Measure leakage current by holding a semiconductor in its specified off state, applying a controlled voltage across the relevant terminals, and recording current after the reading settles. For a diode, that is typically reverse current versus reverse voltage; for a MOSFET or IGBT, define the gate condition and measure drain-source or collector-emitter cutoff current. Reliable nanoamp measurements depend just as much on the fixture, guarding, settling time and safety controls as on the source-measure unit (SMU).
What leakage current means—and which terminals to measure
Leakage current is the current flowing between specified terminals while a device is intended to be off. It contributes to standby power, and its value depends on applied terminal voltage and temperature. A leakage result is meaningful only when it states the terminals, polarity, off-state control conditions, voltage and temperature.
Diodes
For a diode, the usual characterization is reverse current as a function of reverse voltage. State which terminal is biased positive, the voltage steps or sweep, and the temperature. Do not treat a single current reading as a complete leakage curve.
MOSFETs and IGBTs
For a MOSFET, specify the gate-to-source condition and measure drain-to-source current with the device off. For an IGBT, specify the gate-to-emitter condition and measure collector-to-emitter cutoff current. State the voltage polarity and terminal convention so the result can be reproduced and compared with the device’s specified test conditions.
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How to measure leakage current safely and reproducibly
- Mount the device in an enclosed, interlocked fixture. Confirm that the fixture, leads and connections are rated for the planned voltage and stored energy. A Keithley example uses a magnetic-relay interlock that turns the SMU output off when the cover is opened; an interlock is a safety feature, not a substitute for appropriate discharge provisions.
- Write down the test conditions before applying bias. Define the device terminals and polarity, gate bias, test temperature, voltage sweep or steps, current limit (compliance), and dwell or settling time. Record the instrument range, integration time and whether auto-zero is enabled. State any preconditioning used.
- Measure the empty fixture and cabling. Before inserting the device, step the voltage through the planned test range and measure the system current. This establishes whether the fixture or cabling contributes current that could be mistaken for DUT leakage. Investigate an unstable or unexpectedly high baseline before proceeding.
- Use guarding and shielding for very low currents. For measurements below 1 µA, use triaxial cabling, electrostatic shielding and the instrument guard where supported. The guard diverts leakage around the measurement terminal, reducing the current that reaches the measurement circuit. Keep insulation clean and control humidity, since surface contamination and moisture can create competing leakage paths.
- Apply bias within all ratings and observe the current response. Sweep or step the voltage only within the device and fixture ratings, using a suitable current limit. Continue to the specified breakdown-current criterion or stop when compliance is reached. Do not identify breakdown from one noisy reading; look for a sustained change in the current-voltage behavior.
- Repeat at required temperatures and retain the full conditions. Report current at each stated voltage and temperature, along with uncertainty, preconditioning, instrument settings and delays. Temperature-dependent results are not interchangeable with room-temperature measurements.
Choosing an SMU or test method
Choose equipment against the full test, not just a headline voltage rating. Keysight describes controlled high-voltage sweeps for IGBT leakage and breakdown characterization up to 10 kV; that capability is an example of a test range, not a universal requirement or a claim that every analyzer supports it. Model availability, connectors and suitability must be checked for the intended setup.
| Selection factor | What to verify |
|---|---|
| Voltage and current compliance | That the source can reach the required test voltage and that current limiting can be set appropriately for the DUT and fixture. |
| Current resolution and uncertainty | Whether the measurement can resolve the target leakage with useful uncertainty on the chosen range. |
| Guarding and connections | Support for triaxial connections, guard and electrostatic shielding in the actual fixture. |
| Sweep and measurement control | Control of voltage steps or sweeps, settling delays, integration time, range and auto-zero settings. |
| Temperature control | Whether the setup can establish and record the temperatures required for the characterization. |
| Safety and fault handling | Interlock, current limiting, discharge provisions and suitability for the energy that may remain after a DUT failure. |
| Data traceability | Ability to export readings together with test conditions and instrument settings. |
Continuous sweeps, stepped tests and heating
A continuous sweep can show how current changes across voltage, but high-voltage, high-current measurements can challenge conventional curve tracers through power compliance and self-heating. Stepped or pulsed approaches may reduce self-heating; the method and timing must be documented because they affect the measured current. Add source and measurement delays when device or fixture capacitance, or very low current, makes readings settle slowly. Longer settling is often necessary as the expected current decreases.
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How to interpret the current-voltage curve
A stable low-current region is consistent with normal off-state leakage under the stated conditions. A progressive rise as voltage increases may reflect field-assisted generation, surface or junction leakage, punchthrough, or approach to avalanche. The curve alone does not identify the mechanism: interpretation depends on device structure and temperature. IEEE analysis of power VDMOSFETs identified dislocation-induced punchthrough and channel shortening as a root cause of excessive drain-source leakage.
Published examples are not universal limits
- SiC MOSFET modules: An IEEE study published in 2017 reported approximately 100 nA leakage at room temperature for 10 kV SiC MOSFET power modules. The figure is tied to that study’s devices and conditions; the cited summary does not give the voltage associated with that particular reading, so it is not a general acceptance limit or a stand-alone benchmark.
- IGBT example: A Keithley/Electronic Design example describes a 4,000 V IGBT with a specified maximum cutoff current of 100 µA at 4,000 V. In that example, current was still not rising rapidly at 4,500 V, indicating rapid breakdown had not yet begun. The retrieved page does not state a publication year. These values describe that example, not IGBTs generally.
Common sources of misleading readings
- Fixture leakage comparable to DUT current: At nanoamp and sub-microamp levels, leakage through cables, insulation or fixture surfaces can be comparable to the device current. Use an empty-fixture baseline, guarding, shielding, clean insulation and humidity control.
- Reading before the circuit settles: Capacitance and low current can delay stable readings. Record source and measure delays as well as integration settings; otherwise another engineer cannot reliably reproduce the result.
- Unstated instrument configuration: Range, integration time and auto-zero affect how readings are acquired. Include them with the reported data rather than treating the number alone as the result.
- Self-heating or a different drive method: Power dissipation and whether bias is continuous, stepped or pulsed can change measured current. Document the method and timing, and do not compare results as though the conditions were identical.
- Unsafe breakdown testing: A DUT breakdown can leave hazardous potential on SMU inputs. Use current limiting, an interlocked enclosure, discharge paths and equipment rated for the maximum stored energy. Do not open or handle a high-voltage fixture until the system has been made safe according to its design.
Keithley’s application guidance describes high-voltage semiconductor characterization as typically including breakdown-voltage and leakage-current study. In practice, a useful result is not simply the lowest current displayed: it is a traceable current-versus-voltage measurement with stated terminal conditions, temperature, settling and instrument configuration, supported by a fixture baseline and appropriate safety controls.
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