Professor Sungjae Cho, from the Department of Physics, and his research team has successfully developed a new type of transistor, Black Phosphorus Tunnel Field Effect Transistor (BP TFET), with a power consumption ten times lower and operation speed two times faster than that of the traditional Metal Oxide Semiconductor Field Effect Transistor (MOSFET). While low-power transistors have been developed before, this is the first instance of a low-power transistor being faster than the MOSFET. 

Professor Sungjae Cho (left) and PhD student Seungho Kim, who is credited as the first author of the published paper (right)

Unlike the MOSFET, which is mainly fabricated from silicon (Si), this transistor uses black phosphorus (BP) as the semiconductor material. A special property of BP is that its band gap changes depending on the material’s thickness, allowing for less energy needed for activation. Furthermore, a monolayer hexagonal boron nitride material (ML hBN) was used in the drain of the transistor to increase the operation current. Until now, low-power transistors were considered impractical compared to the MOSFET due to the slower operation speed caused by low currents. 

In order to reduce a transistor’s power consumption, both its activation voltage and current during idle states should be reduced. This means that while the current is small when not activated, a small voltage should increase the current drastically to activate the transistor. Therefore, the subthreshold swing (SS) of the transistor, which measures the change in voltage required to increase the current by ten times, must be reduced for a low change in voltage to cause a large increase in current. 

The mechanism by which a MOSFET operates limits the SS value to be no smaller than 60 mV/dec. In January, Professor Cho’s research team used a different mechanism to develop a BP-based transistor with an SS value below 60 mV/dec. However, at this point, the current was too small for practical applications.

By using the ML hBN material in the drain of the BP-based transistor, the research team developed the BP TFET. This successfully overcame the aforementioned limitation, being able to reach higher current values with low voltages compared to the MOSFET. 

Professor Cho expects that based on the results of this research, various applications in semiconductor research as well as in semiconductor industries will be possible.

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