Intrinsic Semiconductors:
Semiconductor Material has a resistance between that of a conductor and
that of an insulator. In the Intrinsic semiconductor the amount of charge carriers is characteristic
of the material itself. Thermal excitation causes electrons to be raised
from the valance band to the conduction band. Note that
this is different than the case of a metal conductor, which always has electrons
in the conduction band. The intrinsic semiconductor has an equal number of
electrons and holes. When an electron in an intrinsic semiconductor jumps
into the conduction band a hole is formed in the valance band. An
increase in temperature causes an increase in conductivity of the semiconductor
by exciting more electrons into the conduction band and thus creating more
holes. Holes and electrons are the charge carriers in a
semiconductor. The positive holes move in the direction of the E field
and the negative electrons flow in the opposite direction of the E field.
Impurity Semiconductors:
Silicon and Germanium semiconductors can have impurities added to make them
more conductive. N-type material is used to dope the Silicon crystal to
increase its conductivity by increasing the availability of free electrons.
Adding P-type material decreases the availability of electrons and increases availability
of holes. The surface created by bonding P-type and N-type together is
called a PN junction. A small contact potential (.3 volts for Germanium
and .6 volts for Silicon) keeps the holes and the electrons separated and
controls the conduction of the PN junction. When a voltage across the PN
junction forces the electrons and the holes to move towards the junction, than conduction
occurs. Conduction consists of electrons and holes moving
towards the junction, where electrons fill the holes. When the polarity of
the voltage across junction is reversed, than no current flows, but electrons and holes
are pulled away from each other a very short distance. This small separation
of charge at the junction is referred to as the depletion zone. When junction
is reversed biased no current flows, When the depletion zone area increases,
a corresponding decrease in the capacitance of PN junction occurs. Thus, a reversed biased PN junction
can be used as a variable capacitor. The ability of a PN junction to conduct only in one direction makes a PN junction a rectifier.
Semiconductor Diodes:
A semiconductor diode does not have to have a PN
junction. A diode is simply a semiconductor with two leads attached.
The PN junction diode can be simple rectifier, Zener Diode, Varactor Diode, or
other types of junction diode. Non Junction diodes are manufactured for
there intrinsic semiconductor bulk properties, such as sensitivity to temperature
or light.
Semiconductor Diode Rectifier circuit:
The schematic of a full wave rectifier is at top
of the page. The rectifier circuit output is displayed on the oscilloscope below it. The filter
capacitor is varied from one to a thousand micro-farads, and the ripple voltage
varies inversely to rise in capacitance. When the capacitance is one
micro-farad the output waveform appears as an unfiltered rectified positive DC
voltage. Note that the rectified output voltage has two positive peaks per
cycle of AC input. One peak of what is called pulsating DC occurs at
the positive peak of the AC input, and the other peak of the pulsating DC occurs
at the negative peak of the AC input voltage. Current flows through CR1
for the first half cycle of the input voltage and through CR2 during the last
half of the input cycle. The rectifier circuit is called a Full Wave
rectifier, because current flows during both the positive and negative cycles of
the input waveform.
