[Technical Master Test Notes Series]Ten: Perform breakdown voltage and leakage current measurements on today’s high-voltage semiconductor devices

“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 drive requirements. The gate voltage (Vgs) required by SiC is much higher and turns off at a negative bias voltage. The threshold voltage (Vth) of GaN is much lower, requiring strict gate drive design. Due to the physical characteristics of wide band gap (WBG) devices, the body diode voltage drop is higher, so the control requirements for idling time and on/off jumps are more stringent.

“

Author: Tektronix 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 drive requirements. The gate voltage (Vgs) required by SiC is much higher and turns off at a negative bias voltage. The threshold voltage (Vth) of GaN is much lower, requiring strict gate drive design. Due to the physical characteristics of wide band gap (WBG) devices, the body diode voltage drop is higher, so the control requirements for idling time and on/off jumps are more stringent.

Accurate power supply and measurement tests are critical to characterizing these high-voltage devices so that correct design decisions can be made in a timely manner. Increasing design margins and over-designing will only drive up costs and lead to performance degradation. In addition, these devices generally involve high voltages exceeding 200 V, so it is critical to ensure personal safety and prevent electric shock.

High voltage device testing

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 have to apply an increasing reverse voltage to the device under test until a certain test current is reached, indicating that the device is broken down. Figure 1 shows the use of a source measurement unit (SMU) instrument to perform a breakdown measurement on a high-voltage diode, such as a Keithley 2470 high-voltage source meter SourceMeter® SMU instrument. Note how the SMU instrument is connected to the cathode of the diode to apply the reverse voltage. For high voltage diodes, use safety triaxial cables and a properly grounded safety wiring box.

Figure 1. Typical breakdown voltage measurement of a high-voltage diode using a 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 to ensure that the device under test is robust and reliable. SMU instruments (such as the 2470 with 1100 V source capability) are generally high enough to test today’s SiC and GaN devices and future device designs.

Personal safety considerations

When carrying out high-voltage tests, personal safety is of the utmost importance and must be prevented in advance to avoid electric shock:

• Seal the device under test (DUT) and any exposed connections in a properly grounded fixture, such as the fixture shown in Figure 2.

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

• Use cables and connectors rated up to the maximum voltage of the system. Keithley’s TRX-1100 V high-voltage triaxial cable is specially designed for 2470 and meets today’s high-voltage safety standards.

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

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

Leakage measurement

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

Semiconductor researchers are looking for a variety of materials to make higher-quality switches and produce high-power devices with low leakage current. SMU instruments (such as Keithley 2470) provide precision weak current measurement functions with a measurement resolution of as low as 10 fA.

Is measuring

The Links:   LB104V03-TJ01   2DI100Z-120