Novel transistors make robust thin-film logic

UK researchers are proposing to replace FETs with source-gated transistors (SGTs, see below) in thin-film digital circuits to improve noise margin, power-delay product, and robustness.

Source gated transistor from the University of Surrey

Championed by the University of Surrey, SGTs can be made on the same thin-film processes as FETs, but are far less sensitive to the geometric errors inherent in printing and other non-lithographic fabrication techniques – because SGTs are hardly sensitive to drain-source distance, while FETs are extremely sensitive.

For this reason, advocates are proposing them for printed electronics and displays, particularly on flexible substrates.

Source gated transistor from the University of Surrey“Performance doesn’t really change when the quality of patterning changes,” Dr Radu Sporea of Surrey’s Advanced Technology Institute, told Electronics Weekly.

Like for like, they also exhibit lower saturation voltage and higher gain.

The downside is that they are slower and can handle less current than similar FETs.

In a paper published in Nature’s Scientific Reports, Surrey scientists led by Sporea modelled two-transistor inverters made 4µm source-drain gap poly-silicon FETs or SGTs – using models calibrated from real devices.

Operating with a 5V supply, SGT inverters showed improved gain and noise-margin.

FET and SGT inverter output characteristicsThe output voltage graph is much ‘squarer’ with SGT inverters, and on/off current ratio is over 10^5.

“Coupled with the almost-standard fabrication process, this circuit robustness recommends SGT as reliable cost-effective implementations for applications such as remote sensing and data processing,” said the University.

According to Surrey, even though switching is slower than with FETs, the high gain of SGTs means the inverter spends less time in the linear region, where both top and bottom transistors are partially-on. “Switching power is lower, as is the power-delay product”.

Source gated transistor from the University of SurreyAs a result of this modelling, Sporea is advocating the use of SGTs for thin-film digital circuits where robustness in the face of noise and fabrication vagueries are more critical that speed.

Previously, along with Philips, the Surrey team has put a lot of work into SGT analogue circuits.

Source gated transistor from the University of Surrey

Output characteristics from the
first batch of Surrey/Philips SGTs

Gain, linearity and dynamic range are high.

The second characteristic makes them particularly suited to OLED pixel drivers where, said Sporea, organic FETs might need a 10V overhead to work well with a 2V OLED, while SGTs need only 1.5V. “The 10V can be reduced using sub-threshold FETs, but by the time you have added sub-threshold compensation you need 10x transistors in the pixel.”

OLEDs need current drive, which favours FETs.

To this, Sporea responds that printed FETs need greater source-drain distance (channel length) to avoid short-channel effects. SGT can have very short channels, and used toe space saved for extra channel length to increase current capability.

The structure of an SGT is similar to that of a FET, and either can be made in almost any semiconductor.

SGT/FET research structure

SGT/FET research structure

To the right is a diagram of a research structure that is a bottom-gate FET (by leaving the ‘source barrier’ open circuit and treating the left drain as the source), or as an SGT (by leaving the left drain open circuit and connecting to the source barrier. In the SGT, the critical thing is that the source contact is conductor-semiconductor, and the gate is on the opposite side of the semiconductor layer from the source.

In a FET, gate voltage modulates conductivity in the device’s channel.

SGT and equivalent circuitIn an SGT, gate voltage modulates reverse leakage through a diode formed between the source contact and an inactive channel.

Unlike in FETs, the SGT source contact must form a diode with the device’s semiconductor channel. A Schottky junction is used at Surrey, although many structures will work.

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