[Technical Master Test Note Series]Eighth: MOSFET Characterization in the Low Power Range

The semiconductor industry is always on the lookout for new specialty materials, dielectric solutions, and new device shapes to reduce device size even further. For example, the lateral and vertical heterostructures of 2D materials have led to the creation of new disruptive small low-power Electronic devices.

Author: Andrea Vinci, Tektronix Technologist

The semiconductor industry is always on the lookout for new specialty materials, dielectric solutions, and new device shapes to reduce device size even further. For example, the lateral and vertical heterostructures of 2D materials have led to the creation of new disruptive small low-power electronic devices.

When producing accurate reports on the electrical characteristics of semiconductor devices, such as special NANO-FETs, a common problem is faced by researchers, scientists and engineers in the industry. This problem is exacerbated when it is necessary to demonstrate that these parameters can actually be controlled in an easy, repeatable manner.

[Technical Master Test Note Series]Eighth: MOSFET Characterization in the Low Power Range
4200A Electrical Parameter Characterization System Controlled from Touch Screen Display

A typical problem with electrical characterization in the low current range is the need to determine the achievable device performance of a low power/low leakage MOSFET under different conditions.

Measurements are critical because they identify concrete metrics (FoMs) that either confirm or deny valid behavior within a particular app. For example, n-type FETs require evaluation of turn-on and turn-off drain currents at different source, drain, and gate voltage values. FoMs may vary from application to application, but the metrics are obtained in basically the same way: provide a precisely controlled voltage or current that varies in a certain way, while accurately obtaining voltage and current measurements, associated with each specific variable .

In practice, this problem can be solved by using a certain number of source measure units (SMUs), specialized instruments capable of supplying current or voltage while measuring current and voltage. But while a practical solution looks ready, there are many hidden “details” that can lead to problems and misleading results, let’s take a look.

These Key Questions You Should Ask Yourself

It is increasingly common for engineers to fall into the trap of forgetting to take a closer look at the test system as a whole. Or better yet, they clearly see their devices, they clearly see their instruments, but they can’t see anything in between. For example, I often see oscilloscope users forget to use the probe to reach a specific test point to measure the board. Engineers who are reminded to consider the effects of probes on signals still generally forget about the effects of probe leads on measurements, as well as issues related to signal coupling.

“So, does it really matter?” they would ask. Sadly, it does matter, and we have to consider these effects.
For DC characterization applications, the risks are similar. Even though we use complex and expensive probe station system components for physical probing, SMUs still have to force voltages, measure currents, and connect to probe cards via cables. Does this mean we should think that the cable might affect our measurements?

Whatever the answer, it’s important that you ask yourself the question before proceeding. More importantly, make sure the answer is correct.

Precision measurements in CMOS fabrication are a prime example of the importance of connectivity. In fact, the ability to connect means adding capacitance to the test system. Since today’s MOSFETs are characterized over a wide extended frequency range, any effects caused by the added capacitance must be carefully considered.

Let’s start by looking at the effect of connections on capacitance. Parametric (automated) test equipment is typically connected using triax cables, which is a very typical example of a low-noise connection between a source-measure test unit and the device under test. Triax cable is a special type of coaxial cable that insulates the signal-conducting portion with an additional outer copper Faraday shield. Even though the Faraday shield reduces the distributed capacitance of the cable, the added capacitance of the cable can still affect the measurement when the overall cable length becomes meaningful.

Let’s look at a practical application where a test system must characterize n-MOSFET transistors. In this application, we use an SMU-based test system to trace the so-called IV curve, which is sometimes referred to as the “output characteristic” or “transfer characteristic.” We program the gate voltage to scan forward and backward (using the SMU as previously described) while measuring the drain current (also using the SMU).

From these curves, we can gather useful data to accurately Model transistor conduction activation and deactivation, analyze when these characteristics exhibit linearity or enter saturation behavior, and determine how much self-heating effects may shift these parameters and curves.

When characterization requires modeling the behavior of carriers, electrons, or pores (jumping between states, modifying their mobility depending on a variety of conditions), the measurement system connects to the DUT in a four-wire (or remote sensing) configuration and uses Triax cable.

Looking at the triax cable connection in a four-wire configuration, the total length corresponds to the sum of the Force Hi and Sense Hi cable lengths. According to the capacitance/meter (pF/m) index of the triaxial cable, we can calculate that the SMU is connected to the device terminal with two triaxial cables, the length is 20 meters (10 meters + 10 meters), the protective capacitance It is about 2 nF and the shield capacitance is about 6 nF.

In these cases, the sensitivity of the SMU is meaningless when measuring the transfer characteristics of weak currents (typically in the nanoamp range) because capacitive cable loading can cause oscillations. Not only must the SMU be sensitive, it must also be able to maintain the effective capacitance due to cable loading, or the loading of any leads that connect the SMU to the DUT.

Otherwise, the sensitivity is useless and the SMU will just produce noisy oscillatory readings.

[Technical Master Test Note Series]Eighth: MOSFET Characterization in the Low Power Range
Comparison of Id-Vd curves of FETs measured through the switch matrix using two SMUs and two 4211-SMUs.

It is becoming increasingly critical to be able to determine if the test capacitance is affecting the measurement. In these cases, Keithley application engineers can provide invaluable consulting services to ensure customers avoid pitfalls. When there are long connecting cables in the setup, or when there is a switch matrix between the measurement system and the DUT, or when the DUT or chuck requires nanoamp measurements, it is a good idea to review the setup and seek counsel and advice.

The latest solution for critical ranges

Faced with these very challenging special cases, it is necessary to use specific SMUs Modules in the measurement. Keithley has introduced a special-purpose SMU that can be used in parametric analysis systems like the 4200A-SCS parametric analyzer.

[Technical Master Test Note Series]Eighth: MOSFET Characterization in the Low Power Range
SMUs are ideal for connecting LCD test stations, probes, switch matrices or any other large or complex testers.

The 4201-SMU medium power SMU and 4211-SMU high power SMU (with optional 4200-PA preamplifier) ​​support stable weak current measurements, even when long cable connections result in high test connection capacitance.

In fact, these modules can power and measure systems 1,000 times more capacitive than today’s. For example, if the current level is 1 ~ 100 pA (picoampere), then the latest Keithley modules will settle on a load capacitance of 1 µF (microfarad). In comparison, the largest load capacitance competitor can only tolerate 1,000 pF (picofarads) before measurement stability degrades, in other words, 1,000 times worse than Keithley’s modules.

[Technical Master Test Note Series]Eighth: MOSFET Characterization in the Low Power Range
CV Measurements for High Impedance Applications

Summarize

Continuous improvement of measurement technology is essential to optimize semiconductor materials for low contact resistance, specialized shapes and unique structures in integrated transistors. The success of GaN transistors in future power electronics is closely related to the nanostructures employed in the casting process. On the one hand, the capacitance in the gate-width configuration is lower, so any other meaningful capacitance effects, such as those from cables and connections, are considered. On the other hand, they also overcome these problems by improving the SMU’s ability to tolerate high capacitances, providing higher measurement stability.

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