Electrical Characterization of Graphene TFTs (Thin-Film Transistors)

Table of Content

Challenges in Characterization of TFTs
Solution from Imina Technologies
Experimental Procedure
About Imina Technologies


Although thin-film transistors (TFT) are critical elements of flat-panel displays, they are still the subject of intense research and advanced to continue expanding the fields of applications. It is a challenging task to perform electrical characterization of the TFT because of the potential softness when organic materials are employed and small thicknesses of electrodes.

Challenges in Characterization of TFTs

The penetration of the electrical probes into the thin electrodes results in improper characterization and causes damage to the device. This article discusses the method of addressing this issue. The experiment discussed in this article involves the testing of a TFT with a graphene channel. The source and drain comprise 100nm thick conductive electrodes.

Figure 1. Schematic of the device under test, showing three thin film transistors with graphene channels.

The source, drain, and channel are all made up on a 100nm thick flexible insulating material that is deposited on a conductive substrate, as shown in Figure 1. Earlier attempts to characterize the transistor by means of manual probes were found it difficult to characterize the current modulation of the device due to the penetration of the probes into the extremely thin source/drain electrodes and gate dielectric.

Solution from Imina Technologies

Figure 2. The miBot micromanipulators probing a thin film transistor on the miBase under the objective of an optical microscope

The aforementioned issue is addressed by simply positioning three miBot micromanipulators on a miBase under the objective of a Nikon Eclipse L200 optical microscope, as depicted in Figure 2. probes are rapidly placed in position on the thin electrodes devoid of penetrating into the conductive substrate, thanks to the nanometer positioning resolution of the miBot.

It took around five minutes to position the probes in place prior to starting the measurements. While lowering the probe over the target area, the tip reached the same focal plane position as the sample prior to contact. By increasing the resolution of approach at this point to 00 nm, the approach was perfectly controlled, enabling the visual detection of the contact. There was no damage caused to the gate dielectric because of this approach.

Experimental Procedure

Figure 3. Testing for open circuit behaviour between the drain electrode and the substrate (gate electrode) to verify that the probe has not penetrated through the gate dielectric.

The condition of the gate dielectric and probe contact was verified by performing three tests. The first involved the positioning of one probe on the drain electrode and another probe on the substrate (gate electrode) to measure the resistance between them. As is needed for a successful characterization, the resistance was that of an open circuit, which means that the gate dielectric had not been compromised (Figure 3).

Figure 4. Testing the resistance across a section of the drain electrode to ensure that the probes are making good electrical contact. A low resistance is expected.

Resistances between two probes on the same drain electrode were then measured and found to be 50 Ω. This value is as anticipated because it represents both the sheet resistance of the electrode and contact resistance of the probes (Figure 4).

The final test is the measurement of the channel resistance between the source and drain electrodes without gate bias so as to ensure the proper functioning of the device and probing setup. The device was chosen for characterization when channel resistance was in the range of 2 kΩ.

Figure 5. The probing assembly with the miBot probes on the source and drain displayed on the microscope monitor.

This operation was performed by placing one probe on the source electrode, second probe on the drain electrode, and third probe on the conductive substrate (gate electrode), as shown in Figure 5. Current-voltage sweeps were produced using a HP 4156A semiconductor parameter analyzer for the gate-source bias (VGS) and the drain-source bias (VDS), and then the I-V curves were measured.


Figure 6. IDS - VDS sweep with VGS = 0. Linear behavior indicates a good Ohmic contact. The slope of this I-V curve yields the channel resistance (plus the contact and electrode surface resistances).

The results clearly show the ability of miBot micromanipulators to address one of the major problems in electrical characterization of thinfilm transistors, i.e. placing probes on the film devoid of penetrating it. This assertion was confirmed by measuring the open-circuit resistance between the conductive substrate and the drain electrode.

The resistances across the channel and those measured on the drain electrode corroborated that there was a good electrical contact between the probes and the electrodes (Figure 6). In addition, when compared to conventional manual probers, the miBot micromanipulator’s virtually unrestricted range of motion helped the experiment to be carried out in a much lesser time.

About Imina Technologies

Imina Technologies is a privately held company founded in 2009 to exploit more than ten years of research in high precision robotics at the Swiss Federal Institute of Technology in Lausanne, Switzerland (EPFL). With many years of experience in precision engineering, micro-robotics and nanomanipulation, Imina's interdisciplinary team is geared up for the needs of the most demanding users. Their unique combination of know-how enables them to propose complete solutions for even the most specific applications.

Imina Technologies introduces a new type of micro- and nanomanipulators. Based on a novel motion technology, the miBot is the world's smallest manipulator. It combines nanometer resolution of positioning, unprecedented ease-of-use and flexibility in an ultra compact design. Complete solutions of nanomanipulation are provided to quickly integrate the systems in electron microscopes (SEM/FIB) and light microscopes. The handling of samples like nanowires, MEMS or cells is effortless, speeding-up the production of experimental results.

This information has been sourced, reviewed and adapted from materials provided by Imina Technologies.

For more information on this source, please visit Imina Technologies.

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