[Professor Yong-Young Noh’s research team at POSTECH develops p-channel transistors through halide anion engineering.]

[The new technology realizes a threshold voltage of 0 V and is hysteresis-free and high performing.]

Robot vacuums, a major household appliance that make life easier, are convenient but they often stumble on door thresholds that are not even very high. A similar threshold voltage exists in a transistor through which current flows. As long as the voltage exceeds the threshold voltage, the output impedance of the transistor is sharply lowered and current flows easily, improving its performance.

Recently, a POSTECH research team led by Professor Yong-Young Noh and Ph.D. candidates Huihui Zhu and Ao Liu (Department of Chemical Engineering), in collaboration with Samsung Display, has developed a p-channel^*1 perovskite thin film transistor (TFT) with a threshold voltage of 0 V.

Despite the impressive development of metal halide^*2 perovskites in diverse optoelectronics, progress on high-performance transistors employing state-of-the-art perovskite channels has been limited due to ion migration and large organic spacer isolation

In this study, the research team constructed a methylammonium-tin-iodine (MASnI3) semiconductor layer by mixing the halide anions (iodine-bromine-chlorine) to increase the stability of the transistor. The device made using this semiconductor layer showed high performance and excellent stability without hysteresis^*3.

In experiments, the TFTs realized a high hole mobility^*4 of 20cm2V-1s-1 and 10 million on/off current ratio^*5, and also reached the threshold voltage of 0 V. A P-channel perovskite transistor with a threshold voltage of 0 V is the first such case in the world. By making the material into a solution, the researchers also enabled the transistors to be printed, lowering their manufacturing cost.

Through this study, the research team demonstrated that the primary cause of the hysteresis that lowers the performance of the perovskite TFTs is the minority carrier trapping, not the ion migration. By lowering the threshold voltage, the movement of electrons and holes is undisturbed, enabling the current to flow smoothly.

Furthermore, the research team succeeded in integrating the perovskite TFTs with commercialized n-channel^*6 indium gallium zinc oxide (IGZO) TFTs on a single chip to construct high-gain complementary inverters through a circuit-printing method.

This study is drawing attention from academic circles as a technology applicable to the development of OLED display driving circuits, P-channel transistors of vertically stacked devices, and neuromorphic computing for AI calculations.

Recently published in Nature Communications, this study was conducted with the support from the Mid-Career Researcher Program of the National Research Foundation of Korea, and from the Samsung Display Corporation. The POSTECH research team and Samsung Display have already applied for domestic and international patents for this technology last year..

1. P-channel semiconductor
A semiconductor that generally has holes as charge carriers. A semiconductor in which the number of electron-carrying holes is much greater than the number of electrons.

2. Metal halide
a substance containing a metal and a halogen.

3. Hysteresis
A phenomenon in which a physical state depends on the process of change of state it has been through. That is, the aftereffect of the impact remains for a prolonged period and cannot be normalized.

4. Hole Mobility
An empty space created by the loss of one electron is called a hole. If an electron has a charge of -1, a hole is a charge carrier with a charge of +1.

5. On/off current ratio
The ratio between the maximum current when the transistor is operating (On state) and the minimum current when it is turned off (Off state).

6. N-channel semiconductor
A semiconductor that generally has electrons as charge carriers. A semiconductor in which the number of electrons is much greater than the number of holes.