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Unfortunately for power IC manufacturers, the rate of technology improvements derived from silicon-based devices is continuously decreasing. In order to meet today’s higher efficiency goals, researchers are increasingly looking to new semiconductor materials, including gallium nitride (GaN) and silicon carbide (SiC), wide bandgap materials with several performance advantages over traditional silicon. The use of these materials has allowed designing devices with lower leakage currents (for better switching performance), lower ON resistances (which increases energy efficiency by reducing the power consumed by the device and delivering more to the load), higher power density (so they can deliver more power using less physical space), and finally, higher safe operating temperatures (which reduces the end product’s cooling requirements).

In order to quantify the performance advantages these new designs offer, devices must be thoroughly characterized, a task that was traditionally performed with a curve tracer. Today, however, that has some problems, not the least of which is that production of curve tracers ceased in 2007. This has left test engineers hanging on to their instruments and struggling to keep them operational for as long as possible. However, the still bigger problem is that the traditional curve tracer has several significant limitations when it comes to testing modern power semiconductor devices. Fortunately, the Source Measure Unit (SMU) offers a more capable test solution. SMUs combine the source capabilities of a precision power supply with the measurement capabilities of a high accuracy digital multimeter (DMM) to create a single instrument that can both source and measure. SMUs offer all the features required for testing today’s semiconductor devices, including power, dynamic range, accuracy and flexibility.

SMUs and power

Modern semiconductor devices are capable of handling extremely high power levels and characterizing these devices demands test equipment capable of delivering those same high levels. Testing them in the ON-state requires instruments that can deliver power at very high currents; testing devices in the OFF-state requires instruments that can deliver power at very high voltages. A traditional curve tracer has two separate supplies for testing at these high levels of current and voltage. Today, there are SMUs capable of reaching these high levels as well.

Testing devices in the ON-state requires delivering a lot of current to the device. Figure 2 compares the power envelope of one traditional curve tracer’s high current supply with that of a pair of

Figure 2. Power envelope of a traditional curve tracer’s high current supply vs. that of a high current SMU

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high current SMU instruments wired together in parallel. Together, the pair of SMUs covers most of the power envelope of the curve tracer. The curve tracer’s high current supply can put as much as 30V across the device at 0A, but this maximum voltage decreases as current increases. The curve tracer can output up to 400A, but it can only do this if there is 0V across the device. When a voltage is put across the device, the maximum current output is reduced. This phenomena is known as the load-line effect, which is caused by resistance in the output of the supply creating a voltage drop as current flows out of the supply.

This resistance has been put into the output of the curve tracer’s supply intentionally as a way to sense how much current is flowing. The effects of this resistor can be seen in the data collected by the curve tracer in the characteristic I-V curves of a transistor (Figure 3). Note how as current increases in each curve, the maximum voltage reached at that end of the curve decreases. Even

Figure 1. Traditional curve tracer block diagram

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