Diodes: A Diode is
an electronic device that allows
current to flow in one direction only. It is a a
semiconductor that consists of a p-n junction. They are used
most commonly to convert AC to DC, because they pass the
positive part of the wave, and block the negative part of
the AC signal, or, if they are reversed, they pass only the
negative part and not the positive part. |
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Both p-type and n-type silicon will conduct electricity just like
any conductor; however, if a piece of silicon is doped p-type in one
section and n-type in an adjacent section, current will flow in only
one direction across the junction between the two regions. This
device is called a diode and is one of
the most basic semiconductor devices.
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A diode is called
forward biased
if it has a positive voltage across it from from the p- to n-type
material. In this condition, the diode acts rather like a good
conductor, and current can flow, as in following figure:
Fig - 1 :
Forward Biased
Diode |
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There will be a small voltage across the diode, about 0.6 volts for
Si, and this voltage will be largely independent of the current,
very different from a resistor.
If the polarity of the applied voltage is reversed, then the diode
will be
reverse biased and will appear non-conducting. Almost no
current will flow and there will be a large voltage across the
device.
Fig - 2 : Reverse Biased
Diode
The non-symmetric behavior is due to the detailed properties of the
pn-junction. The diode acts like a one-way valve for current and
this is a very useful characteristic. One application is to convert
alternating current (AC), which changes polarity periodically, into
direct current (DC), which always has the same polarity. Normal
household power is AC while batteries provide DC, and converting
from AC to DC is called rectification. Diodes are used so commonly
for this purpose that they are sometimes called rectifiers, although
there are other types of rectifying devices.
Figure 3 shows the input and
output current for a simple half-wave rectifier. The circuits gets
its name from the fact that the output is just the positive half of
the input waveform. A full-wave rectifier circuit (Figure 4)
uses four diodes arranged so that both polarities of the input
waveform can be used at the output. The full-wave circuit is more
efficient than the half-wave one.
Fig - 3 :
Half-Wave Rectifier
Fig - 4 : Full-Wave Rectifier
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