Chapter Five

Introduction to Active Microwave Devices

and Networks

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 Microwave Semiconductor Diodes
Small Signal Diodes
Diodes used for signal detection and mixing are the most
common microwave semiconductor devices.
Because of the relatively large surface area of the junction,
typical semiconductor diodes exhibit a high capacitance
which prevents normal operation at microwave frequencies.
Two types of widely used microwave diodes are:
 Point-contact diode
 Schottky barrier or hot-carrier diode
 
 
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Small Signal Diodes: Point-Contact Diode
The oldest microwave semiconductor device is the
point-contact diode, also called a crystal diode.
A point-contact diode is a piece of semiconductor
material and a fine wire that makes contact with the
semiconductor material.
Point-contact diodes are ideal for small-signal
applications. They are extremely delicate and cannot
withstand high power and are easily damaged.
They are widely used in microwave mixers and
detectors and in microwave power measurement
equipment.

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4
Parameter Germanium Silicon Comments
     
Depletion layer p.d. 0.15V 0.6V Germanium can be
useful for low voltage
applications.
Forward current A few milli-Amperes Tens of Amperes Silicon much better for
high current
applications.
Reverse leakage A few micro-amperes A few nano-amperes Germanium 1000 times
current more leaky than silicon.
Max. reverse voltage Volts Hundreds of volts Silicon the only real
choice for high voltage
applications
Temperature stability Poor Good Germanium more sensitive
to temperature. Can be a
problem or can be useful.

Junction capacitance Very low (point Comparatively high This is a useful feature
contact)   for high frequency use.
Note: low capacitance
silicon diodes are also
available but their
capacitance is still
higher than point
contact type. 5
Small Signal Diodes: Hot Carrier Diodes
For the most part, point-contact diodes have been
replaced by Schottky diodes, sometimes referred to as
hot carrier diodes.
Like the point-contact diode, the Schottky diode is
extremely small and has a tiny junction capacitance.
Schottky diodes are widely used in balanced
modulators and mixers.
They are also used as fast switches at microwave frequencies.
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Small Signal Diodes: Hot Carrier Diodes (Schottky
Diode)
Increasing the doping level of the semiconductor, the width
of the depletion region drops.
Low voltage drop and a very fast switching action.

0.15 – 0.45V voltage drop – this can provide higher

switching speed and better system efficiency.
Schottky diodes do not have recovery time, as there is
nothing to recover from lower forward resistance and lower
noise generation.
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 Frequency-Multiplier Diodes: Varactor Diodes
A varactor diode is basically a voltage variable
capacitor.
When a reverse bias is applied to the diode, it acts
like a capacitor.
A varactor is primarily used in microwave
circuits as a frequency multiplier.
Varactors are used in applications in which it is
difficult to generate microwave signals.
Varactor diodes are available for producing
relatively high power outputs at frequencies up to
100 GHz.
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Frequency-Multiplier Diodes: Step-Recovery Diodes
A step-recovery diode or snap-off varactor is widely
used in microwave frequency-multiplier circuits.
A step-recovery diode is a PN-junction diode made
with gallium arsenide or silicon.
When it is forward-biased, it conducts as any diode,
but a charge is stored in the depletion layer.
When reverse bias is applied, the charge keeps the
diode on momentarily and then turns off abruptly.
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 This snap-off produces a high intensity reverse-current
pulse that is rich in harmonics and with relatively high
amplitude.
 SRDs can also be used circuits like that of the varactor
diode tp produce multipliers with power ratings up to 5
and 10W.
 Power ratings of 50W can be obtained.

 Operating frequencies up to 100 GHz are possible with

an efficiency of 80 percent or better
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Oscillator Diodes

Three types of diodes other than the tunnel diode

that can oscillate due to negative resistance
characteristics are:
Gunn diode

IMPATT diode

TRAPATT diode

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Oscillator Diodes: Gunn Diodes
Gunn diodes, also called transferred-electron
devices (TEDs), are not diodes in the usual sense
because they do not have junctions.
A Gunn diode is a thin piece of N-type gallium
arsenide (GaAs) or indium phosphide (InP)
semiconductor which forms a special resistor when
voltage is applied to it.
The Gunn diode exhibits a negative-resistance
characteristic.
Gunn diodes oscillate at frequencies up to 150 GHz.

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 When it is so biased, the time it takes for the electrons
to flow across the material is such that the current is
180 degrees out of phase with the applied voltage.
 If the Gunn diode so biased is connected to a cavity
near the frequency determined by the electron transit
time, the resulting combination will oscillate.
 It is primarily used then as a microwave oscillator.

