Tags: CMOS, resistor, silicon

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Resistor parameters:

  • square resistance Rsquare

  • tolerance

  • temperature coefficient (remark: voltage divider is independent of temperature)

  • voltage coefficient

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The resistance of any material depends on the its resistivity and its dimensions. The resistance is given by the equation:

resistance equations

where:

  • ρ – resistivity

  • t – thickness

  • L – length

  • W – width

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In a semiconductor process, thickness of any material is fixed (for example, the depth of n-well). A designer has control over length L and width W of a chosen material.

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resistance of a corner = 0.6 x Rsquare

In order to avoid corners in serpentine pattern:

http://images.slideplayer.com/13/4027754/slides/slide_77.jpg

lay out resistors as some unit resistors connected by metal layer:

http://image.slidesharecdn.com/bdwcpb9auzrthvhfnpdniexpires1260163132awsaccesskeyid1z5t9h8pq39v6f79v8g2-12601640670973-phpapp01/95/analog-layout-and-process-concern-71-728.jpg?cb=1260142992

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Types of resistors

List of resistors:

  • p+ poly

    • parameters:

      • Rsquare: 5-80 Ohms/square

      • tolerance: +/- 15%

      • temperature coefficient TCR1: -500 – -1500 ppm/oC

      • voltage coefficient VCR1: 300 ppm/V

    • usually small parasitic capacitance – usage in high speed amplifiers

    • can be placed on n-well to improve noise immunity

    • high (unsilicided) poly can be used for higher Rsquare values (550-2000 Ohms/square)

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  • n-well

    • parameters:

      • Rsquare: 1k-5k Ohms/square

      • tolerance: +/- 20%

      • temperature coefficient TCR1: ~4000 ppm/oC

      • voltage coefficient VCR1: LARGE

    • voltage on either side of resistor must be high enough in order not to turn p-substrate – n-well diode

    • minimum size: set by quality of patterning the resist

    • minimum spacing: set by the parasitic npn (horizontal) transistor

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  • source/drain diffusion

    • parameters:

      • Rsquare: 20-80 Ohms/square

      • tolerance: +/- 15%

      • temperature coefficient TCR1: -500 – -1500 ppm/oC

      • voltage coefficient VCR1: to be defined

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  • MOS

    • transistor biased in triode region

    • large resistance (Mega Ohms) obtained by small transistor

      • resistance is nonlinear, thus such resistors are rather restricted to small swing circuits or DC circuits

    • parasitic capacitance: large (gate capacitance)