Using Source Measure Units to Characterize High-Power Semiconductors (Part 1)




Power Semiconductor Product Circuit Elements





ON-State Characterization



Output Characteristics


DC testing can cause devices to selfheat, altering their measured characteristics, so pulsed testing is common in power semiconductor testing. Imple men ting a pulsed test with multiple SMUs requires precise timing of source adjustments and measurements, and is typically computer controlled. For consistent results, it is important to validate the system by sourcing a single pulse through it and measuring the response at the device. Capturing the complete pulse waveform as a function of time allows selecting appropriate source and measure delays so that the device turns on properly and measurements are made after the system settles.

Figure 4 shows the results of a pulse transient characterization of collector voltage and current vs. time of an IGBT. For this example, the subsequent DC measurement was delayed for 100μs after the start of the pulse to ensure the system had settled.

Power semiconductors are often high gain devices; oscillation is common when characterizing them, resulting in erratic measurements. Using pulse transient characterization enables the test engineer to identify oscillation. Resolvi ng this oscillation involves adding a resistor in series with the device control or input terminal; for example, the gate of a MOSFET or IGBT.

ON-State Voltage

A semiconductor device’s ON-state voltage directly impacts conduction loss. Examples of ON-state voltages include the forward voltage of a power diode (VF), the ON-state saturation voltage of a BJT or an IGBT (VCEsat), and the ONstate voltage of a thyristor (VT). Power consumed by or lost in the device is equal to the product of the ON-state voltage and the load current. This power is not delivered to the device. Typically, device manufacturers want to characterize how the ON-state voltage and, by extension, the conduction loss, varies with temperature and load current. SMUs are widely used in these characterizations.

To measure the ON-state voltage, a high-current SMU is configured to output current to the device and measure voltage. For BJTs and IGBTs, a second, lower-power SMU is used at the base or gate terminal to place the device in the ON state. Because power semiconductors are typically high-current devices, ON-state voltage is generally measured using a pulsed stimulus to avoid changes in device parameters from device selfheating. Two key elements help ensure a successful ON-state voltage test: (1) accurate voltage measurement, and (2) proper cabling and connections. Accurate voltage measurements are crucial because ON-state voltage varies with temperature. For instance, a few millivolts of difference in the forward voltage of a power diode can indicate a change of several degrees in the temperature at the device.

For power diodes, BJTs, and IGBTs, typical test currents can range from 100mA to tens of amps, while ON-state voltages of 1–3V are very common. Thyristors have very low ON-state voltages (<2V) and can conduct currents that could be greater than 100A. During testing of these devices, such high currents can generate voltage drops across test leads and test lead connections between the instrument and the DUT. These additional voltages create errors in the voltage measurement. Four-wire or Kelvin connections eliminate most of these voltage errors from the measurement by using separate test leads for the voltmeter. Minimal current flows in these leads, creating virtually no voltage drops between the instrument and DUT, so the instrument measures the voltage seen at the DUT. Using low-inductance cables helps ensure pulse fidelity (i.e., short rise and fall times) when testing high-current devices, which provides more time for accurate DC measurement within a given pulse width.

Transfer Characteristics

A device’s transfer characteristics allow evaluating its transconductance and therefore its current carrying capability. Transfer characteristics have an indirect relationship to determining switching time and estimating switching losses. The transfer characteristics are often monitored vs. temperature in order to gauge temperature’s effect on the device’s maximum current handling capability. Two SMUs are required for measuring the transfer characteristics: one sweeps the input voltage on the control terminal of the device, and the second biases the output terminal and measures the output current. Typical transfer characteristic measurements include the gate voltage vs. drain current plot for a MOSFET (VDS–ID), the gate voltage vs. collector current plot for an IGBT (VGE–IC), and the Gummel plot for a BJT (VBE vs. IC, IB).

In some cases, a wide range of output current is measured, such as in a Gummel plot of a BJT, where several orders of magnitude of current are traversed. Figure 5 is a Gummel plot generated using a high-current SMU on the collector and a low-current SMU on the base.


The figure of merit for a power MOSFET is the product of ON-resistance (RDS(on)) and gate charge (QG). The ON-resistance is the key determinant of the conduction loss of the power MOSFET. The conduction loss is equal to ID * RDS(on). Newer devices have ON-resistances in the range of a few milliohms to tens of milliohms at high current. This requires very sensitive voltage measurement capability at the drain terminal. Measuring ON-resistance re quires two SMUs: one to drive the gate into the ON state, and a second to pulse a defined current at the drain and measure the resulting voltage.

ON-resistance is often characterized as a function of drain current or gate voltage. Both SMUs can be triggered and swept to perform this measurement within a single test. Figure 6 shows the calculated RDS(on) vs. drain current results. For very-high-current devices, two high-current SMUs can be used in parallel to generate current pulses up to 100A.

ON-resistance increases with breakdown voltage, so any process adjustments made to increase the breakdown voltage will involve later testing of ONresistance in order to assess the overall impact of process changes. Obtaining more efficient devices at higher voltages is one of the drivers for further research on SiC and GaN devices, which offer ON-resistances smaller than those of silicon devices at high withstand voltages.

Part 2 of this article addresses the use of SMUs in OFF-state characterization of highpower semiconductors. View Part 2 online at www.techbriefs.com/august/feature2.

This article was written by Jennifer Cheney, Staff Applications Engineer at Keithley Instruments in Cleveland, OH (www.keithley. com), part of the Tektronix test and measurement portfolio.