What is a source measure unit (source-meter)?

The basic idea of a source measure unit (also called a SMU or source-meter) is very simple - it outputs a voltage and measures the current that flows. In this respect it is just like a bench-top power supply - however it is programmable and allows the user to sweep the voltage over a defined range. It is also a million times more accurate than a regular power supply (literally 1,000,000 times more accurate in the case of the Xtralien X100).

With a bench power supply you usually use a dial to select the voltage you want to generate, and then look at the display to read off how much current is flowing through your circuit. Usually a bench-top power supply will output a voltage from zero to 12 or 24 volts and measure the current to the nearest milliamp (1 thousandth of an amp) or so. This is great when measuring the current used by motors or light bulbs or high power devices. However, if your want to make precise scientific measurements, then a milliamp is actually a huge amount of current - very often it is necessary to have a precision of microamps (1 millionth of an amp) or nanoamps (1 billionth of an amp) to characterize many electronic devices.

How is a source measure unit different from a multimeter?

It is important to separate the function of a source measure unit from a regular multimeter;  both are very useful but have different purposes. A standard multimeter can measure voltage and it can also measure current - but not at the same time. It  also doesn't output a voltage. A good handheld multimeter will be able to measure voltage with an accuracy of a few hundred micro-volts and current to an accuracy of a micro-amp or so - much better than a bench-top power supply. As such, you could also build a source-measure unit with moderate accuracy by using a bench-top power supply to output a voltage/current and two good quality multi-meters - one to measure voltage and the other to measure the current.  However, this still wouldn't be programmable and also wouldn't allow negative voltages to be measured very easily (both of which are important for many applications).

Why is it important to have a programmable source measure unit?

For some applications it might not be important to have a programmable instrument - you may just want to read off the value once or a small number of times. However, in many cases you might want to collect lots of data so that you can plot a graph or measure the performance of something over time, or link several pieces of apparatus together. However doing this manually is time consuming and difficult.

There are also lots of different experiments that require automated data collection to get faster or more accurate measurements, or to take measurements over a long time-scale (months or even years). Here, you will certainly need a computer to collect your data and export it to a spreadsheet or database for analysis.

Why is it important to have negative voltages?

Not all experiments will need negative voltages and in some cases you can avoid this, however many different types of device work differently if a positive or a negative voltage is applied. To fully understand how such devices work, we need to be able to change the sign of the voltage applied.

As an example, consider a diode - a device that only allows electricity to pass through it in only one direction. In order to evaluate if a diode is working we need to see if it can pass electricity in both directions. We can do this in one of two ways. We can measure the diode in one direction, and then manually turn it around and then measure it the other direction and then 'stitch' the data sets together. More simply however, we can just measure current flow when we apply either a positive or negative voltage. In fact, this technique is so useful it is used to characterize many types of devices that have diode-like behavior - solar cells and light emitting diodes are both very good examples of this.

What are the uses of a source measure unit?

Many everyday objects will have been tested with a source-measure unit as part of the production and quality control process. If you use LEDs to light your home or have solar panels on your roof, these will have been tested with a source measure unit as part of the quality control process.

The X100 is designed for use by scientists and engineers who are working on the next generation of electronic devices. Understanding how a vast number of materials and devices conduct electricity - ranging from carbon nanotubes and quantum well heterostructures to biomembranes and biosensors requires a source measure unit. In short, you can use the X100 to understand the electrical characteristics of any device at DC or low frequency over a voltage range from -10 V to + 10V, recording current flow from 1 nano-amp (nA) to 100 milliamps (mA) using the X100 source measure unit.

Can you give me a specific example of a measurement that needs a source-measure unit?

Lets take the example of a solar cell. In research labs around the world, researchers are looking at ways to make better, more efficient, cheaper to produce solar cells. In order to understand how a solar cell is working they will often produce a small scale test device - maybe a few square mm to a few square cm in size and then characterize its performance. As such, these devices are much too small to generate any usable power (for example to boil a kettle) but they are more than big enough to understand the basic operating principles.

The key characteristic of a solar cell is how efficiently it converts sunlight energy into electrical power. This can be done by shining a known amount of light power over the area of the cell and calculating the electrical power produced per unit area. Since power is simply voltage multiplied by current, then a starting point is to measure the applied voltage (V) and current produced (I) - or specifically the current per unit area (J).

We can measure the voltage generated simply by placing a multimeter across a solar-cell terminals while it is illuminated. Similarly we can also measure the current using a multimeter and if we divide this by the area of the solar-cell we get the current density (J). However, there is a subtle problem; If you multiply the voltage by the current (or current density) then this only tells us how much power (or power per unit area) we can generate if we had a perfect device. The reason for this is that a good voltmeter has a very high (near infinite) internal resistance, and when we measure the voltage by itself no current can flow. As such, there is actually zero power being generated (measured voltage x zero current = zero). Similarly, when we place the multimeter across the terminals to measure the current, we testing the device when it has been short-circuited (this is because a good ammeter has near zero internal resistance). Here there is current flowing but no applied  voltage - so again there is no power generated (measured current x zero voltage = zero).

For any practical (real) solar cell, the voltage that it outputs will depend upon how much current is being produced and this is why we need a source measure unit so that we can vary the voltage and measure the change in current.

The below graph, shows a typical JV curve for a particular type of prototype solar cell (in this case a perovskite solar cell, one of the latest generation of new solar cell technologies). The J in JV stands for current density and the V stands for voltage. The JV curve tells us how the voltage and current are affected by one another and allows us to calculate the actual amount of power that a solar cell generates.

 

If we multiply the voltage by the current density then we get the power density produced by the solar cell as plotted below. The peak of the graph is the point at which maximum power is generated (the so-called maximum power point).

 

Notice that in the above graphs, we also measure the solar cell when a negative voltage has been applied (so-called reverse bias). This gives us some useful information;  firstly it tells us that the device doesn't break down under reverse bias. This is a sign that the device is of good quality. Secondly, it tells us whether there is any extra current available that we are not making good use of, since by applying a negative voltage we can effectively "suck" charges out of the device that wouldn't otherwise be extracted. While these "sucked" charges can't be used to generate power (we're actually putting power into the device at this point in the graph rather than extracting it), it allows us to understand some of the photocurrent loss mechanisms. Thus measuring JV curves is one of the most important tools used in solar-cell development and optimization. Similarly taking IV and JV curves is hugely important to understanding a wide range of other device types including LEDs and OLEDs, transistors, sensors and many more.