Staircase sweeps, also known as “Integrated Pulsed Sweeps”, are a traditional method for generating an I-V plot. In a staircase sweep, the current level is increased in steps, and measurements are made at each step.
Staircase Sweep functions are now available with SpikeSafe Performance Series Current Sources and Source Measure Units. Leveraging Vektrex’s repeatable, near-perfect, square pulse capability, the addition of the Pulsed Sweep function makes Performance Series Current Sources a more-complete solution. Hardware-integrated Pulsed Sweeps execute faster than traditional staircase sweeps and greatly reduce device heating, allowing high-power devices to be tested without the need for active cooling.
The SpikeSafe SMU has 3 modes that may be used to duplicate a staircase sweep test:
- Modulated DC mode may be used. In this case a sequence is programmed to step the current from minimum to maximum. Up to 20 steps can be programmed.
- DC Dynamic mode may be used with an external software program that sweeps the current in steps. This method can support and arbitrary number of steps, but timing accuracy depends on software delays that are not always predictable.
- Pulsed Sweep can be used to approximate a staircase sweep. If the off time is short compared with the on time, then the device heating is similar to the heating of a staircase sweep. For example at 20ms on, 9us off, step heating is within 0.05% of the heating of an equivalent staircase sweep. This is the mode Vektrex recommends.
Early source meters were incapable of pulsed sweeps and so labs had to use staircase sweeps. Now that better equipment is available, Vektrex recommends that labs test using a pulsed sweep with short on time for best accuracy, especially when doing I-V or L-I sweeps. To allow maximum settling time for each step, the SMU digitizer should be set to capture one 2us sample on each step falling edge.
Pulsed sweeps are routinely used for threshold detection, kink detection, etc. Our research has shown that the only drawback is that short pulses produce less heating and that means I-V and L-I sweeps performed with short pulses will differ from those measured on an early source/measure instrument using a staircase sweep . To overcome this, as mentioned above, program a pulsed sweep with the on time equal to the original staircase sweep step time, and an off time of 9us, the minimum programmable off time. Measurements should be taken during the on time at a point that duplicates the measurement of the original staircase sweep. If this is done, the pulsed sweep results will precisely match the original staircase sweep results.
Performance Series Current Sources that include the Bias upgrade will also be capable of an industry-first, integrated Pulsed Sweep with Bias. Pulsed Sweep with Bias allows junction temperature (Tj) and forward voltage (Vf) to be measured after each pulse, producing invaluable data and graphs that reduce time to market for devices.
All SpikeSafe Performance Series Current Sources ordered after August 2017 will include these new functions.
The Pulsed Sweep function outputs a specific number of current pulses of increasing or decreasing amplitude. Pulses may be as short as 10µs. Pulsed Sweeps improve the accuracy and execution time of common device tests such as voltage vs current plots, light vs current plots, and maximum pulse withstand tests.
Hardware-Based Timing Improves Repeatability
The SpikeSafe’s hardware-based timing precisely aligns voltmeter or spectrometer measurements to the current pulses. This precision alignment means sweep plots are perfectly repeatable without the variations seen with source-measure instruments that utilize firmware-based timing systems.
Short Pulses Reduce Heating, Improving Accuracy
Utilizing Pulsed Sweeps instead of the typical staircase sweeps found in source-measure instruments greatly reduces device junction heating during testing. This means high-power devices may be tested without using active cooling. It also means plots generated with Pulsed Sweeps show reduced droop at high currents, meaning a more accurate plot overall.
Emulates Staircase Sweeps for Compatibility
While Pulsed Sweeps more accurately show device characteristics, sometimes it is necessary to duplicate measurements made with previous-generation source-measure instruments that employ non-pulsed or “staircase” sweeps. Staircase sweeps induce significant junction heating, so the measurements made with them don’t match a typical pulsed sweep measurement. To replicate these measurements, the SpikeSafe’s pulsed sweep may be programmed with long on times and very short off times. The resulting waveform duplicates staircase sweep junction heating, enabling direct comparison of old and new measurements.
Optimize Sweep Speed Using Sweep with Bias Mode
The industry’s first Pulsed Sweep with Bias current mode combines a Pulsed Sweep with a DC bias current. The bias current keeps the semiconductor junction forward biased between pulses so that its forward voltage may be measured. Changes in this voltage correspond to changes in temperature. Plots of this post-pulse forward voltage are easily generated and viewed using the SpikeSafe Control Panel software application.
Simplify Pulse Withstand Testing with Failure Level Capture
Sometimes sweeps are used for destructive testing or current-limit testing. For example, an LED’s pulse withstand capability may be tested using a pulsed sweep that extends above the device’s absolute maximum pulsed current. This type of testing is supported in two ways – by allowing higher output and capturing the actual current level at the moment of failure and report this current level via a SCPI error message.
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