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DEVICE TESTING


Consider another example, of a power MOSFET with an Rds(on)


of 1.5mΩ specified at 30A IDS . A


calculation based on Ohm’s Law (V = I×R) reveals that only 45mV of voltage is required to reach 30A on this device (V = 30A ×1.5mΩ = 45mV). Given that a curve tracer typically only has a programming resolution of 30mV, sourcing 45mV would be pretty tough. To be fair, sourcing 45mV accurately, particularly at these high current levels, is difficult for any instrument; in contrast, measuring 45mV is relatively easy, so it’s generally highly preferable to source current and measure voltage when measuring low resistances.


Although curve tracers can only source voltage, SMUs have the ability to source both voltage and current. Sourcing current allows SMUs to make extremely accurate Rds(on)


measurements. SMUs


like the Keithley Model 2651A can source current with resolution as small as 2pA and its high sensitivity voltmeter can measure with resolution as small as 1µV. This allows for highly accurate resistance measurements, even at the sub- milliohm level.


ON-state testing requires sourcing high current; OFF-state testing requires sourcing extremely high voltage. Today’s devices are commonly capable of withstanding >1200V and IGBT devices capable of 2500V are readily available. The traditional curve tracer is capable of sourcing up to 3000V, so it’s still suitable for breakdown testing of modern devices. SMUs like the Keithley Model 2657A High Voltage System SourceMeter instrument are also capable of sourcing 3000V, but unlike a curve tracer, it can deliver a large amount of current at high voltage. At 3000V, the amount of current that the curve tracer can deliver is miniscule. The Model 2657A can deliver up to 20mA at 3000V or up to 120mA at 1500V, expanding its capability. Like the high current SMU, the high voltage SMU is capable of operating in quadrants II and IV, which not only expands its capabilities but increases test system safety. Devices in the OFF state have very high impedance and therefore very low leakage.


These devices also have some capacitance as well. In OFF-state testing, this capacitance gets charged to a very high voltage, but due to the very low leakage of the device, this charge remains on the device for a significant amount of time after the test voltage is removed. The SMU’s ability to sink current increases test system safety because it allows the SMU to discharge the device at the end of the test very quickly, preventing shocking an operator who comes in contact with the device too soon after testing.


SMUs and dynamic range Characterizing a device accurately demands test equipment capable of measuring both large and small currents precisely. For example, Figure 5 shows the characteristic curve for a typical diode.


In the regions between the reverse breakdown voltage (Vbr


) and the forward voltage (Vf ), currents


are very small. However, in the regions below Vbr and above Vf


, the currents are several orders of


magnitude larger, especially in the forward region. An instrument with an extremely wide dynamic range is essential to characterize this device accurately, and SMUs offer some of the widest dynamic ranges in the T&M industry.


Forward or ON-state testing of a device is usually performed with a high current supply. In the forward region of a diode (below the forward voltage), the level of current is very small. The device has not turned on yet and so the current flowing through it may be just nano-amps or less. Above the forward voltage, the current quickly gets much larger, reaching several amps or even tens of amps. For an SMU, measuring over this range of currents is no problem. For example, the Model 2651A offers a measurement capability that stretches from 50 amps (or 100 amps if two units are connected in parallel) all the way down to 1pA or 14 decades of current. In sharp contrast, the high current supply of a curve tracer only has about 4 decades of current measurement capability and can only measure with far less resolution.


Reverse or OFF-state testing is usually performed with a high voltage supply. In the reverse region of the diode, at voltages smaller than the breakdown voltage, currents are very small. At and above the breakdown voltage, they rapidly become several orders of magnitude larger. In today’s devices, OFF-state leakage currents can be down in the pico-amp range, but the generally accepted breakdown current is 250 micro-amps.


Accurate characterization requires an instrument capable of measuring the extremely small currents found before the breakdown with sufficient dynamic range to measure the larger currents that occur at breakdown. With the ability to measure currents as low as 1fA and as high as 120mA, the


Figure 4. Curve tracer gfs measurement results


Issue IV 2013 www.siliconsemiconductor.net 45


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