Diodes and Transistors

A device that blocks current in one direction while letting
current flow in another direction is called a diode. Diodes
can be used in a number of ways. For example, a device that
uses batteries often contains a diode that protects the
device if you insert the batteries backward. The diode simply
blocks any current from leaving the battery if it is
reversed -- this protects the sensitive electronics in the
device.

A semiconductor diode's behavior is not perfect, as shown in
this graph.

When reverse-biased, an ideal diode would block all current.
A real diode lets perhaps 10 microamps through -- not a lot,
but still not perfect. And if you apply enough reverse
voltage (V), the junction breaks down and lets current
through. Usually, the breakdown voltage is a lot more voltage
than the circuit will ever see, so it is irrelevant.

When forward-biased, there is a small amount of voltage
necessary to get the diode going. In silicon, this voltage is
about 0.7 volts. This voltage is needed to start the
hole-electron combination process at the junction.

Another monumental technology that's related to the diode is
the transistor. Transistors and diodes have a lot in common.

Transistors

A transistor is created by using three layers rather than the
two layers used in a diode. You can create either an NPN or
a PNP sandwich. A transistor can act as a switch or
an amplifier.

A transistor looks like two diodes back-to-back. You'd
imagine that no current could flow through a transistor
because back-to-back diodes would block current both ways.
And this is true. However, when you apply a small current to
the center layer of the sandwich, a much larger current can
flow through the sandwich as a whole. This gives a transistor
its switching behavior. A small current can turn a larger
current on and off.

A silicon chip is a piece of silicon that can hold thousands
of transistors. With transistors acting as switches, you can
create Boolean gates, and with Boolean gates you can create
microprocessor chips.

The natural progression from silicon to doped silicon to
transistors to chips is what has made microprocessors and
other electronic devices so inexpensive and ubiquitous in
today's society. The fundamental principles are surprisingly
simple. The miracle is the constant refinement of those
principles to the point where, today, tens of millions of
transistors can be inexpensively formed onto a single chip.