5-6. Diffusion & Ion Implantation

4.7.1 Sheet-Resistance Definition
L 1
R = r r =
A s
s = q( m n
n + m p)
p
Figure 4.13
Resistance of a block of material having uniform resistivity. A uniform current distribution is
entering the material perpendicular to the end of the block. The ratio of resistivity to thickness is
called the sheet resistance of the material.
o Using W and t as the width and the thickness of the sample:
R = ( ρ /t )(L /W ) = R s (L /W ).
- R s = ( ρ /t ) : sheet resistance of the layer of material.
- Given R s , a circuit designer need to specify only L and W of the reisistor to define
its value.
- The unit for R s is the ohm, since the ratio L/W is unitless. To avoid confusion
between R and R s , R s is given the special descriptive unit of ohms per square.
- The ratio L/W can be interpreted as the # of unit squares in the resistor.
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