Page 145 - Radar Technology Encyclopedia

diode, mixer diode, positive-negative (PN) 135

meter waveband. The basic types and features of diodes used shocks. These diodes also exhibit a high reverse current and
as mixer diodes are the same as for detector diodes. IAM low dielectric strength.
Ref.: Van Voorhis (1948), p. 141; Gassanov (1988), p. 79. i ( A)
m
A modulator diode is one that is used to change the parame-
1.0
ters of a continuous or pulsed signal. In radar modulators, the
modulation diodes are usually used for mixing at high power.
0.5
For these purposes diodes with nonlinear capacitance are used
(PN diodes and high-power PIN diodes). IAM -1.5 -1.0 -0.5
u (V)
Ref.: Gorbachev (1968), p. 6. 0 0.5 1.0
A multiplier diode is used in a frequency multiplier or
divider. In varactor multipliers and frequency-shift mixers
that operate at up to several watts, p-n diodes in which the
Figure D38 Voltage-current characteristic of a silicon point-
capacitance varies nonlinearly with voltage are usually used. contact diode (after Gassanov, Fig. 3.2, p. 74).
The limiting frequency is 30 to 300 GHz, and the breakdown
Point-contact diodes were used in older microwave
voltage in silicon or gallium-arsenide p-n multiplier diodes is
detectors and mixers.
up to 100 volts. In simple low-power frequency multipliers
Related to point-contact diodes are back diodes devel-
one finds Schottky-barrier diodes, which have a nonlinear
oped with microalloy technology, which are distinguished by
impedance. In devices with large multiplying factors, charge
stable parameters and a high current sensitivity. IAM
storage diodes are used, due to the ability to obtain a short
(0.1-ns) pulse of reverse current, which is rich in higher-order Ref.: Gassanov (1988), p. 75; Fink (1975), p. 9.60.
harmonics. Such diodes are limited to 10 to 20 GHz (for sili- A positive-intrinsic-negative (PIN) diode has strongly
con) and 200 to 300 GHz (for gallium-arsenide), with power alloyed p- and n-regions, separated from one another by an
on the order of tens of milliwatts being possible at these fre- extended i-region with a carrier concentration close to the
quencies. IAM concentration of fundamental carriers in a semiconductor.
Ref.: Fink (1982), p. 9.69; Gassanov (1988), p. 82. They are characterized by a small forward impedance and a
large reverse impedance and a low capacitance with a weak
An oscillator diode is used in a solid-state microwave oscilla-
voltage dependence.
tor. It is differentiated from other devices in that it requires
They are used in switching diodes, attenuators, amplitude
low power and is small in size, but exhibits an internal noise
modulators, continuous and graduated microwave phase
level several times higher than other devices. Oscillator
shifters, and also as powerful rectifier diodes.
diodes are usually avalanche transit-time diodes, tunnel
With a wide i-region (0.1 to 0.5 mm), microwave PIN
diodes, or Gunn diodes. IAM
diodes may be used at voltages exceeding 1 kV and pulse
Ref.: Gorbachev (1968), p. 7.
power higher than 10 kW. IAM
A parametric diode is a variant of the varactor used in low- Ref.: ITT (1975), p. 19.5; Fink (1982), p. 9.69; Andrushko (1981), p. 89;
noise parametric amplifiers and weak-signal mixers, in low- Brookner (1977), pp. 330, 384.
noise frequency multipliers and dividers, in microwave limit-
A positive-negative (PN) diode has a p-n junction, formed
ers, and for electronic tuning of semiconductor microwave
between two parts of a semiconductor that conduct holes and
generators.
electrons. Such a diode conducts current in one direction only
The limiting frequency for parametric diodes is 150-750
(see Fig. D39). The resistive loss is on the order of an ohm,
GHz. The usual operating mode is with a reverse bias and no
and the junction capacitance depends on the voltage. Such
conductance current, due to which the low noise level is
diodes are distinguished by rather high breakdown voltages
obtained.
and are the diodes most commonly used in radar equipment.
Parametric diodes may be Schottky-barrier diodes with
IAM
power dissipation on the order of tens of milliwatts or p-n
Ref.: Gorbachev (1968), p. 10; Jordan (1985), p. 18.10.
diodes with dissipation on the order of tenths of a watt. IAM
Ref.: Gassanov (1988), p. 81. i ( A)
m
A point-contact [-junction] diode has a metal-semiconduc- 1.0
tor junction in which a tungsten or phosphor-bronze electrode

is clamped against a semiconductor crystal. There are germa-
0.5
nium, silicon, and gallium arsenide point-contact diodes. The
desired voltage-current characteristic (see Fig. D38) is -10 -8
u (V)
obtained through selection of the point of contact and the 0 0.5 1.0
clamping force. The clamped contact results in a large varia-
tion of the junction parameters and mechanical instability,
making point-contact diodes sensitive to vibration and Figure D39 Typical voltage-current response for a PN diode.