Characterizing field effect transistors using two SMUs

Introduction

This document describes a test and measurement method for characterizing Field Effect Transistors using Aim-TTi SMU4000 Series instruments, accessories and software.

The characterization of three leaded components (such as a field effect transistor) requires two SMU output channels; one to stimulate the Gate of the FET and the other provides a drain to source voltage (Vds) for which current measurements can be made.

SMU4000 Series instruments provide a single output, however a fully functioning two channel SMU can be achieved with the addition of a second SMU, connected via the DIO terminals. Simply adding an additional SMU to the test setup can result in a slow and arduous test, therefore the following accessories are available to make this type of test quick and simple to setup and perform:

Linking two SMUs via the DIO terminals enables them to perform simultaneously via a handshake triggering system, effectively creating the fully functioning two channel SMU.

The ‘SMU LINK’ adaptor accessory is available To make this ‘link’ far simpler and convenient to utilize, The link adaptor connects to both DIO terminal ports to synchronise the two SMUs together, without the need to wire each DIO terminal individually.

The whole process can be made far simpler via the use of the free ‘Test bridge SMU’ PC software. This software allows for complete control of both instruments and provides powerful intuitive graphing features for plotting I/V curves with measurement data provided by both SMUs.

Using the Link Adaptor

This SMU link adaptor is designed to connect two Aim-TTi SMUs together via the DIO terminal ports on the rear panel

Figure 1: Two Aim-TTi SMUs and a link adaptor, connected via the DIO terminals

It can also be used as a DIO Expander for one or two instruments, removing the need to wire each DIO terminal individually. See ‘SMU Link Adaptor Instruction Manual’ for more details on using the link as a DIO expander.

Figure 2: SMU link adaptor from Aim-TTi

A fully functioning two channel SMU can be created with two SMUs via a handshake triggering system using the link adaptor to connect the trigger DIO lines.

When using the link adaptor to connect two SMUs, the adaptor internally connects the ‘Trigger In’ of each SMU to the ‘Trigger Out’ of the other. 'Trigger Out' will be set once all measurements associated with the level / shape are complete.

This combination of 'Trigger in' and 'Trigger out' allows for the handshaking of multiple instruments without further wiring.

For full synchronization of two SMUs, consider the following settings with the 'Manual setup' of the SMU:

• To initialize the test, both SMUs must be ‘RUN’.
• The DIO must all be set to either active high or active low on both instruments.
• CNFG > [System] Interfaces > [DIO] Pin Action
• A trigger (either level or shape) must be set on both instruments, A shape other than steady must be selected to set the trigger.
• CNFG > [Source Measure Action] Manual Setup > [Timing] Trigger
• If both sets of results are intended to be plotted on one graph, the total number of measurements on both SMUs must match.
• To maintain synchronisation, set the same number of levels/ shapes on both SMUs

Figure 3: Example setup - using level triggering to synchronise two SMUs

The time for Trigger In/ Trigger Out to respond will typically take:

Figure 4: Trigger Out / In typical timings

Test setup

Figure 5: FET test setup example, using two Aim-TTi SMUs and a Link Adaptor

Equipment required

• Two Aim-TTi SMUs
• Aim-TTi SMU Link adaptor (ribbon cables included)
• Test cables and clips, at least two per SMU. Additional cables are required for 4 wire measurement or for utilizing 2 wire plus guard
• FET for characterization

Characterization test concept

To perform an IV characterization measurement of a Field Effect Transistor requires one SMU to stimulate the Gate of the FET by providing a gate to source voltage (Vgs) whilst the other SMU provides a drain to source voltage (Vds) for which current measurements can be made.

The gate source voltage is swept from a start voltage to an end voltage in discrete voltage steps, at each of these steps a drain current measurement is performed. The gate voltage can then be plotted against the drain current as an IV curve. This can then be repeated at different drain to source voltages.

There is a huge abundance of different FETs, coming in all different forms and sizes, therefore no one setup configuration suits all. As such the set levels and limits in this document are very specific to the device under test. However, there are key settings required for the synchronised test to work as intended.

Example settings

Figure 6: Example Settings table.[Voltage and Current settings are FET specific]

Firstly, and most importantly is the ‘Trigger’ setting within the manual setup. This must be set to ‘Level’. This essentially controls the state of the global trigger, setting it to ‘Level’ means that a global input trigger is required prior to setting the next level, once all measurements are complete for the set level, the global output trigger is set. Without this no synchronisation handshaking between the units takes place.

In order for both the SMUs to remain in synchronization requires that both source the same number of levels to ensure that there is a measurement of drain current for each gate voltage step. This effectively means that the start and end level will likely remain the same for the Vds sourcing SMU. (i.e. the Vds voltage remains the same for the whole test, however there is a triggered measurement on each step level).

In most applications the Measurement Count should be set to one, this ensures that for each gate level voltage measurement there is a single drain current measurement.

