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Microwave Integrated Circuits
UNIT - I
Microwave Integrated Circuits
Microchip for Microwave frequencies.
 It can incorporate innumerable
components of different types, (passive
and active) into a small chip to form a
complete microwave subsystem.
 Size, weight and cost are reduced much.

Types of Microwave circuits
Discrete circuit:
 Packaged diodes/transistors mounted in coaxial and waveguide
assemblies.
 Devices can usually be removed from the assembly and
replaced.
Hybrid MIC:
 Diodes/transistors, resonators, capacitors, circulators etc., are
fabricated separately on most appropriate material and then
mounted into the microstrip circuit and connected with bond wires
MMIC :
 Diodes/transistors, resistors, capacitors, microstrip etc., are
fabricated simultaneously, including their interconnections, in
semiconductor chip
HMIC
Hybrid MICs have only one layer of
metallization
for
conductors
and
transmission lines and discrete components
like resistors, capacitors, diodes and
transistors, etc. are bonded to the substrate.
 Alumina, quartz and Teflon fiber are
commonly used substrates.
 Transmission line conductors for hybrid
MICs are typically copper or gold.

Hybrid Microwave Integrated Circuit (HMIC)
Photograph of one of the 25,344 hybrid integrated T/R modules used in
Raytheon’s Ground Based Radar system. This X-band module contains phase
shifters, amplifiers, switches, couplers, a ferrite circulator, and associated control
and bias circuitry.
Microstrip Circuit elements commonly used in HMIC
The components that can be fabricated as part of the
microstrip transmission line are:
Matching stubs and transformers
Directional couplers
Combiners and dividers
Resonators
Filters
Inductors and capacitors
Thin film resistors
Components Added After Micro strip
Fabrication
The MIC Components that are fabricated separately and
added to the micro strip circuits are:
Bond wire
Chip resistor
Chip capacitors
Dielectric resonators
Circulators
Diodes and transistors
Coupled line filter
Hybrid coupler
Branch line coupler
Microstrip coupler
Typical spiral inductor and interdigitated capacitor
Loop inductor
High impedance transmission line inductor
Figure: Microstrip elements used in HMIC
Bond wires
Dielectric resonator
Chip capacitor and resistor
MMIC


The substrate of an MMIC must be a
semiconductor material to accommodate the
fabrication of active devices and devices
consisting, several layers of metal, dielectric
and resistive films.
Potentially, the MMIC can be made at low cost
because the manual labour in the fabrication of
hybrid MICs is eliminated and that a single
wafer can contain a large number of circuits,
all of which can be processed and fabricated
simultaneously.
Monolithic Microwave Integrated Circuit (MMIC)
Photograph of a monolithic integrated X-band power amplifier. This circuit
uses eight heterojunction bipolar transistors with power dividers/combiners
at the input and output to produce 5 watts.
Courtesy : Internet
Advantages and Disadvantages of HMIC
Advantages:
1-Each
component can be designed for optimal
performance:
 Each transistor can be made of the best material.
 Other devices can be made of the most appropriate
material.
 The lowest loss microwave components can be made by
choosing the optimal micro strip substrate.
2- It has high power capability since the high power
generating elements can be optimally heat-sinked.
3- Standard diodes and transistors can be used and made to
perform different functions by using different circuit
design.
4- Special-purpose devices for each function are not
required.
5- Trimming adjustments are possible.
6- The most economical approach when small quantities, up
to several hundred, of the circuits are required.
Disadvantages:
1-Wire bonds cause reliability problems. Each circuit
element that is not part of the microstrip assembly must
be attached to the microstrip by a wire bond.
2-The number of devices that can be included is limited by
the economics of mounting the devices onto the circuit
and attaching them by a wire bonds. The circuit is usually
limited to a few dozen compartments.
Advantages and Disadvantages of MMICs
Advantages:
1- Minimal mismatches and minimal signal delay.
2- There are no wire bond reliability problems.
3- Up to thousands of devices can be fabricated at one time into a
single MMIC.
4- It is the least expensive approach when large quantities are to
be fabricated.
Disadvantages:
1- Performance compromised, since the optimal materials cannot
be used for each circuit element.
2- Power capability is lower because good heat transfer materials
cannot be used
3- Trimming adjustments are difficult or impossible.
4- Unfavorable device-to-chip area ratio in the semiconductor
material.
5- Tooling is prohibitively expensive for small quantities of MMIC.
Materials used for MIC

Substrate materials
 sapphire, alumina, ferrite/garnet, silicon, RT/duroid, quartz, GaAs,
Inp, etc.,

Conductor materials
 copper, gold, silver, aluminum, etc.

