The composition dependence of the structural and electronic properties in chalcogenide glasses suggests that there exists a structural phase transition at the average coordination number of 2.67. Materials having smaller coordination numbers are characterized by molecular structures, and otherwise three-dimensional networks govern the properties. The result is discussed in light of topological and percolative arguments.
Optical and electrical properties of sputtered Ge2Sb2Te5 films in amorphous and crystalline states have been studied. The optical band-gaps of amorphous, cubic (NaCl-type), and hexagonal Ge2Sb2Te5 are 0.74, 0.5, and 0.5 eV, respectively. Electrically, the amorphous and cubic states behave as semiconductors with activation energies of 0.45 and 0.14 eV, while the hexagonal state is metallic. The resistivity decreases slightly at the melting point of ∼600°C. All the states show p-type thermoelectric power, in which the amorphous and the cubic state have the activation energies of 0.3 and 0.14 eV. Carrier parameters and electronic densities-of-states are estimated and considered.
oscillator circuits. For an inverter oscillator (Fig. 5), we coupled five inverters together, with the o u t~u t voltage of one inverter providing the input for the following. If the initial input to the first inverter is 0 V (-V,), then the output of the fifth will be -V, (0 V). Returning the output of the fifth inverter as the i n~u t for the first inverter causes all of the inverters to sequentially change logic state. Consequently, the circuit oscillates with a characteristic frequency f,,,. If all transistors are identical, fClSc is %o of the switching delay frequency of an individual inverter. In reality, fc,,, is limited by the slowest gate. For pentacene ring oscillators, f,,, is in the range of 100 to 500 Hz, and for PTV ring oscillators, fc,,, is in the range of 10 to 50 Hz. These values are dominayed by the RC time constant of the channel resistance R of the load transistor (the nonswitching transistor), which is giv-
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