10 Doping Processes for MEMS 757
a thermal runaway event, with large currents producing high temperatures, which
excite higher intrinsic carrier densities enabling even higher currents, and eventual
catastrophic results for the device.
Silicon single crystals have a diamond lattice structure illustrated in Fig. 10.2,
with an underlying face-centered cubic structure and tetrahedrally bonded Si
atoms. Many other technically important semiconductors have a similar diamond
or zincblende structure, such as Ge, GaAs, or β-SiC [3]. In order for impurity atoms
to function as electrically active dopants in the silicon structure, they must be capa-
ble of contributing a mobile electron or hole without acting as an electronic trap
or recombination center [4]. For Si crystals, viable doping impurities are primarily
found in either group III or group V of the periodic table (Fig. 10.3).
Group III atoms such as boron, aluminum, and gallium, when incorporated into
the silicon semiconductor lattice, form an empty localized electronic state in the
semiconductor bandgap located just above the valence band edge. Electrons in the
valence band are easily promoted into this state at moderate temperatures, resulting
in a nonlocalized, mobile hole state in the valence band. This hole then acts as a
free carrier in the semiconductor crystal, allowing it to support a current flow. These
group III dopants are known as electron acceptors, and a silicon crystal with an
excess of acceptor levels is defined as p-type.
Group V atoms such as phosphorus, antimony, and arsenic also form localized
electronic states in the silicon bandgap, but these impurities form filled states just
below the conduction band edge. At moderate temperatures, electrons occupying
these states are easily promoted into the conduction band of the semiconductor,
where they act as free electronic carriers. These dopants are known as electron
donors, and a crystal with an excess of donor levels is defined as n-type. It is possible
for a semiconductor crystal to be doped with both donor- and acceptor-type dopants
simultaneously. The donors and acceptors then effectively cancel each other out
(compensation), and the crystal type is essentially determined by which dopant type
predominates.
Fig. 10.2 Atomic
arrangement of a silicon
crystal illustrating the
diamond lattice structure