Dielectric films
 SiO, SiO2,…etc

Resistive films
 Nichrome (cNiCr), tantalum (Ta)
Substrate Choice for HMIC
1. The cost of the substrate must be justifiable for the application
2. Is the technology to be thin- or thick film?
3- The choice of thickness and permittivity determines the achievable
impedance range and the usable frequency range.
4- There should be low loss tangent for negligible dielectric loss
5- The substrate surface finish should be good (~ 0.1 mm), with relative
freedom from voids, to keep conductor loss low and yet maintain good
metal-film adhesion
6- There should be good mechanical strength and thermal conductivity.
7- No deformation should be occur during processing of circuit
8- A substrates with sufficient size are for the particular application and
complexity should be available.
Commonly used substrate materials
1. Organic PCBs (Printed Circuit Boards)

FR4
1) Low cost, rigid structure, and multi-layer capability.
2) Applications for operation frequency below a few GHz.
fop  Loss 

RT/Duroid
1) Low loss and good for RF applications.
2) Board has a wide selected range for permittivity. e.g. RT/Duroid 5870 with r
=2.33, RT/Duroid 5880 with r =2.2, and RT/Duroid 6010 with r =10.2.
3) Board is soft leading to less precise dimensional control.
2. Plastic substrate
1) This is suitable for experimental circuits operating below a few GHz and
array antennas operating up to and beyond 20 GHz.
3. Alumina
1) Good for operation frequency up to 40 GHz.
2) Metallic patterns can be implemented on ceramic substrate using thin-film or
thick-film technology.
3) Passive components of extremely small volume can be implemented because the
ceramic substrate can be stacked in many tens of layers or more, e.g. low
temperature co-fired ceramic (LTCC).
4) Good thermal conductivity.
5) Alumina purity below 85% should result in high conductor and dielectric losses
and poor reproducibility.
4. Quartz
1) Production circuits for millimetric wave applications from tens of GHz up to
perhaps 300 GHz, and suitable for use in finline and image line MIC structures.
2) Lower permittivity of property allows larger distributed circuit elements to be
incorporated.
5. Sapphire

The most expensive substrate with following advantages:
1) Transparent feature is useful for accurately registering chip
devices.
2) Fairly high permittivity (r =10.1~10.3), reproducible ( all pieces
are essentially identical in dielectric properties), and thermal
conductivity (about 30% higher than the best alumina).
3) Low power loss.
 Disadvantages:
1) Relatively high cost.
2) Substrate area is limited (usually little more than 25 mm
square).
3) Dielectric anisotropy poses some additional circuit design
problems.
6. Beryllia (BeO) and Aluminium Nitride (AlN):
 Ceramic substrate.
 Excellent thermal conductivity – high power applications.
 Dangerous to handle – Its dust is toxic and must not be
machined.
7. GaAs:
 Suitable for MMICs. Lownoise MESFET, Power MESFET,
Schottky diodes are fabricated on GaAs.
Conductor Materials
Properties:
1. High conductivity
2. High coefficient of thermal expansion
3. Low resistance at RF/microwaves
4. Good adhesion to the substrate
5. Good etch ability and solder ability
6. Easy to deposit or electroplate
Example:
HMIC: Cr/Au, Pd/Au, Ta/Au
MMIC: Cr/Au, Ti/Pd/Au, Ti/Pt/Au
Properties of Conductors :
Dielectric Films
Reproducibility
2. High breakdown voltage
3. Low loss tangent
4. Ability to Process without developing
pinholes.
1.



Capacitors
Protective layers for active devices
Insulating layers for passive circuits
Sio2 Vs GaAs
Properties of Dielectrics :
Resistive Films
1.
2.
3.
Good stability
Low Temperature Coefficient of Resistance
(TCR)
Sheet resistivity (10-2000 /square)
Terminations
Attenuators
Bias Networks
Examples:
Cr, NiCr, Ta, Cr-Sio, Ti
Properties of Resistive Films :
Properties of Various Manufacturing Technology