Power Engineering. Соколова Е.В. - 34 стр.

      1.Were semiconductor diodes used as active devices? 2. When was the tunnel
diode developed? 3. What does the operation of parametric diode depend on? 4. Of
what nature is the parametric diode?

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      Parametric and tunnel diodes

       1. The history of the semiconductor diode is held to have begun in the early
days of radio when "crystals" were used as signal detectors. However, the operating
mechanism of the semiconductor diode remained obscure until the introduction of
modern transistor physics. Furthermore, semiconductor diodes were not used as ac-
tive devices until a p-n junction diode was discovered to be an attractive element for
parametric amplifiers. Such a diode, characterized by a variable capacitance, is
known to be called a parametric diode. It has a voltage-dependent junction capaci-
tance. In some commercial forms it is also known as a "varactor" diode.
       2. Immediately after the introduction of parametric diodes, another semicon-
ductor diode known as the tunnel diode was developed. The tunnel diode is a semi-
conductor p-n junction somewhat similar to the parametric diode, although its physi-
cal principles of operation are entirely different. The operation of the tunnel diode is
based on quantum-mechanical tunneling. Parametric diodes and tunnel diodes may be
compared as follows:
       a) They are both p-n junctions. Physically, they can be manufactured in much
the same manner.
       b) The operation of parametric diode depends on its nonlinear capacitance,
controlled by the applied voltage, while the operation of a tunnel diode depends on its
negative resistance, made possible by the tunneling current.
       c) They are both two-terminal, negative-resistance devices. In parametric am-
plifiers using parametric diodes the negative resistance is derived from an r-f pump
through a nonlinear interaction, while in tunnel-diode amplifiers, negative resistance
is produced by a simple d-c source.
       d) In a tunnel-diode amplifier the gain-bandwidth product is inversely propor-
tional to the product of the diode capacitance and negative resistance, while in a pa-
rametric-diode amplifier the gain-bandwidth product is inversely proportional to the
Q of the associated idler circuit.
       e) The parametric diode is of a reactive nature, and thus generates very small
excess Johnson noise. The noise from a parametric amplifier originates essentially
from the associated circuitry. As the tunnel diode is dependent on a tunneling stream
of electrons, it has an inherent shot noise.

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