Passives
The great benefits of designing chips using Silicon-on-Sapphire technology are the high quality passive components (inductors, capacitors, resistors) that can be integrated.
This makes it ideal for mixed signal applications - especially RF circuitry.
Inductors
In RF integrated circuits, inductors are required for:
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Impedance matching
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Resonant circuits (VCO) and filters
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Transformers, baluns
Inductors are therefore a critical limiting component in highly integrated RFICs.
High Q inductors are very beneficial because they have:
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Lower losses in matching networks
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Better frequency selectivity
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Lower VCO phase noise
The following is the equivalent circuit of an inductor on a semi-insulating substrate:
- Q = |Im[Y11]/Re[Y11] = wLs/(Rs+w2RpCp2[(Rs2+w2Ls2)/(1+w2Rp2Cp2)]
- The effect of substrate resistance (Rp) is very high
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Q of 3-4 in bulk Si technology is common
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If Rp is very large (UTSi case), Q is totally dominated by Rs and Q is simply = wLs/ Rs.
- In SoS, inductor designs focus on reduction of Rs only.
- In SoS, QL > 40 (e.g. at 2 GHz for 5 nH inductor) is very achieveable.
The following graph compares inductors on Bulk CMOS and SoS process technologies.
