Optimize instrument setup to ensure quality measurements

Once the system is built, its functionality must be tested and the instrumentation setup optimized to obtain the best measurements.

In most of the on-state characterizations performed today, pulsed signals are applied to the device to achieve minimum heating. In addition, many power semiconductor devices are also operated under pulsed conditions in their final applications. Passing the output pulse of the source measurement unit (SMU) through the test system and capturing the device port response qualifies the test system for pulsed signal capabilities. Keithley's Model 2651A High Power System SourceMeter contains high-speed analog-to-digital converters (ADCs) that can digitize both current and voltage waveforms. These analog-to-digital converters (ADCs) are very helpful in determining the pulse performance of the system. If the pulse shape is abnormal, check the cable to ensure that the lead inductance is minimized. Use low-impedance coaxial cable provided by Keithley whenever possible, and minimize the inductive loop area of ​​other leads.

For Off-State Characterization

Understand the source and measurement settling time of the system. Device off-state measurements are usually made using a high voltage source where the current is low and the device is in a high impedance state. However, there may be unavoidable system capacitance and the device itself has some capacitance. Power semiconductor transistor output capacitance is typically above 100pF. In the off state, the device resistance may be above 1GΩ, making the single RC time constant 100ms or more. Note the voltage-time curve of the charging capacitor in Figure 1. In order to generate a settling reading, you must wait at least 4 to 6 time constants (4 time constants = 99%), which may mean a settling time of nearly 1 second. Consider this settling time when estimating production throughput or the time required for long tests such as device reliability testing.