Swapping Thin-Film Transistors for Source-Gated Transistors Yields Simpler Display Circuits
By using source-gated transistors in place of TFTs, researchers have created a new circuit design that promises great improvements in large-area electronics.
Over the past couple of decades, CMOS-based electronics have undergone incredible improvements thanks to Moore's law and the mass integration of transistors. Other non-CMOS-based electronics have not benefited from this trend and have historically struggled to keep up with the pack.
One specific field—display screens—has been severely limited in growth due to one of the technology’s fundamental devices: the thin-film transistor.
Thin-Film Transistors and Their Limitations
A thin-film transistor (TFT) is a transistor whose active, current-carrying layer is a thin film—usually a film of silicon. This differs from a MOSFET, which is made on silicon wafers and uses the bulk-silicon as the active layer.
The main advantage of TFTs over CMOS, according to research published in IEEE Sensors, is their ability to be manufactured on large substrates at a low-cost per unit area and at low processing temperatures. This allows them to be directly integrated onto a variety of flexible substrates.
TFT displays are the most popular form of color display on the market right now. In this type of display, the transistors are arranged in a matrix and the TFT acts as a switch for each given pixel.
Silicon structure of a TFT. Image used courtesy of Nianduan Lu et al.
According to X. Guo et al., TFT scalability is significantly hindered by materials and substrate process limitations. This results in poor performance that needs high operation voltage and supports only low-frequency circuits. This limitation has resulted in inefficient circuit designs relying on increased complexity to account for device shortcomings.
A Solution: the Source-Gated Transistor
Recent research from the University of Surrey, the University of Cambridge, and the National Research Institute in Rome has addressed these issues using source-gated transistors (SGTs). Published in IEEE Sensors Journal, the researchers used source-gated transistors in place of TFTs to create a new circuit design that promises great improvements in large-area electronics like display screens.
An SGT is created by combining a TFT and a Schottky diode. According to R.A. Sporea et al., three major aspects determine the operation of an SGT:
- The source electrode purposely comprises a potential barrier
- The gate overlaps the source
- Drain bias is able to deplete the semiconductor fully at the source edge
Current is controlled by the reverse-biased source barrier, as opposed to the gate in conventional CMOS.
A source-gated transistor structure. Image (modified) used courtesy of Jiawei Zhang et al.
The resulting advantages include high intrinsic gain, low-voltage saturation, insensitivity to channel length and semiconductor quality, and improved stability.
Simpler Designs for Large-Area Electronics
The study showed promising results. Researchers found that the functionality of 12 TFTs was achieved by using only 2 SGTs. This result is significant because it opens doors for less complex circuit designs, higher performance, less waste, and, importantly, lower cost.
The hope is that these improvements can lead to a new generation of flexible and versatile display technology.
Lead author of the study Dr. Radu Sporea says, “Our design offers a much simpler build process than regular thin-film transistors. Source-gated transistor circuits may also be cheaper to manufacture on a large scale because their simplicity means there is less waste in the form of rejected components."
He continues, "This elegant design of large-area electronics could result in future phones, fitness tracker or smart sensors that are energy-efficient, thinner and far more flexible than the ones we are able to produce today.”
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