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Oscillator Diodes: IMPATT and TRAPATT
Diodes
Two microwave diodes widely used as oscillators are
the IMPATT (IMPact Avalanche Transit Time) and
TRAPATT (TRApped Plasma Avalanche Triggered
Transit) diodes.
Both are PN-junction diodes made of silicon, GaAs,
or InP.
They are designed to operate with a high reverse bias
that causes them to avalanche or break down.
IMPATT diodes are available with power ratings up
to 25 W to frequencies as high as 300 GHz.
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 IMPATT are preferred over Gunn diodes if higher
power is required.

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 An IMPATT diode is reverse biased above the
breakdown voltage.
 Applied voltage causes momentary breakdown once
per cycle
 This starts a pulse of current moving through the
device
 Frequency depends on device thickness

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 Pin Diodes:

 A PIN diode is a special PN-junction diode with an

I (intrinsic) layer between the P and the N sections.
 The P and N layers are usually silicon, although
GaAs is sometimes used and the I layer is a very
lightly doped N-type semiconductor.
 PIN diodes are used as switches in microwave
circuits.

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 Microwave Transistors
The GaAs MESFET, a type of JFET using a Schottky
barrier junction, can operate at frequencies above 5
GHz.
A high electron mobility transistor (HEMT) is a
variant of the MESFET and extends the range beyond
20 GHz by adding an extra layer of semiconductor
material such as AlGaAs.
A popular device known as a heterojunction bipolar
transistor (HBT) is making even higher-frequency
amplification possible in discrete form and in integrated
circuits.

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Small-Signal Amplifiers
A small-signal microwave amplifier can be made up of a
single transistor or multiple transistors combined with a
biasing circuit and any microstrip circuits or components
as required.
Most microwave amplifiers are of the tuned variety.

Another type of small-signal microwave amplifier is a

multistage integrated circuit, a variety of MMIC.
 
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 Small-Signal Amplifiers: Transistor
Amplifiers
A low-noise transistor with a gain of about 10 to 25
dB is typically used as a microwave amplifier.
Most microwave amplifiers are designed to have input
and output impedances of 50 Ω.
The transistor is biased into the linear region for class
A operation.
RFCs are used in the supply leads to keep the RF out
of the supply and to prevent feedback paths that can
cause oscillation and instability in multistage circuits.

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 Ferrite beads (FB) are used in the collector supply
lead for further decoupling.
A single-stage class A RF microwave
amplifier

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From the figure:

TLs are microstrip sections that act as tuned

circuits, inductors or capacitors
TL1 acts as an impedance matching section
TL2 and TL3 form a tuned circuit; set the
bandwidth of the amplifier input
TL4 – another impedance matching section
C2 and C3 are variable capacitors that allow
some tuning of the bandwidth.
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 TL9 and TL10 – are inductors, forming part of
the substantial decoupling networks used on
the base bias supply Vbb and Vcc.
 Vbb and R1 – set the base bias (class A
amplifier).

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Small-Signal Amplifiers: MMIC Amplifiers
A common monolithic microwave integrated
circuit (MMIC) amplifier is one that
incorporates two or more stages of FET or
bipolar transistors made on a common chip to
form a multistage amplifier.
The chip also incorporates resistors for biasing
and small bypass capacitors.
Physically, these devices look like transistors.

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 Another form of MMIC is the hybrid circuit,
which combines an amplifier IC connected to
microstrip circuits and discrete components.

Small-Signal Amplifiers: Power Amplifiers

 A typical class A microwave power amplifier
is designed with microstrip lines used for
impedance matching and tuning.
 Input and output impedances are 50 Ω.

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 Typical power-supply voltages are 12, 24, and
28 volts.
 Most power amplifiers obtain their bias from
constant-current sources.
 A single-stage FET power amplifier can
achieve a power output of 100 W in the high
UHF and low microwave region.
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Class A Microwave Power Amplifier

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 From the figure:
Microstrip lines are used for impedance
matching and tuning
L-type matching networks with a low-pass
configuration
Small wire loop inductors and capacitors are
used to form the decoupling networks to prevent
feedback through the power supply which would
cause oscillation
L and C - keep the microwave energy out of the
bias circuit
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A constant-current bias supply for a linear
power amplifier.

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From the figure:
R1, R2, and R3 – voltage divider to set the base
voltage on Q1
R4 – the voltage drop at R4 divided by R4 gives the
value of the current supplied by Q1 to the transistor in
the microwave amplifier
Diode – provides some temperature compensation for
variations in the emitter-base voltage that occur in Q1
R1 – is adjustable to set bias current to the precise
value for optimum power and minimum distortion.

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An FET Power Amplifier

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 Can achieve power output of 100W in the high UHF
and low microwave region
 FET does not conduct with drain voltage applied (for
enhancement mode)
 Zener – supplies the gate bias

 Potentiometer R2 – adjusts the gate voltage. Allows

the bias to set for linear class B or C operation

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 LC tuned circuits and microstrip – used for
tuning and impedance matching
 L1 – tiny hairpin loop of heavy wire

 L2 – larger inductor made of multiple turns of

wire to form a high impedance for decoupling

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Thank You!!!

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