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A semiconductor is a device can act as either a conductor or resistor / insulator depending upon certain conditions. The most common semiconductor material is silicon, but there are other materials used depending upon the characteristics required. The semiconductor allows a circuit to act differently depending upon different conditions and is essential for all but the most basic electronic circuits. An integrated circuit provides a means of encapsulating multiple devices into a single package (in some cases the equivelant of many thousands of individual components), but discrete devices are still heavily used as they are usually able to handle higher power loads than integrated circuits.
The diode is the basic building block of the active electronic circuit. The basic characteristic of a diode is to allow current to flow in one direction, but not the other. The two leads are called the anode and the cathode. When the anode is more positive than the cathode then it will conduct, when it is reversed then it will not.
This on it's own is a useful feature as it can be used to protect circuits against the power being reversed and can be used to convert AC voltage to DC voltage. Depending upon how the diode is manufactured some of the other characteristics of the diode can be exploited for different uses.
A rectifier diode uses the normal characteristic of the diode allowing current to flow in one direction and blocking current flow in the reverse direction. The diagram below shows the effect of a single rectifier diode on an AC input. In this mode the forward voltage is transferred through the diode, but the reverse is blocked which results in only half of the AC phase being made available. The important things to identify for a rectifier diode is the maximum reverse voltage VR and the maximum forward current IF. There will be some reverse leakage current, but this is normally negligable for a rectifier circuit.
Four diodes can be connected into a configuration as a bridge-rectifier. This circuit will take an AC signal and convert it into a DC signal at the output. The bridge rectifier can be made using 4 discrete diodes or it is available as a single bridge rectifier package.
When combined with a capacitor between the output this will result in a fairly smooth DC output voltage. Normally thi sis an electrolytic capacitor with a value of between 470μF and 2200μF. A voltage regulator is normally used as well to ensure a consistant voltage.
The zener diode exploits the reverse breakdown voltage of the diode. The zener diode is connected in reverse in a circuit and when the breakdown voltage is reached the diode will conduct. This can be used directly across the output of a circuit, but is normally used as a component in a stabiliser circuit.
The light emitting diode, usually known as LED, is a special diode constructed so as to emit light when a current flows through it. These are discussed further in the optoelectronic component section
Transistors are a common components used in electronic circuits. These are available as discrete components as discussed here, but are often used on integrated circuits where multiple transistors can be used to make a more complex circuit.
This entry is based around a bipolar transistor, which is often known as a "transistor". FET / MOSFET transistors are also available, which are discussed below.
Transistors work as current amplifiers. They are available as NPN or PNP types depending upon the direction of the current flow at the base. The NPN is the most common and is the one discussed here; the same applies for the PNP transistor except that the base current is reversed. Transistor have 3 terminals; named collector (c), emitter (e) and base (b). When there is no input signal (current) at the base then no current will flow between the collector and emitter, but by introducing a signal to the base then the transistor will allow a current to flow. Depending upon the current at the base the transistor can amplify the signal or can be used as an on-off switch.
Different transistors are available and have different characteristics that are more suited to certain functions. It may be possible to use a different type of transistor, but the characteristics may not be as suitable.
A FET is a Field Effect Transistor. There are different types of FETs with different names depending the technology used. The most common are MOSFET (Metal-Oxide Silicon FET) and JFET (Junction FET).
The FET differs from the bipolar resistor in that it is controlled by the input voltage rather than input current. It has three terminals kwown as Drain Gate and Source.
FETs are often compbined within integrated circuits creating logic gates and more complex digital circuits.
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