![]() ![]() ![]() For the hot electron transistors, when an electron is emitted into the base, the energy difference between the electron and the Fermi energy level (or the bottom of conduction band) of the base is transformed into electron kinetic energy. The heterojunction bipolar transistors have achieved great development toward the terahertz (THz) operation 6, 7, 8, 9, however, their f α is ultimately limited by τ b. In the past 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 and hot electron transistors. f α is inversely proportional to the delay time, which includes the emitter charging time τ e, the base transit time τ b, and the collector delay time τ c 2, 3, 4, 5. During operation, electrons are emitted from the emitter, diffuse through the base, and eventually are collected by the collector 1.įor a BJT, a main figure of merit is the alpha cutoff frequency f α, which is used to represent the upper frequency limit when a BJT is biased in the common base mode. In an n-p-n BJT, a p-type base semiconductor is sandwiched by n-type emitter and collector semiconductors, forming an emitter junction between the emitter and the base, and a collector junction between the base and the collector. In 1947, the first transistor, named a bipolar junction transistor (BJT), was invented in the Bell Laboratory and has since led to the new age of information technology. With further engineering, the semiconductor-graphene-semiconductor transistor is expected to be one of the most promising devices for ultra-high frequency operation. Such Schottky emitter shows a current of 692 A cm −2 and a capacitance of 41 nF cm −2, and 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. Here we demonstrate a vertical silicon-graphene-germanium transistor where a Schottky emitter constructed by single-crystal silicon and single-layer graphene is achieved. To overcome this issue, a graphene-base heterojunction transistor has been proposed theoretically where the graphene base is sandwiched by silicon layers. However, generally used tunnel emitters suffer from high emitter potential-barrier-height which limits the transistor performance towards terahertz operation. Graphene-base transistors have been proposed for high-frequency applications because of the negligible base transit time induced by the atomic thickness of graphene. ![]()
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