The first active communications satellite placed in orbit by the United States, Telstar (1962), failed when its transistors were exposed to unexpected levels of ionizing radiation. Ionizing radiation, furthermore, may cause transistors to fail by permanently altering the crystal structure. Usually these effects are unwanted, because one does not want the properties of a circuit to vary with temperature. Heat, light, or ionizing radiation may all increase the semiconductor's electrical conductivity by liberating these electrons to support current. Silicon transistors have far less leakage. They are not bound too tightly to break loose if given a small amount of extra energy, but cannot wander easily through the crystal. Germanium transistors make leaky switches substantial leakage currents can flow when these devices are supposedly in their off state. Covalent bonding also prevents these outermost electrons from moving through the crystal (i.e., flowing as an electric current) as easily as do the conduction electrons in metals. This sharing holds the atoms together by the process termed covalent bonding. In the 1990s a revolution in bipolar transistor design occurred with the emergence of silicon-germanium SiGe Heterojunction Bipolar Transistors. That is, each atom is held in its place in the crystal's orderly structure because each atom shares its four outermost electrons with the outermost electrons of four nearby atoms. The forward bias voltage of silicon is 500 to 700mV, which is much higher and more difficult for a guitar to activate. Silicon has seen extensive use as a semiconductor material since the 1950s. This has been made possible by adding small amounts of Germanium (Ge) and Carbon (C) to Silicon (Si) to create new material properties. In the late 1950s, interest grew in using silicon instead of germanium because of its. Each atom in a silicon or germanium crystal lattice has four atoms as close neighbors. Silicon Germanium:Carbon (SiGe:C) has extended the life of silicon technology for high performance and high speed/ frequency applications. The invention of the transistor transformed the possibilities for.
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