Device design and fabrication. a A Si–Gr–Ge transistor is built by directly stacking a Si membrane, single-layer graphene and a Ge substrate. b Optical image of a Si–Gr–Ge transistor (scale bar: 20 μm). c SEM image of a Si membrane on graphene (scale bar: 4 μm). d Illustration of the cross-section of the transistor. e Illustration of the basic operating principle of the transistor. Credit: Nature Communications

In 1947, the first transistor, a bipolar junction transistor (BJT), was invented in the Bell Laboratory and has since led to the age of information technology. In recent decades, there has been a persistent demand for higher frequency operation for a BJT, leading to the inventions of new devices such as heterojunction bipolar transistors (HBT) and hot electron transistors (HET). The HBTs have enabled terahertz operations, but their cut-off frequency is ultimately limited by the base transit time; for the HETs, the demand of a thin base without pinholes and with a low base resistance usually causes difficulties in material selection and fabrication.

Recently, researchers have proposed graphene as a base material for transistors. Because of the atomic thickness, the graphene base is almost transparent to electron transport, leading to a negligible base transit time. At the same time, the remarkably high carrier mobility of graphene will benefit the base resistance compared with a thin bulk material. Graphene-based transistors (GBTs) generally use a tunnel emitter that emits an electron through an insulator. However, the emitter potential barrier height seriously limits the cut-off frequency. Theoretical study has indicated that a Schottky emitter may solve this potential barrier limitation.

A team of researchers at the Institute of Metal Research, Chinese Academy of Sciences, has built the first graphene-based transistor with a Schottky emitter, which is a silicon-graphene-germanium transistor. Using a semiconductor membrane and graphene transfer, the team stacked three materials including an n-type top single-crystal Si membrane, a middle single-layer graphene (Gr) and an n-type bottom Ge substrate.

Compared with the previous tunnel emitters, the on-current of the Si-Gr Schottky emitter shows the maximum on-current and the smallest capacitance, leading to a delay time more than 1,000 times shorter. Thus, the alpha cut-off frequency of the transistor is expected to increase from about 1 MHz by using the previous tunnel emitters to above 1 GHz by using the current Schottky emitter. THz operation is expected using a compact model of an ideal device. The electrical behavior and physical activity of the working transistor are discussed in detail in the published paper in Nature Communications.

With further engineering, the vertical semiconductor-graphene-semiconductor transistor is promising for high-speed applications in future 3-D monolithic integration because of the advantages of atomic thickness, high carrier mobility, and the high feasibility of a Schottky emitter.

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More information: Chi Liu et al. A vertical silicon-graphene-germanium transistor, Nature Communications (2019). Journal information: Nature Communications Chi Liu et al. A vertical silicon-graphene-germanium transistor,(2019). DOI: 10.1038/s41467-019-12814-1