Staircase Sweep

Staircase sweeps are a traditional method for generating a I-V, V-I or LIV curves. In a staircase sweep, the current level is increased in steps, and voltage and other measurements are made at each step.

Staircase sweeps are easily generated with SpikeSafe Performance Series Current Sources and Source Measure Units. Staircase sweeps have been the de-facto standard for generating IV curves but, there is a better method and this method is called the Pulsed Sweep.

SpikeSafe Performance Series Current Sources and Source Measure Units offer hardware integrated pulsed sweeps. With pulsed sweeps, the current amplitude is increased in steps and voltage or other measurements are made at each step. With pulsed sweeps, device heating is reduced, and cumulative heating may be avoided. With pulsed sweeps, high power devices may be tested without the need for active cooling.

Staircase sweeps, also know 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.

10 Step Pulsed Sweep
10 Step Pulsed Sweep

Pulsed Sweeps

Leveraging Vektrex’s repeatable, near-perfect, square pulse capability, pulsed sweeps are easily generated using a series of Single Pulse commands. 

However, a simpler and better method is to use the hardware-integrated pulsed sweep function. Use one software command to define the pulsed sweep parameters and another to start the sweep. With other instruments, many operator actions or commands are needed to generate one pulsed sweep that may not be repeatable due to software timing.  

200mA Step Pulsed Sweep

SpikeSafe SMU hardware integrated sweeps are easy to produce, reliable and repeatable.  

Improve Repeatability with Hardware Based Timing

Measurement standards recommend using hardware triggers with language like: “Triggers used to coordinate the precision pulsed current source, the spectrometer, and the digitizer shall be hardware-based to minimize timing variation.”

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 or software-based timing systems.

Short Pulses Reduce Heating, Improving Accuracy

Use the hardware-integrated pulsed sweep with short pulse on time for the most accurate IV curves during characterization and other testing. A good rule of thumb is to use 20µs pulses at currents above 10% of maximum and 200µs pulses for currents below 10% of maximum. Use an off time that gives a 1-2% duty cycle.

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.

I-V Plot Generated with Pulsed Sweep
I-V Plot Generated with Pulsed Sweep
20 Step, 100µs Pulsed Sweep Used For Cree Plot
20 Step, 100µs Pulsed Sweep Used For Cree Plot

Emulated 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 will not perfectly match a typical pulsed sweep measurement. Emulating a staircase sweep with the SpikeSafe SMU is easy.  There are three modes available. 

A) 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.

B) 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.

C) Pulsed Sweep can be used to approximate a staircase sweep. If the off time is short compared with the on time, then device heating is similar to the heating of a staircase sweep. This is the mode Vektrex recommends 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.

20ms on, 300us off 20 points 0.1 to 1A
20ms on, 300us off 20 points 0.1 to 1A
Staircase Sweep Generated Using 30us Off Time
Staircase Sweep Generated Using 30us Off Time

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. 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.

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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