Transparent transistors: Low power, high performance

Oxide thin-film transistors with low operating voltages show promise for low-power ‘see-through’ electronics.

Oxide-based thin-film transistors have emerged as very promising components for applications in displays and ‘see-through’ electronics because of their transparency and high performance. Indium gallium zinc-oxide (InGaZnO₄) is widely used as the active layer in transparent transistors because it has high electron mobility and thus high performance, and also because it can be deposited at room temperature. Unfortunately, this material requires high operating voltages, and the consequently high power consumption is far from ideal for portable battery-powered applications.

Fig. 1: Transfer curve for transparent transistors and a photograph of an array of the transparent devices.

Qing Wan and colleagues from Hunan University in China¹ now report a new approach to reducing the operating voltage of InGaZnO₄ transistors. Their devices operate at an ultralow voltage of just 1.0 V, exhibit good transistor characteristics, and are transparent throughout the visible light spectrum (Fig. 1).

The key feature of these low-power devices is their mesoporous silica dielectric layer, which is positioned between the InGaZnO₄ active layer and the gate electrode of the transistor. A transistor acts like a switch, where the flow of current through the active layer between the source and drain electrodes is controlled by applying a voltage to the gate electrode. In previous InGaZnO₄ transistors, a high gate voltage was needed to push enough charge into the oxide channel to allow current to flow between the source and drain.

However, in the devices fabricated by Wan and his colleagues, the mesoporous silica dielectric layer was treated with silane (SiH₄) plasma, thereby encapsulating mobile protons. When a positive voltage was applied to the gate electrode, the protons moved to the interface between the dielectric and the InGaZnO₄ channel, inducing the formation of mirror-image charge in the oxide channel — a layer of electrons on the other side of the interface that balanced the positive proton charge. This layer of protons and charge-balancing electrons is known as an ‘electric double layer’, and the presence of the layer of electrons allows current to flow easily along this layer between source and drain, even at low voltages.

“These ultralow-voltage transparent thin-film transistors were processed at room temperature and are very promising for low-power, portable, battery-powered applications,” says Jie Jiang, lead author on the study. “In the future, since our fabrication process is at room temperature, we will fabricate low-voltage flexible transparent transistors on plastic substrates.”