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Energy Conversion Mechanisms in Solid-State and Vacuum Microwave Devices


I.V. Lebedev

All electron devices, whether they depend for their operation on the motion of free electrons in high vacuum or solid state behaviour, are basically energy converters. In an amplifier or generator, the energy of mobile charge carriers (electrons, holes) is taken in simplest case from a direct current source. The part of this energy is to be converted in the energy of output oscillations of desired frequency spectrum. It is known, that conventional vacuum microwave devices employing electron transit-time effects are based on two principally different mechanisms of energy conversion. The operation of the so-called O-type vacuum microwave devices (klystrons, traveling-wave tubes) is described in terms of the concept of electrons giving up their kinetic energy to the retarding electric r. f. field. However, in the M-type vacuum devices (multicavity magnetrons, amplitrons) the drifting electrons give up to microwave electric field rather potential than kinetic energy at nearly constant average drift velocity. Until now, however, there is apparently no settled opinion on the energy conversion physical mechanism in solid-state devices which are similar in function to vacuum devices and also employ the transit-time effects of charge carriers. The purpose of this article is to describe and discuss shortly the way in which a r. f. electric field interacts with moving charge carriers in solid-state diode structures; to compare this interaction mode with the processes in the high-vacuum microwave devices and to estimate the maximum possible power efficiencies. A simplified one-dimensional model is considered to analyse the operation of the IMPATT diode generator or amplifier and the Gunn diode in the LSA mode of oscillations. The change of the energy of electrons in the interaction space of these solid-state structures is estimated with the aid of the time-to-distance diagrams which are effectively used in the explanation and analysis of vacuum microwave tubes. It is shown that the mechanism of power conversion in the considered solid-state devices is similar to M-type vacuum devices mechanism. It differs principally from the mechanism of O-type vacuum devices. The drifting electrons reaching the anode contact of the diode, just as in a vacuum magnetron, lose their potential energy with respect to the anode of the device. The energy captured from the electron stream is partly delivered to the high- frequency electric field and partly wasted in the crystal body due to dissipation processes. The efficiency of this energy conversion is limited by the known physical characteristics of semiconductor material and can not be increased by the depressed collector recuperation method which is widely used in vacuum traveling-wave tubes and in some types of klystron amplifiers. The transfer of the potential energy from drifting electrons to the h.f. electric field in solid-state devices may be compared with the change of potential energy of physical body falling in the atmosphere of Earth. The retarding microwave electric field plays in the semiconductor devices the role of additional effective friction. The article may be of interest in the understanding of principles of microwave electronics and can help to the development of new efficient microwave devices.

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