Tab Electronics Guide to Understanding Electricity ... - Sciences Club

Field-Effect Transistors
307
Static Electricity: An Unseen Danger
The introduction of MOS (metal oxide semiconductor) devices brought
on a whole new era in the electronic world. Today, MOS technology has
been incorporated into discrete and integrated components, allowing
lower power consumption, improved circuit design and operation, higher
component densities, and more sophisticated operation. Unfortunately, a
major problem exists with all MOS devices. They are very susceptible to
destruction by static electricity.
Inadvertent static electricity is usually caused by friction. Under the
proper conditions, friction can force electrons to build up on nonconductive
surfaces, creating a charge. When a charged substance is brought in
contact with a conductive substance of lesser charge, the charged substance
will discharge to the other conductor until the potentials are equal.
Everyone is “jolted” by static electricity from time to time. Static electrical
charges can be built up on the human body by changing clothes, walking
over certain types of carpeting, sliding across a car seat, or even friction
from moving air. The actual potential of typical static charges is surprising.
A static charge of sufficient potential to provide a small “zap” on touching
a conductive object is probably in the range of 2000 to 4000 volts!
Most MOS devices can be destroyed by static discharges as low as 50
volts. The static discharge punctures the oxide insulator (which is almost
indescribably thin) and forms a tiny carbon arc path through it. This
renders the MOS device useless.
The point is that whenever you work with any type of MOS device,
your body and tools must be free of static charges. There are many good methods
available to do this. The most common is a “grounding strap,” made
from conductive plastic, that might be worn around the wrist or ankle
and attached to a grounded object. Soldering irons should have a grounded
tip, and special “antistatic” desoldering tools are available. Conductive
work mats are also advisable. MOS devices must be stored in specially
manufactured small parts cabinets, antistatic bags, and conductive foam.
NOTE Do not try to make your own grounding straps out of common wire or
conductive cable of any type!
This is very dangerous. It is like working on electrical equipment
while standing in water. Specially designed grounding straps, for the
removal of static charges, are made from conductive plastic exhibiting