Semiconductor atoms have four valence electrons. Both germanium (Ge) and silicon (Si) are examples of semiconductor materials.
A pure semiconductor material with only one type of atom is called an intrinsic semiconductor. An intrinsic semiconductor is neither a good conductor nor a good insulator.
An extrinsic semiconductor is a semiconductor with impurity atoms added to it through a process known as doping. Doping increases the conductivity of a semiconductor material.
n-Type semiconductors have many free electrons as a result of adding pentavalent impurity atoms during the doping process. A p-type semiconductor has many holes or vacancies in its covalent bond structure as a result of adding trivalent impurity atoms during the doping process. A hole exhibits a positive charge.
A diode is a unidirectional device that allows current to flow through it in only one direction.
A diode is forward-biased by making its anode positive relative to its cathode. A diode is reverse-biased by making its anode negative relative to its cathode.
A forward-biased diode has relatively low resistance, whereas a reverse-biased diode has very high resistance.
When testing a silicon diode with an analog ohmmeter, the ratio of reverse resistance, RR, to forward resistance, RF, should be at least 1000:1.
The first approximation treats a forward-biased diode like a closed switch with a voltage drop of 0 V. The second approximation includes the barrier potential, VB, which is 0.7 V for a silicon diode.
Both the first and second approximations of a diode treat a reverse-biased diode like an open switch with zero current.
The third approximation of a diode includes both VB and the bulk resistance when forward-biased. The bulk resistance, rB, of a diode is the resistance of the p and n materials.
The third approximation of a reverse-biased diode takes into account the reverse resistance, RR.
Half-wave, full-wave, and bridge rectifier circuits convert an ac voltage into a pulsating dc voltage.
For an unfiltered half-wave rectifier, the dc output voltage is 0.318 x Vout(pk). For an unfiltered full-wave rectifier, the dc output voltage is 0.636 x Vout(pk).
When a filter capacitor is connected to the output of a half-wave or full-wave rectifier, the dc output voltage is approximately equal to the peak output voltage from the rectifier.
The ripple frequency at the output of a half-wave rectifier is the same as the frequency of the ac input voltage. The ripple frequency of a full-wave rectifier at the output is twice the frequency of the ac input voltage.
A diode that emits light when forward-biased is called a light-emitting diode (LED). LEDs are doped with elements such as gallium, arsenic, and phosphorus because these elements emit different colors of light such as yellow, red, green, and orange.
The voltage drop across a forward-biased LED ranges from about 1.5 to 2.5 V. When making calculations, an approximate voltage of 2.0 V can be assumed for a forward-biased LED. The breakdown voltage rating of an LED is typically 3 to 15 V.
A zener diode is a special diode designed for operation in the breakdown region. The most common application of a zener diode is voltage regulation.
