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DEVICE TESTING


Model 2657A‘s measurement capability spans 14 decades, making it easy to measure off-state currents. In contrast, the curve tracer offers far less resolution. With this limited low current measurement capability, the curve tracer is really only suitable for finding the breakdown voltage and is unable to measure picoamp-level leakage on today’s devices.


SMUs and flexibility


SMUs offers the high flexibility needed to perform tests that would be difficult or even impossible with a curve tracer. Gate leakage measurements are difficult to do on a curve tracer but very easy to perform with SMUs. Measuring gate leakage requires sourcing a voltage on the gate terminal with the drain and source terminals grounded, then measuring the current into the gate. A curve tracer’s step supply (to which the gate terminal is traditionally connected) does not support measurements, so the connections to the device must be changed so that the gate is connected to the curve tracer’s collector/drain supply terminal and the drain and source terminals are connected to common.


However, this doesn’t work very well because measuring gate current requires measuring low currents. Accessing the lowest current ranges on the curve tracer requires using the high voltage supply, but the highest resolution setting on the high voltage supply is 50V/division. Given that most gate leakages are measured at 20V or less, this isn’t very practical. Sourcing only 20V or less is easy to do if the high current supply is used, but the lowest current measure range is 500mA/ division, which is much too large to measure any kind of leakage current. In contrast, with an SMU- based power device characterization system, the gate terminal is connected to an SMU so it can make measurements.


By putting today’s lowest current SMU on the gate terminal, users can source the gate voltage precisely and measure the leakage current with resolution down to 100aA. Creating Gummel plots is another test where SMUs offer a clear advantage over traditional curve tracers. This test, performed on BJTs, requires sweeping the voltage and measuring the current on the base and collector terminals in unison. This test absolutely


cannot be performed using a curve tracer, but thanks to the flexible sweep configurations SMUs offer, this task is trivial.


SMUs and PCTs


As capable as SMUs are and as many advantages as they offer over traditional curve tracers, they are not the whole solution for testing advanced power semiconductor devices. Increasingly, engineers are turning to a new concept known as a parametric curve tracer or PCT, which combines the simplicity of a curve tracer with the high precision of a parametric analyzer. A PCT includes SMU instruments, cables, a test fixture, software, and test libraries to provide measurements at up to 3000 volts and up to 100 amps. As new test needs evolve, a PCT can be upgraded easily in the field, providing a solution that is both scalable and reconfigurable.


Parametric curve tracers offer two modes of operation: trace test mode and parametric test mode. Trace test mode presents an interface similar to the controls and display found on a traditional curve tracer. It allows for rapid generation of device characteristics and for interactive operation based on viewing the results in the graph. It incorporates knowledge of many device types and tests, which speeds and simplifies test setup. An on-screen slider provides real-time control and acts like the knob found on the traditional curve tracer. Trace test mode is very handy for quickly testing whether a device is good or bad or finding its boundaries. This mode is often used during device development or failure analysis.


Parametric test mode offers access to all of the advanced capabilities of the SMUs within the PCT, allowing users to specify exactly how a test is to be performed. Built-in test libraries provide support for all of the most common device and test types; a vector math formulator supports accurate parameter extraction on these devices. Once configured, an entire suite of tests can run autonomously without operator intervention. This mode is often used in device qualification, process monitoring and datasheet generation applications.


Conclusion


Device engineers have long considered the traditional curve tracer their go-to instrument for device characterization, but dwindling availability and new device challenges are making it an increasingly unworkable solution. Fortunately, SMU instruments have assumed many of the roles in the lab that the curve tracer once played. With the extended capabilities that parametric curve tracers add to those provided by SMU instruments, engineers can be assured their evolved characterization needs won’t be left unaddressed.


Figure 5. Characteristic curve for a typical diode


46 www.siliconsemiconductor.net Issue IV 2013


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