The DIO must all be set to either ‘Active High’ or ‘Active Low’ on both instruments.

Running the test

Once the test setup and configuration are complete the IV characterisation can be performed.

Figure 7: Running the FET test setup

To perform the test, enable SMU 1 (Vgs), wait for the first measurement to be made, then enable SMU 2 (Vds). The two SMUs will then perform the test and synchronise themselves together via the SMU link.

Once the test is complete the results from each SMU can be exported via the USB flash drive or over remote to a PC to combine the results.

Figure 8: FET test results, IV curve exported from Aim-TTi Test Bridge SMU

This is an example for how to characterize a FET with one I/V curve at a single constant drain to source voltage. However, the test can be repeated and the Vds sweep voltage of SMU 2 changed to build up a series of results at different drain to source voltages.

This can be done as individual tests or can be combined into a test sequence by importing multiple test setups into the sequence mode.
For more information on test sequence setups, see the ‘SMU4000 Series Instruction Manual’.

When performing this type of test, the first measurement of SMU 1 and the last measurement of SMU 2 may be ‘unexpected’ for each step or setup, causing a similar result to that shown:

Figure 9: FET test results, IV curve exported from Aim-TTI Test Bridge SMU

The first and last measurement results are created during the initiation of the handshaking process. There are FET test results, IV curve exported from Aim-TTI Test Bridge SMUsolutions that can alleviate the impact of this:

Figure 10: Handshaking process using two Aim-TTi SMUs and a Link Adaptor

Add an additional single point to the setup of SMU 1; allowing SMU 2 to take the final measurement before the SMU 1 sweep finishes.

Increase the total number of points to compensate for these results, for example if 100 measurements are required, select 101 points. The measurement results can be exported and the unwanted measurements removed as required.

Set ‘Measurement Control’ to constant level; the final measured level will be recorded with the output of SMU 1 still running, the output of SMU 1 will need to be disabled manually.

Using test bridge SMU

The whole process of Characterizing a FET using two SMUs can be made far simpler and quicker via the use of the free ‘Test Bridge SMU’ PC software. This package is available as a free download from: www.aimtti.com/support. This software allows for complete control of both instruments and provides powerful but intuitive graphing features for plotting IV curves with measurement data provided by both SMUs.

Test Bridge SMU setup features

Figure 11: Aim-TTi Test Bridge, SMU setup

When initializing the connection, each SMU can be given an individual name for ease of identification- for example ‘Gate Voltage’ and ‘Current Drain’, this name is then associated with all exported results from that instrument. Allowing the results to be quickly and efficiently organized for analysis.

Once identified, the setups for each SMU can be shown together on one screen, allowing all settings to be displayed and edited for each instrument.

From this window, setups can also be saved and loaded from one SMU to another, allowing identical setups to be run at once without the need to further configure.

Test bridge SMU sequence features

Test bridge allows sequences to be created from one place, opposed to the multiple screens on the front panel. Simply change the mode to sequence using the ‘Mode’ drop down box to enable the Sequence Mode editor.

Figure 12: Aim-TTi Test Bridge, SMU sequence setup

Once a step has been added to the sequence, setups can be created directly from this window in the same way as previously explained, using expanders below the step or loaded from an external file. This provides a simple platform for the same setup to be repeated at different drain to source voltages.

Up to 25 steps can be added to a sequence. Each sequence step can be named and reordered as required.

Once the test is complete, a pop-up window will appear with the options to save/ analyze the data.

Analysing results with Test Bridge

Test bridge provides powerful features to simplify the extraction and analysis of results, from one or two SMUs.

After running a test, Test Bridge will extract the results from the SMUs and load them into the ‘Results’ tab (if ‘Analyze Data’ is checked in the pop-up window).

Figure 13: Aim-TTi Test Bridge, SMU results analysis

When characterising a FET, Test Bridge can plot the gate voltage results from SMU 1 against the drain current results of SMU 2 as an IV curve on the graph, using the ‘Two Datasets’ feature.

The selected data combined in the graph can be directly exported to a file, creating a concise new set of results from the two tests which can be analyzed further in an external program if required. Image files can also be saved directly from the graph, as shown in figures 8 & 9.

Figure 14: Aim-TTi Test Bridge SMU IV graph using two datasets

If the results are part of a sequence, Test bridge provides a feature called ’Grouping’. Grouping is designed to be used with measurement data collected from the SMU when in Sequence Mode, the recorded measurement data must contain steps and/or repeats for grouping to work. Each step within the dataset can be shown as a new series on the X Axis.

Grouping also allows a dataset to be split when a step repeats or changes. Figure 15, shows each step and repeat within the dataset as a new series on the X Axis.Advanced zooming and panning functions are also available to further analyse the results, alongside many othe features. For more information, see the ‘Test Bridge SMU Instruction Manual’.

Figure 15: Aim-TTi Test Bridge SMU I/V graph using two sequenced datasets