10-11-2012, 04:11 PM
Microwave Integrated Circuits
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What are Microwave Integrated Circuits?
•Microwave integrated circuits, or MICs, are a type of microchip
semiconductor designed to operate specifically over microwave frequencies,
often at 1 GHz or higher.
•The specific ability to transmit over microwaves is usually what differentiates
MICs from other types of integrated circuits.
•MICs are widely-used in small electronic devices that function via electronic
and electromagnetic frequencies, such as cell phones, GPS devices, remotecontrolled
systems, and imaging devices.
•Their small size allows them to be used in multiple handheld, wireless
devices, and the nature of the MIC is such that one chip can operate as a selfcontained
device, using only a single semiconductor wafer.
•MICs were first designed in the 1940s, and evolved from the basic microwave
circuit as demand and technologies of the time enabled advances in waferbased
circuit fabrication.
•Since then, microwave integrated circuits have matured from simple, singlefunction
circuits to complex, multi-function circuits of increasingly small sizes
and complex capabilities.
•They form the root of the microwave and semiconductor manufacturing
industries, and many types of MICs can be cheaply and efficiently massproduced
for use in consumer electronics, science, and industry.
Monolithic Microwave Integrated Circuit (MMIC)
A Monolithic Microwave Integrated Circuit, or MMIC (sometimes pronounced
"mimic"), is a type of integrated circuit (IC) device that operates at microwave
frequencies (300 MHz to 300 GHz).
These devices typically perform functions such as microwave mixing, power
amplification, low noise amplification, and high frequency switching. Inputs and
outputs on MMIC devices are frequently matched to a characteristic impedance of 50
ohms.
This makes them easier to use, as cascading of MMICs does not then require an
external matching network. Additionally most microwave test equipment is designed
to operate in a 50 ohm environment.
MMICs are dimensionally small (from around 1 mm² to 10 mm²) and can be mass
produced, which has allowed the proliferation of high frequency devices such as
cellular phones.
MMICs were originally fabricated using gallium arsenide (GaAs), a III-V compound
semiconductor. It has two fundamental advantages over Silicon (Si), the traditional
material for IC realisation: device (transistor) speed and a semi-insulating substrate.
Both factors help with the design of high frequency circuit functions. However, the
speed of Si-based technologies has gradually increased as transistor feature sizes
have reduced and MMICs can now also be fabricated in Si technology. The primary
advantage of Si technology is its lower fabrication cost compared with GaAs. Silicon
wafer diameters are larger (typically 8" or 12" compared with 4" or 6" for GaAs) and
the wafer costs are lower, contributing to a less expensive IC.
Substrate for MICs
• The first step in the design of any MIC is the choice of substrate.
• Selection of the substrate material depends on three major factors-
– Type of fabrication technology, i.e., hybrid or monolithic
– Type of transmission line used
– The frequency band of operation
• Other factors includes the desired electrical and mechanical properties and
cost.
• MMICs use essentially semiconductor substrates.
• Hybrid MICs use dielectric substrates which are generally isotropic although
anisotropic dielectrics are also used for certain special applications.
• Isotropy comes from the Greek iso, for equal (as in isolateral triangles) and
tropos, for direction. Isotropy means that a particular material property is
equal in all directions (X, Y and Z).
• Anisotropy means that the material property might vary depending on
direction. Anisotropy can be caused within Angstrom dimensions (in a crystal
lattice which is not uniform in all directions, for example), or at micron
dimensions (where material grain boundaries are oriented in a particular
direction), or at millimeter dimensions (when materials are created in a
sandwich format, such as glass cloth reinforced substrates).