Semiconductor devices are rapidly gaining in output-power capabilities, whether used in continuous-wave (CW) circuits or in pulsed applications. Because they have continued to increase power levels while remaining at such smaller sizes, power semiconductors have made vacuum-tube devices such as klystrons, magnetrons, and travelling-wave tubes (TWTs) all but forgotten except for designs at the highest power levels. But packing so much power into such small semiconductor chips and packaged devices poses the problem of properly testing these devices, for such parameters as breakdown voltage, leakage power, maximum output power, and typical operating lifetimes. Last month’s blog reviewed ways to perform capacitance-voltage (CV) measurements; this month’s blog will look at high-power semiconductors and some of the best and most reliable ways to test them for their most essential characteristics.
Of course, “high” is a relative term and there is no generally accepted definition for what constitutes high output power for any type of transistor, such as a bipolar or a field-effect transistor (FET). Each type of power semiconductor device, whether on wafer in chip form or in packaged form, must be tested for a set of characteristics by which a device supplier or circuit designer can determine whether that device can perform under various current and voltage (I-V) conditions. Typically, a power semiconductor is swept through a range of minimum to maximum current and voltage values, with constant or pulsed power, during a process known as curve tracing, testing for trends in device behavior over those ranges.
To do this, high-power semiconductor test equipment must serve as a very precise power supply, with current and voltage limits that typically exceed the range of the power supply to be used in a desired application. The test equipment must also be capable of accurately measuring the voltage, current, and power at each of a transistor gates or ports of other semiconductors, such as diodes or integrated circuits (ICs). Traditional test tools have provided supply and analysis functions as separate instruments although current trends in integration and modularity have resulted in more high-power semiconductor testers that provide precision power sources and high-power analyzers within the same instrument enclosure, with high-resolution display screens and software to automate the measurements.
One of the more essential tests performed on any power semiconductor is a determination of its breakdown voltage. Quite simply, breakdown voltage is the maximum amount of voltage that can be applied to a two- or three-terminal semiconductor device, such as a diode or transistor, respectively, without destroying it. Voltages beyond the breakdown voltage turn a semiconductor’s insulator materials to conductors, with destructive results. For an FET, for example, the breakdown voltage is measured from the drain to the source; as it approaches the breakdown voltage for a device under test (DUT), the current will begin to significantly increase. For a bipolar transistor, the breakdown voltage is measured from the collector to the emitter, with the base terminal open.
The breakdown voltage of a transistor is a function of the transistor type and semiconductor material. Materials with wide bandgaps, such as such as gallium nitride (GaN) or silicon carbide (SiC), are capable of higher breakdown voltages than silicon-based devices, requiring test gear with wider voltage ranges to measure a device’s I-V characteristics, find its breakdown voltage range, and determine performance parameters such as optimum voltage range, gain under different conditions, and maximum output power.
Testing for Power
High-power device testing calls for precise control of the power (I and V) to and from a DUT, with the enough power range to properly “exercise” a device. In some cases, power remains constant, at a DC voltage and current, and in some cases, the power to a DUT is pulsed to achieve higher power levels. For evaluating power semiconductors under pulsed conditions, a tester must be capable of emulating the operating conditions, with suitable control of pulsed parameters, such as pulse width, pulse repetition frequency (PRF), and pulse repetition interval (PRI). Whether working with its own integrated microprocessor or connected to an external PC, automated test equipment can take advantage of test software offered by many equipment suppliers to quickly and repeatably perform curve tracing of I-V trends and breakdown voltage characteristics.
The Keysight B1505A Power Device Analyzer and Curve Tracer highlighted in last month’s blog, in addition to C-V measurements, is quite capable of characterizing high-power semiconductors when equipped with either the company’s N1265A or N1268A expanders. The B1505A features a modular format, adding functionality as needed by plugging in one or more test modules.
The N1268A expander is one such module, extending covering the B1505’s maximum ranges for current (1500 A) and voltage (10 kV) to 1500 A and 10 kV, respectively, for enough test source power to reach a DUT’s breakdown voltage range and with the measurement capability to measure voltage and current (and power) at those high levels. The N1265A expander is a plug-in module that also works over those extended current and voltage ranges with a built-in test fixture for handling packaged power semiconductors. The B1505A is available with or without a PC instrument controller with the company’s I/CV 2.1 Lite automated test software to automate and simplify high-power device testing. Keysight also offers its model 4155C Semiconductor Parameter Analyzer in modular form. Its DC voltage range of ±100 V can be extended to ±200 V pulsed operation with a model 4150B source measure unit (SMU) and pulse generator expander module.
For basic curve tracing and DC parameter characterization of high-power devices, the Tektronix 371A High Power Curve Tracer offers continuous high-voltage and pulsed high-current operating modes. In high-voltage mode, testing can be performed to 3 kV at peak current of 40 mA (30 W maximum power). In high-current mode, pulses with as much as 400 A can be sent through at DUT at peak voltage of 30 V (3 kW maximum power).
The Keithley 2410 SourceMeter combines many tightly coupled measurement functions in one instrument, with maximum DC voltage of 1100 V and maximum current of 1 A (20 W power). For higher power testing, the Keithley 2430 Pulse Mode SourceMeter provides 1 kW maximum test power using as much as 10 A pulsed current at 100 V. Keithley’s 2651A SourceMeter SMU provides both DC and pulsed test modes, with as much as 200 W DC power and 2 kW pulsed power. A single unit can provide pulses to 50 A while two units can linked to generate 100-A pulses. When higher DC and pulsed power is needed, such as for characterizing high-power GaN and SiC transistors, Keithley’s 2657A SourceMeter SMU delivers as much as ±3000 V at 20 mA.
These are some of the instruments available for characterizing high-power semiconductor devices. An important part of performing those tests will be the availability of low-loss probes for on-wafer testing and test fixtures for packaged devices. Higher-power semiconductors continue to shrink in size as they reach higher output levels, fitting within surface-mount and other miniature housings that require suitable test fixtures as an interface to the test equipment. Suppliers of quality test equipment, such as Axiom Test and the manufacturers mentioned here, can help in specifying optimum probes or test fixtures when testing a specific device type.
To rent or purchase these or other test equipment, please visit Axiom’s homepage to view our inventory. If you would like help selecting the right equipment for your project, contact Axiom Test Equipment’s sales department at firstname.lastname@example.org, or call an Axiom sales representative at 760-806-6600.