What are the different SMU operation modes and when do you use each one?
While some source/monitor units (SMUs) only have one operation mode, modular SMU products such as the M96xxA family of PXI SMUs and the PZ2100A family of modular SMUs support multiple modes of operation. Those modes are normal, power supply, high capacitance, and laser diode. In this post, I shall discuss each of these in turn and give examples as to when you should use each one.
Normal SMU operation mode (as its name implies) is the conventional mode of SMU operation. In normal mode, the SMU can apply and measure voltage or current within the user-specified limit (or compliance) value. For most common source and measurement applications, normal mode is the best choice. However, when SMUs operate in normal mode they have characteristics that may not meet the needs of specialized applications. For example, in normal mode, SMUs must adhere to the user-specified limit value. This means that the SMU controls its rise times to ensure that it does not overshoot and exceed its limit value. Obviously, if the user needs ultra-fast rise times then operating in normal mode may not be the best choice.
Conversely, when driving large capacitive loads SMUs can sometimes ring or oscillate if the output rises too quickly. In this case to avoid ringing or oscillation the SMU needs to decrease its output rise time. Finally, when making pulsed measurements some SMUs support a special mode that enables them to produce shorter and cleaner voltage and current pulses than the SMU can generate in normal mode. Therefore, while as initially stated normal SMU mode is best for general purpose SMU applications, the other SMU modes exist to support specialized measurement needs.
Power supply SMU operation mode provides faster slew rates than does normal SMU operation mode, although it only supports operation in voltage source/measure current configuration. All of the M96xxA PXI SMUs support power supply mode, as do all of the SMUs in the PZ2100A family of modular SMUs. As its name implies, power supply mode is useful if you want an SMU to function more like a power supply than an SMU. When operating in power supply mode the inrush current flowing from the SMU into the DUT can briefly exceed (for about 100 microseconds) the set limit value in order to provide a faster rise time. Power supply mode supports fixed, auto, and seamless measurement ranging, although when using fixed measurement ranging not all of the measurement ranges are available (see product manuals for more information).
The fast pulsing SMUs (M9602A, M9603A, PZ2120A, and PZ2121A) are the only SMUs that support the high capacitance and laser diode modes. I shall discuss the high capacitance mode first because it is easier to understand. When driving loads greater than 0.01 microfarad, oscillations or ringing can occur when the SMU is in normal mode. In these cases, switching the SMU to high capacitance mode can help to stabilize the output (although rise times will decrease). In high capacitance mode the SMU can remain stable for loads of up to 100 microfarad. However, high capacitance mode only works when the SMU is in a force voltage and measure current configuration, and it only supports fixed and seamless measurement ranging. In addition, only a subset of the fixed measurement ranges are available (10 mA to 5 A). The usage and benefit of high capacitance mode should be apparent.
The added functionality of laser diode is not immediately obvious, and it is a reasonable question to ask why it exists at all. In other words, why do we need a special SMU mode to achieve fast pulsing when testing laser diodes? To explain this we first need to review basic SMU operation. SMUs have internal feedback circuitry that continuously monitors their output and adjusts the sourcing circuitry to compensate for unexpected changes in load conditions. This allows SMUs to maintain accurate sourcing and to remain within the user-specified limit (or compliance) value. However, preserving this control limits the rise and fall times of the SMU’s output (i.e., there is a trade-off between accuracy/stability and speed).
When driving a nonlinear device (such as a laser diode) at low levels of voltage or current, the bias resistance can be quite large. This large resistance can cause the SMU’s feedback circuitry to take a long time to stabilize. The SMU laser diode mode optimizes performance when driving non-linear devices by prioritizing speed when changing the voltage or current, after which it prioritizes accurate and stable sourcing. Another difference between laser diode mode and normal mode is that, in laser diode mode, it is possible to briefly exceed the user-specified limit value (resulting in overshoot). Rise time and overshoot exist in a trade-off relationship, but you can tweak waveform shapes by changing the limit value and/or utilizing the integrated SMU output filter’s time constant. Therefore, although the differences between normal mode and laser diode mode are somewhat subtle, it should be clear that laser diode mode has definite benefits when testing non-linear devices.
For example, here are 10 microseconds current pulses with amplitudes of 100 mA, 200 mA, and 300 mA generated using normal mode:
As you can see, the pulses in this case do not have very good definition. In contrast, here are 10 microseconds current pulses with amplitudes of 100 mA, 200 mA, and 300 mA generated using laser diode mode:
In this example the laser diode mode gives much cleaner pulse definition.
As previously mentioned, using the SMU output filter in laser diode mode can also improve the shape of a pulse. The following graph shows a 10 microseconds current pulse with an amplitude of 2.5 A with SMU filter time constant settings ranging from 0.5 microseconds to 3.0 microseconds.
It should be evident that some trial and error is necessary to optimize the waveform shape.
In conclusion, for the vast majority of SMU applications standard mode works fine. However, in situations where you want the SMU to act like a power supply (forcing voltage) and fast response time is important, then using an SMU that supports power supply mode is beneficial. In addition, for special cases where you are driving high capacitive loads or non-linear devices, having an SMU that supports high capacitance mode or laser diode mode is essential. When choosing a solution from either the PXI SMUs (M96xxA family) or the PZ2100A modular SMUs, it is important to keep in mind which SMUs support these different modes so that you select the optimal SMU for your application. The following table summarizes the modes supported by the PZ2100A SMU modules.
