• Home
  • Download PDF
  • Order CD-ROM
  • Order in Print
Varactor Devices - Continued - 14183_130
Bulk-Effect Semiconductors - Continued - 14183_132

Neets Module 11-Microwave Principles
Page Navigation
  114    115    116    117    118  119  120    121    122    123    124  
2-51 Q-53.   What limits the usefulness of high-gain, tunnel-diode frequency converters? Q-54.   The varactor is a pn junction that acts as what type of electronic device? Q-55.   The underlying principle of operation of the parametric amplifier is based on what property? Q-56.   What is the most important feature of the parametric amplifier? Q-57.   How is amplification achieved in the circuit shown in figure 2-43? Q-58.   What is the purpose of the pump in a parametric amplifier? Q-59.   The pump signal frequency must be of what value when compared to the input signal of a simple parametric amplifier? Q-60.   What is the primary difference between the pump signal of a simple parametric amplifier and the pump signal of a nondegenerative parametric amplifier? Q-61.   In a nondegenerative parametric amplifier the difference between the input frequency and the pump frequency is called what? Bulk-Effect Semiconductors BULK-EFFECT SEMICONDUCTORS are unlike normal pn-junction diodes in both construction and operation. Some types have no junctions and the processes necessary for operation occur in a solid block of semiconductor material. Other types have more than one junction but still use bulk-effect action. Bulk-effect devices are among the latest of developments in the field of microwave semiconductors and new applications are being developed rapidly. They seem destined to revolutionize the field of high- power, solid-state microwave generation because they can produce much larger microwave power outputs than any currently available pn-junction semiconductors. Bulk-effect semiconductors are of two basic types: the transferred-electron devices and the avalanche transit-time devices. TRANSFERRED-ELECTRON SEMICONDUCTORS.—The discovery that microwaves could be generated by applying a steady voltage across a chip of n-type gallium-arsenide (GaAs) crystal was made in 1963 by J.B. Gunn. The device is operated by raising electrons in the crystal to conduction-band energy levels that are higher than the level they normally occupy. The overall effect is called the transferred-electron effect. In a gallium-arsenide semiconductor, empty electron conduction bands exist that are at a higher energy level than the conduction bands occupied by most of the electrons. Any electrons that do occupy the higher conduction band essentially have no mobility. If an electric field of sufficient intensity is applied to the semiconductor electrons, they will move from the low-energy conduction band to the high- energy conduction band and become essentially immobile. The immobile electrons no longer contribute to the current flow and the applied voltage progressively increases the rate at which the electrons move from the low band to the high band. As the curve in figure 2-48 shows, the maximum current rate is reached and begins to decrease even though the applied voltage continues to increase. The point at which the current on the curve begins to decrease is called the THRESHOLD. This point is the beginning of the negative-resistance region. Negative resistance is caused by electrons moving to the higher conduction band and becoming immobile.






Western Governors University

Privacy Statement
Press Release
Contact

© Copyright Integrated Publishing, Inc.. All Rights Reserved. Design by Strategico.