Author: Tyco technology expert

After years of research and design, silicon carbide (SIC) and gallium nitride (GAN) power devices are becoming more and more practical. Although these devices have high performance, they also bring many challenges, including gate driving requirements. SiC requires a much higher gate voltage (VGS) and turns off when the bias voltage is negative. The threshold voltage (Vth) of Gan is much lower, which requires strict gate drive design. Due to the physical characteristics of wideband gap (WBG) devices, the fuselage diode voltage drop is high, so the control requirements for idle time and on / off jump should be more strict.

Accurate power supply and measurement are very important to characterize these high-voltage devices, so that correct design decisions can be made in time. Increasing design margin and over design will only increase cost and lead to performance degradation. In addition, these devices generally involve a high voltage of more than 200 V, so it is very important to ensure personal safety and prevent electric shock.

High voltage device test

The basic characterization of high voltage semiconductor devices generally requires the study of breakdown voltage and leakage current. These two parameters help device designers quickly determine whether the device is manufactured correctly and whether it can be effectively used in the target application.

Breakdown voltage measurement

When measuring the breakdown voltage, we should apply a continuously increasing reverse voltage to the tested device until a certain test current is reached, indicating that the device is broken down. Figure 1 shows the breakdown measurement on the high voltage diode using the source measurement unit (SMU) instrument, such as Keithley 2470 high voltage source meter SourceMeter ® SMU instrument. Note how the SMU instrument is connected to the negative pole of the diode and apply a reverse voltage. For high-voltage diodes, use safe triaxial cables and properly grounded safety distribution boxes.

  

Figure 1 Typical breakdown voltage measurement of high voltage diode using 2470 high voltage SMU instrument.

When judging the breakdown voltage, it is generally measured at a level far higher than the expected rated value of the device under test, so as to ensure the robustness and reliability of the device under test. SMU instruments (such as 2470 with 1100 V source function) are generally high enough to test today’s SiC and Gan devices and future device designs.

Personal safety considerations

  

Figure 2: test fixture correctly grounded. Figure 3: location of the safety interlock on the rear panel of Keithley 2470 SMU instrument.

During high-voltage test, personal safety is very important and must be prevented in advance to avoid electric shock:

·Seal the DUT and any exposed connections in a properly grounded fixture, as shown in Figure 2.

·Ideally, the SMU instrument should have a safety interlock, as shown in the 2470 rear panel in Figure 3. 2470 can be fully interlocked, and the high voltage output will be closed when the interlock is invalid (interlock switch is closed). The interlock circuit of the SMU instrument shall be connected to the normally open switch. The switch will be closed only when the user access point in the system is closed, so as to ensure that the operator will not touch the high voltage connection of the DUT. For example, once the test fixture cover is opened, the switch / relay will open and disengage the interlock of 2470 SMU.

·Use cables and connectors rated to the maximum voltage of the system. Jishili trx-1100 v high voltage triax cable is specially designed for 2470 and meets today’s high voltage safety standards.

·Always wear correct safety gloves when handling high voltage on energized components, as shown in Figure 4.

  

Figure 4 Use correct safety gloves when handling high voltage on energized components.

Leakage current measurement

In typical power conversion applications, semiconductor devices are used as switches. The leakage current measurement shows that the semiconductor is close to the ideal switch. In addition, when measuring the reliability of devices, leakage current measurement is used to indicate device degradation and predict the service life of devices.

Semiconductor researchers are looking for various materials to make higher quality switches and produce high-power devices with small leakage current. SMU instruments (such as Keithley 2470) provide precision weak current measurement function, with a minimum measurement resolution of 10 FA.

When measuring “1” μ A current, in order to prevent unwanted measurement error, triax cable and electrostatic shielding device can be used. Triax cables are important, in part because they allow the carrying of protective terminals from current measuring instruments. The protection function eliminates the influence of system leakage current and makes it bypass the measuring terminal. The use of electrostatic shielding device can make the electrostatic charge avoid the measuring terminal. The electrostatic shielding device is a metal distribution box placed around the circuit and any exposed connections. The safety test distribution box can be used as an electrostatic shielding device.

The SiC power diode is characterized by Keithley 2470 SMU instrument and kickstart software. The combination of 2470 SMU instrument and jishili kickstart software can accurately, safely and quickly test the breakdown voltage and reverse leakage current on high-voltage semiconductor devices.

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