power pulse generators.

References. For more information on techniques for implementing high current test configurations, including cabling and test fixture details, download Keithley's Application Note #3047, "Methods to Achieve Higher Currents from I-V Measurement Equipment," available at www.keithley.com/data?asset=52630.

About the Author

David Wyban is an Applications Engineer with Keithley Instruments, Inc., Cleveland, Ohio. He joined the company in 2006, working on the team that developed Keithley's line of System SourceMeter® instruments. He holds a bachelor's degree in electrical & computer engineering from The Ohio State University.