Page 146 - Radar Technology Encyclopedia

136 diode, pulse diode, varactor

A pulse diode is designed for use in pulse circuits, including diodes with sharp p-n junctions can reach 300 to 400 for sili-
pulse amplifiers and pulse generators. Pulse diodes can be con diodes, and 500 to 800 for gallium arsenide diodes, at a
PIN diodes, Schottky-barrier diodes, tunnel diodes, and also frequency of 1 GHz and a reverse voltage of 4 to 6 volts. The
PN diodes of various types. IAM Q-factor for diodes with extremely sharp junctions is several
Ref.: Erofeev (1989), p. 99. times smaller, limited to about 300, due to the large resistance
of the low-alloy n-base. IAM
A Schottky(-barrier) diode is based on a metal-semiconduc-
tor contact formed through the vacuum deposition of metal on Ref.: Gassanov (1988), p. 83.
the semiconductor (a Schottky junction). Most often, n-type A tunnel diode exploits the tunnel effect in thin, strongly
gallium arsenide is used, with silicon being used more rarely. alloyed junctions. They exhibit a negative active conductivity
Due to the thinness of the n-layer formed by the metal junc- (see Fig. D41) over a wide range of frequencies.
tion, the slope of the voltage-current curve (see Fig. D40) and Tunnel diodes are made from germanium (Ge), silicon
the dielectric strength are both higher than in point-contact (Si), gallium arsenide (GaAs), and gallium antimonide
diodes. The diode’s parameters exhibit low variance and high (GaSb). The parameters of the voltage-current curve for a tun-
stability. Schottky-barrier diodes have almost no injection or nel diode is determined by the materials used, as summarized
accumulation of minority carriers and therefore operate at in Table D7.
higher frequencies and with less noise than PN diodes. Among the parameters of a tunnel diode that are impor-
Schottky-barrier diodes are used as detector and mixer tant in its application in amplifiers and oscillators are the res-
diodes at frequencies up to 300 GHz. The noise figure at 170 onant and cutoff frequencies. Above the cutoff frequency the
GHz is between 4.8 and 5.5 dB. These diodes are also used in resistance of the diode becomes positive, and amplification
frequency multipliers and converters and as low-power, fast- and signal generation become impossible. At the resonant fre-
acting switching diodes. IAM quency, the reactive component returns to 0; this is the most
Ref.: Andrushko (1981), p. 90; Gassanov (1988), p. 75; Fink (1975), dangerous frequency in terms of exciting the amplifier. Tun-
pp. 8.38, 9.62. nel diodes may or may not be encased. IAM
i ( A) Ref.: Rudenko (1971), pp. 92–100; Fink (1975), pp. 8.38, 9.70.
m
1.0 I

0.5
I
p
-10 -8
u (V)
0 0.5 1.0
I
m
-0.5 V
V V V
1 2 3
Figure D40 Voltage-current curve for a GaAs Schottky-
barrier diode.
A small-signal diode is a diode operating in the small-signal
Figure D41 A typical voltage-current curve for a tunnel diode.
mode as a switch, demodulator, limiter, nonlinear resistor,
I = peak current, I = minimum current (after Rudenko, 1971,
m
p
and so forth. SAL
Fig. 4.1, p. 93).
Ref.: Fink (1975), p. 7.34.
A switching diode is used in microwave circuits in those Table D7
cases where small size, fast switching speed, and low control- Characteristics of Tunnel Diodes
ling power are required. Switching diodes may be PN, PIN or Material V , mV V , mV V , mV
1
2
3
Schottky-barrier diodes. IAM
Ge 40-70 270-350 400
Ref.: Gorbachev (1968), p. 6.
TRAPATT diode (see ATT diode). Si 80-100 400-500 700
GaAs 90-120 0-600 1000
A tuning diode is a variant of the varactor diode used for
electronic tuning of microwave filter circuits, amplifiers and GaSb 30-50 200-250 450
oscillators, and also in continuous phase shifters.
A varactor diode (or variable-reactance diode) is a diode that
The basic parameters of a tuning diode include the capac-
uses the change in capacitance of a reverse-biased PN junc-
itance ratio K = C max /C min , its Q-factor, the capacitance for a
c
given bias, the maximum reverse voltage, and the maximum tion as a function of applied voltage. Typical applications of
varactor diodes are harmonic generation, parametric amplifi-
microwave power.
cation, and electronic tuning. SAL
The highest values for K (10 to 15) are obtained in
c
diodes with an extremely sharp PN junction. The Q-factor for Ref.: Fink (1975), p. 9.60.