Fab feedback for new semi development

In addition to static DC I-V tests, this stage of characterisation includes AC impedance parameters, such as various device capacitances (such as CISS, COSS, CRSS). Capacitance can be a tricky measurement in as much as all of the stray impedances in the cables and test fixtures need to be properly compensated. This is usually done with an OPEN and SHORT correction step, but it can be difficult to simulate the open and short conditions in a power device test fixture or probe station. Coaxial cables should be used, and extra care should be taken to make sure the shields of the cables are connected together as close to the device as possible.

Designers also typically measure several other AC and transient characteristics, including rise/fall time (tr, tf), switching delay (tdon, tdoff) and reverse recovery time (trr). For power FETs and IGBTs, numerous charge measurements are performed, including total gate charge (QG), pre- and post-threshold gate charge (QGS1, QGS2), drain charge (QGD) and output charge (QOSS). Curve tracers’ limited applicability becomes increasingly apparent and they have been largely replaced with modern digital semiconductor parameter analysers (SPAs) for these types of measurements. Until recently, even SPAs were limited by insufficient power and capability for such applications. For example, many lacked the speed or power necessary to capture many of the charge characteristics. A difficult test called Dynamic On Resistance (sometimes referred to as “current collapse” in a gallium nitride power HEMT) is one good example. The device must be biased to full breakdown voltage, typically 600V, then switched to full ON current, often as high as 50A, all while monitoring the ON resistance for rapid changes in RDS(ON). These changes can occur as rapidly as microseconds and can continue for milliseconds. Keithley’s newest high power SMUs incorporate numerous digital enhancements, such as high speed digitisers, intended to make it easier for designers to capture important data.

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Other equipment used at the sample testing stage includes pulse generators and oscilloscopes, IR cameras, EMF Test, and wafer probers. In some cases, the devices will be packaged to allow for easier testing in a specially designed test fixture rather than at the wafer level.

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