Using Source Measure Units to Characterize High-Power Semiconductors (Part 1)
- Thursday, 01 August 2013
The proliferation of electronic control and electronic power conversion into a variety of industries (e.g., energy generation, industrial motor drives and control, transportation, and IT) has made efficient power semiconductor device design and test more critical than ever. To demonstrate technology improvements, new device capabilities must be compared with those of existing devices. The use of semiconductor materials other than silicon demands the use of new processes. To be sustainable, these new processes must be tuned to deliver consistent results and high production yield. As new device designs are developed, reliability measurements must be performed on many devices over long periods. Therefore, test engineers must identify test equipment that is not only accurate, but scalable and cost-effective.
Power module design engineers — the consumers of the discrete power semiconductor components — work at the other end of the semiconductor device testing spectrum from the device test engineers. They integrate the discrete components into designs for DCDC converters, inverters, LED controllers, battery management chips, and many other devices. Driven by demands for higher energy efficiency, these engineers need to qualify the devices they receive from their vendors to ensure that they can withstand use in the application, predict how the efficiency of the power modules may be affected by the device, and validate the performance of the end product.
Source Measure Units (SMUs) give both types of engineers the tools they need to characterize new devices quickly. This article highlights some of the most commonly performed tests, the challenges associated with them, and the advantages the newest generation of high-power SMUs offer for characterizing new high-power semiconductors based on both silicon and new widebandgap materials.
Power Semiconductor Product Circuit Elements
The switching power supply is one circuit element commonly used in power management products. Its main components include a semiconductor such as a power MOSFET, a diode, and some passive components, including an inductor and a capacitor (Figure 1). Many also include a transformer for electrical isolation between the input and output. The semiconductor switch and diode alternatively switch on and off at a controlled duty cycle to produce the desired output voltage.
When evaluating a device’s energy efficiency, engineers need to understand both its switching loss (which occurs when the device is changing states) and conduction loss (which occurs when the device is either on or off). This article focuses on conduction loss. Although curve tracers were once the instruments of choice for device characterization, engineers are increasingly turning to solutions that configure one or more SMUs into parametric curve tracer systems to evaluate the device parameters that affect conduction loss. SMUs integrate the capabilities of a precision power supply with those of a high-performance digital multimeter in a single instrument. For example, they can simultaneously source or sink voltage while measuring current, and source or sink current while measuring voltage. They can also be used as pulse generators, as waveform generators, and as automated current-voltage (I-V) characterization systems. The newest SMUs offer higher power capabilities (up to 3kV) to support power semiconductor characterization and test.
Semiconductor devices like thyristors are often employed for overvoltage protection. To achieve that objective, such devices must trigger at the appropriate voltage and current, must withstand the intended voltage, and must behave in circuit with minimal current draw. Highpower instrumentation (including the latest generation of SMUs) is essential to qualify these devices properly.
Static power device parameters can be divided into two broad categories: those that determine a device’s performance in its ON state and those that determine the performance in its OFF state.
ON-state characterization is typically performed using a high-current instrument capable of sourcing and measuring low voltages. If the device has three terminals, a second SMU is used at the device control terminal to place the device in the ON state. Figure 2 illustrates a typical configuration for characterizing the ON-state parameters of a power MOSFET.
Let’s examine the configuration details and measurement challenges of a few ON-state parameters.
A semiconductor device’s data sheet normally includes a plot of its output characteristics that depicts the relationship between the output voltage and current. For a gated power semiconductor switch such as a MOSFET, IGBT, or BJT, output characteristics are commonly referred to as the “family of curves.” Figure 3 shows the results for a commercially available power IGBT.
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.
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.
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.