
259Mechanical Nanosensors
Piezoresistive sensing is realized by detecting the resistance changes through
a piezoresistive NEMS device upon actuation (Ekinci 2005). There are, how-
ever, some obstacles in this method, e.g., in a P-type Si doubly clamped
beam, an optimistic estimate for the resistance change ΔR, as a function of
the displacement, the beam length l, and the resistance R, is ΔR/R ∼ x
2
/l
2
,
indicating that ΔR
max
/R ∼ 1/100. In view of the already high resistance of a
doped semiconductor beam approaching R ∼ 10 kΩ, such a small resistance
change is likely to be obscured at high resonance frequencies by the inescap-
ably a ...