Unfortunately, these smaller devices can't be tested using standard test techniques for a number of reasons, one key reason being their physical size. The nanoscale dimensions of some of the new "beyond CMOS" devices can be susceptible to damage from even small amounts of current used in the measurement process. In addition, traditional DC test techniques are not always adequate to reveal how devices really operate. Consequently, designers need new testing techniques and test tools. One such technique is pulse testing.

Pulse-testing technique

Pulsed electrical testing is a measurement technique that reduces the total energy dissipated in a device. It reduces the Joule heating effects (such as I2R and V2/R) that could potentially damage small nanoscale devices. The DUT (device under test) is excited for a very short interval with a source high enough to produce a quality measurable signal; then the source is removed.

Pulse-test measurements are essential for devices with isothermal limitations, such as SoI (Silicon-on-Insulator) devices, FinFETs and nano devices, in order to avoid self-heating effects that could mask the response that the researcher is seeking. Pulsing also helps the device engineer to understand charge-trapping effects (decreased drain current after a transistor is turned on).

There are two different types of pulse testing: voltage or current pulsing. Voltage pulsing produces much narrower pulse widths than current pulsing. This makes it more suitable for experiments in thermal transport, where the time frame of interest is shorter than a few hundred nanoseconds. High amplitude accuracy and programmable rise and fall times are necessary to control the amount of energy delivered to a nanodevice. Voltage pulsing is useful for transient analysis, charge trapping, and AC stress tests during reliability testing, as well as for generating clock signals and simulating repeating control lines such as in memory read and write cycles. Current pulsing is very similar to voltage pulsing. In this method, a specified current pulse is applied to the DUT, and the resulting voltage across the device is measured. Current pulsing is often used to measure very low resistances or to obtain an I-V (current-versus-voltage) curve without putting significant power levels into the DUT that would otherwise damage or destroy a nanoscale device.

Both voltage and current pulse testing have many benefits but are not without some drawbacks. For example, the speed characteristics of an ultra-short voltage pulse are in the RF domain, so it is very easy to introduce errors in the measurement if the test system is not optimised for high bandwidth. There are three main sources of errors: signal losses due to cables and connectors, losses due to device parasitics, and contact resistance.

Pulse I-V testing

Performing I-V pulse characterisation on nanoscale devices often requires measuring very small voltages or currents due to the necessity of applying a very small current or voltage, respectively, to control power or to reduce the Joule heating effects. Here, low-level measurement techniques become important, not only for I-V characterisation of devices but also for resistance measurements of highly conductive materials. For researchers and electronics-industry test engineers, this power limitation creates challenges for characterising modern devices and materials, as well as future devices.

Unlike I-V curve generation on micro-scale components and materials, measurement on nanoscale materials and devices requires special care and techniques. I-V DC characterisations are typically performed using a two-point electrical-measurement technique. The problem with this method is that if sourcing a current and measuring voltage, the voltage is measured not only across the device but includes the voltage drop across the test lead and contact, as well. If the goal is to measure the resistance of a device using a typical Ohm meter to measure resistance greater than a few Ohm, this added resistance is usually not a problem. However, when measuring low resistances on conductive nanoscale materials or components, obtaining accurate results with a two-point measurement can be a problem even when using pulse testing.

If the pulse I-V characterisation or resistance measurement involves low voltage or low resistance, such as with molecular wires and semiconducting nanowires, a four-wire or Kelvin measurement technique with a probe station will yield more accurate results. With a Kelvin measurement, a second set of probes is used for sensing. Negligible current flows in these probes due to the high impedances associated with the sensing inputs; therefore, only the voltage drop across the DUT is measured. As a result, the resistance measurement or I-V curve generation is more accurate. Source and measurement functions for this measurement technique are typically provided by SMUs (source-measure units), which are electronic instruments that source and measure DC voltages and currents.

Tools for nano testing

With nanoelectronic and semiconducting materials and films, sensitive electrical measurement tools are essential. They provide the data needed to fully understand the electrical properties of new materials and the electrical performance of new device and components. Instrument sensitivity must be much higher because electrical currents and voltages are much lower and many nanoscale materials exhibit significantly improved properties such as conductivity. The magnitude of measured current may be in the femtoampere range, voltage in the nanovolt range, and resistance as low as microohms. Therefore, measurement techniques and instruments must minimise noise and other sources of error that might interfere with the signal.

One such solution is the Keithley Model 4200-SCS (Figure 1) semiconductor-characterisation system with 0.1fA and 1µV resolution. A special pulse I-V Package provides dual-channel pulse generation and measurement for making pulse I-V measurements. Combined with an internally installed high-speed pulse generator and scope, it offers the ability to perform both DC and pulse I-V testing together.

Figure 1: Schematic diagram of Model 4200-PIV test system.

With this package, the researcher can perform both DC and pulse IV testing together to understand device behaviour, for example via the family of curves for a FET device shown in Figure 2.

Figure 2. Pulse I-V versus DC I-V characterisation of a family of FET curves.

Conclusion

Pulse testing provides a key capability needed for the investigation of nanomaterials, nanoelectronics, and today's semiconducting devices. Pulsing a voltage while simultaneously measuring the DC current is the basis for charge pumping, which is valuable for measuring inherent charge trapping in semiconducting and nanoscale materials. Pulsing current and measuring voltage give the researcher the best opportunity for measuring very low resistances or performing I-V characterisation on next-generation devices while protecting these valuable devices from damage.