A typical workflow of a power semiconductor device holds many measurement challenges

With some of the newer technologies, the fabrication process is often being developed at the same time as the devices themselves. In some cases, the device designer will have the fab create simple sub-circuits to test some element of the process. These devices include simple Schottky and bipolar diodes, and even resistors and capacitors. Testing at this level usually involves I-V (current-voltage) characterisation with a simple curve tracer. The designer wants an instrument that is very quick and easy to set up, with interactive control of the voltage and current levels, and fast feedback on the display to allow quick determination of proper voltage limits. Given that exceptional measurement accuracy and sensitivity are typically not critical to such applications, curve tracers remain well suited to this task.

The next step (or the first step in many situations) is to fabricate a real device. Characterisation on this device starts out with taking a simple I-V (current-voltage) curve to determine basic device characteristics. Usually, the designer knows what he or she expects the device to do but not what the device actually does. Again, a curve tracer can be a simple and efficient choice for this task. It allows the designer to apply a controlled voltage carefully, read back the current to determine if junctions and interconnects designed actually exist, and discover the typical breakdown voltage or ON current the device can handle.

Connecting a curve tracer safely to a probe station is a common test problem in this situation. Curve tracers were typically intended to test packaged parts, but it is much more efficient to test devices on the wafer directly, eliminating the cost and time of packaging the part. The newest generation of source measurement units (SMUs), including Keithley’s Models 2651A and 2657A High Power System SourceMeter instruments, provide safer and more accurate connections, and better control over the voltage and current. The designer can control the instrument using the front panel knob and display or via the LXI-compliant web page embedded in the instrument. Another option is to connect the instrument to an external controller running an application created using something like Keithley’s ACS Basic Edition Test Software for Component and Discrete Devices, which provides a simple Trace Mode that provides precise control of the voltage and read back of the current.

You can read the rest of this article in the September issue of Electronic Specifier Design by clicking here.