Page 144 - Radar Technology Encyclopedia

134 diode, avalanche transit-time (ATT) diode, mixer

through the depleted n-region to the cathode and gives energy Detector diodes are often Schottky-barrier diodes (some-
to the applied microwave field during its negative half-period. times back-diodes). The current sensitivity for point diodes is
For certain dimensions of the drift region and the wave period 1 to 5 A/W. Schottky-barrier diodes have a current sensitivity
the microwave energy is amplified, and the resulting mode of of 3 to 10 A/W and a voltage sensitivity of 1 to 50 V/W in the
operation is referred to as “IMPact Avalanche and Transmit millimeter waveband, and 1,000 to 5,000 V/W in the centime-
Time,” (IMPATT). ter waveband. The construction of a detector diode depends
Another mode sometimes used in diodes is “TRapped on the frequency band. For waveguide and coaxial structures
Plasma and Avalanche Triggered Transit” (TRAPATT), in used in the decimeter and centimeter wavebands, diodes in a
which the region of impact ionization, moving along the socketed case are used, and in the millimeter waveband, the
diode, quickly fills the entire depleted region of the electron- diodes are in cases which are a type of waveguide insert. In
hole plasma. The impedance of the diode drops sharply, and a hybrid circuits, unhoused diodes are used. IAM
strong current impulse flows through it; the charge is then Ref.: Gassanov (1988), p. 76; Fink (1975), p. 9.61.
resorbed and the impedance of the diode increases.
A Gunn(-effect) diode has a section with negative impedance
Avalanche transit-time diodes are distinctive in having
at microwave frequencies and is a semiconductor crystal
high noise levels. They are used in medium- and high-power
without a p-n junction. Its operation is based on the Gunn
amplifiers and oscillators at frequencies up to 200 GHz for
effect. The conductor used, most often gallium arsenide, has
IMPATT diodes, and 10 GHz for TRAPATT diodes. IAM
two conductivity zones and does not possess rectification
Ref.: Tager (1968), p.17; Fink (1975), p. 8.38; Skolnik (1980), p. 217.
properties. It is used to generate and amplify microwave
In a back diode, the reverse current exceeds the forward cur- oscillations (see AMPLIFIER, Gunn diode).
rent due to a tunnel effect in the presence of a reverse bias on The operating parameters of a Gunn diode are deter-
18
the diode. The impurity concentration is high (about 10 / mined by the origination and migration of domains in accor-
3
cm ), but less than in a tunnel diode. The voltage/current dance with the diode’s operating mode. The different modes
characteristic is analogous to that of a tunnel diode, without are the transmit mode, in which the period of the generated
the negative impedance portion. Back diodes operate at oscillations is equal to the transit time of the domain, modes
higher frequencies than normal p-n diodes. in which the domains are suppressed or delayed, and a mode
Back diodes can be used as detector diodes, but due to with limited volume charge buildup, which is the most com-
low dielectric strength and difficulty of manufacture, they are mon mode. The operating frequency in this last mode is sev-
rarely used. IAM eral hundred gigahertz, with tuning over an octave band, with
Ref.: Fink (1982), p. 9.65; Zherebtsov (1989), p. 133. continuous-wave power on the order of several watts (at 15 -
20 per cent efficiency) and impulse power on the order of sev-
A charge storage diode has a nonuniform distribution of
eral kilowatts. IAM
acceptor impurity throughout the semiconductor, leading to
Ref.: Gassanov (1988), p. 186; Andrushko (1981), p. 120; Skolnik (1990),
an increased concentration of minority carriers near the
p. 5.11; Fink (1975), p. 9.70.
boundary of the junction when direct current is applied to the
diode. When the polarity of the applied voltage is suddenly IMPATT diode (see avalanche-transit-time diode).
reversed, there arises a short, strongly nonsinusoidal impulse
An integrated diode is the diode implemented on the basis of
of negative current, caused by the stored carriers. Such a
integrated circuit technology when any one of the semicon-
diode is used in frequency-multiplier circuits and is distin-
ductor junctions forming the monolithic circuit structure can
guished by having a high gain. IAM
be used as a diode. SAL
Ref.: ITT (1975), p. 19.5.
Ref.: Fink (1975), p. 8.37.
A converter diode is used in a receiver mixer (see mixer
A light-emitting diode (LED) is a diode using the effect in
diode) or transmitter modulator (see modulator diode).
which the p-n junction can emit visible light when biased into
These applications usually use the nonlinear capacitance of
the avalanche-breakdown region. They find the use in visual
the diode. IAM
displays. SAL
A detector diode is intended to detect (demodulate) signals, Ref.: Fink (1975), p. 7.35.
and is characterized by the following basic parameters: its
A mixer diode utilizes the property of nonlinear resistance.
current and/or voltage sensitivity to applied microwave
The fundamental parameter is the normalized noise factor,
power, which indicates it effectiveness in converting micro-
characterizing the noise properties of the receiver containing
wave oscillations into direct current; the normalized reverse
the mixer with the diode, and the noise ratio, which depends
voltage, at which the reverse current achieves its limiting
upon the noise factor (see detector diode). Other parameters
value; its maximum microwave power dissipation; and the
include the normalized reverse voltage and the maximum dis-
noise ratio, or relative noise temperature, equal to the ratio of
sipated power, along with the usual detector diode parame-
the diode’s noise power in the operating mode to the noise
ters.
power in a matched load.
Typical values for the normalized noise factor are 5 to 8
dB in the centimeter waveband and 10 to 12 dB in the milli-