How to make IV measurements with a source measure unit (SMU)

Blog Post created by CS on Jan 26, 2018

Why Perform Current-Voltage (IV) Measurements?

IV measurements obtain the current vs. voltage or resistance characteristics by providing a voltage/current stimulus and measuring current/voltage reaction. This is a basic electrical measurement and a fundamental way to understand the characteristics of various materials and devices under test (DUT).

Figure 1 shows the IV curves of some common electrical components. In the first graph, we see a linear relationship between voltage and current, so we immediately know that we are looking at a resistor. The graph in the center shows an exponential relationship between voltage and current. This probably means we are looking at a diode (or something that exhibits diode-like behavior). Finally, the graph on the far-right indicates that we are looking at some transistor curve. The characteristic of IV curves are able to provide us with immediate insight into a component, making them crucial in science and engineering.

Figure 1. IV curves of common electrical components

So, how do you make quick and accurate IV measurements? The most common tool for measuring IV curves is a source measure unit (SMU).

What is a Source Measure Unit (SMU)?

An SMU combines the capabilities of a current source, voltage source, current meter and voltage meter into a single unit. This gives it the ability to evaluate the IV characteristics of devices across all four measurement quadrants without the need for any additional equipment. You get substantial cost and space savings compared to having multiple instruments.

Besides being able to output and measure voltage or current very accurately, SMUs also possess a compliance feature that allows a limit to be placed on the voltage or current output to prevent device damage.

As Figure 2 shows, a source measure unit packs an amazing amount of measurement capability into a very small package. SMUs can act as ideal 4-quadrant sources, meaning they will always try to maintain their programmed current or voltage until they reach the limit of their output power or a user-defined limit.

Figure 2. Simplified equivalent circuit (2-wire measurements) in an SMU

In addition, while sourcing current or voltage, SMUs can simultaneously measure both current and voltage. Figure 2 represents a single SMU channel, and this channel can exist as a standalone product or as one SMU channel within a mainframe that supports multiple SMU channels.

Notice that the “Low Force” side of the SMU can be connected to chassis ground via a relay. In fact, the default boot-up state of most SMUs has them configured with their low side grounded. However, it is sometimes convenient to float the “Low Force” side connection (for example when making differential measurements). In those cases, keep in mind that disconnecting the low side from ground is possible.

Why Use an SMU for IV Measurements?

Without an SMU, here are some common test challenges that you may face when performing IV measurements:

• Difficulty in controlling and synchronizing multiple instruments
• Complex cabling and setup
• Difficulty in obtaining accurate, reliable measurements

When using multiple instruments simultaneously for an IV measurement, it is not easy to obtain good performance and accuracy due to the cabling and grounding with various instruments. In contrast, an SMU typically provides superior measurement performance that goes down to sub pA and sub µV resolutions.

SMUs integrate many capabilities into a single channel. It is possible to source voltage and measure both the sourced voltage and current (or source current and measure both the sourced current and voltage). But usually, either only voltage is sourced while current is measured, or only current is sourced while voltage is measured. Hence, the most common SMU use models are shown here (also in Figure 3):

• VSIM – Source voltage and measure current
• ISVM – Source current and measure voltage

Figure 3. Two of the most common modes of operation for an SMU

For why would you need to measure a source if the SMU will always hold its sourced value? When you specify a sourced value, you also must specify some limit on the measure unit value. And if for some reason the measured unit hits its limit, the value you expect to source may not actually be what you set. Below is a simple example to explain this (Figure 4).

Figure 4. IV curve of a sweep voltage

Suppose we are sweeping the voltage applied to a DUT (sourced value) and measuring the current flowing through the DUT. If the IV curve of the DUT hits the set current limit before the applied voltage reaches its stop value, then the applied voltage will remain at the voltage level for the rest of the sweep. In fact, all voltage and current data points beyond the limit value will be the same. However, unless you tell the SMU to measure its applied voltage, you will not be able to see it happening. If you are only measuring current, then all you will see is that the current does not change for the last part of the sweep.

All SMUs have some sort of indicator to tell you that a measurement has hit the limit. However, if you do not see this indicator, then reviewing the measured data is the only other way to catch this.

Source Measure Units Make IV Measurements Easier

So, what are reasons that make an SMU the preferred choice for IV measurements – as opposed to using multiple discrete instruments?

It ultimately boils down to two things:

• Convenience, and
• Form factor.

A good example of this is shown in Figure 5, which shows measurement of current and voltage at the inputs and outputs of a four-terminal DUT. Performing these operations with discrete sources and meters is complicated and time-consuming, and requires many manual connection changes to modify a measurement setup.

In contrast, the same measurements can be performed using a 2-channel SMU with a very 'clean' setup. One channel can be connected to the device input, and the other channel can be connected to the device output. All the necessary connections to measure different current and voltage parameters can be performed without the need to modify any physical connections.

Figure 5. IV measurement setup comparison: using multiple instruments (left) vs. using a 2-channel SMU (right)

Summary

IV measurements are an important part of testing as they provide unique insights into your device under tests. Source measure units (SMUs) are recommended instruments for IV measurements as they are able to provide better measurements, easy to set-up and operate, and take up minimal bench space – which makes them a cost effective investment.

If you'd like to know more about how to make accurate resistance measurements – which is one of the more challenging areas of measurement science – download this Application Note: Resistance Measurements Using the B2900A Series of SMUs. You will learn how you can overcome measurement issues such as residual test lead resistance, thermal electromotive force and leakage currents in the measurement path.