|Notes on Diode
A diode can be thought of as the electronic version of a one-way valve. By restricting
the direction of movement of charge carriers, it allows an electric current to
flow in one direction, but essentially blocks it in the opposite direction. Diodes
may be made from semiconductor materials such as silicon or germanium or may be
fabricated using devices depending on thermionic emission: tubes (US) (or valves
Physical explanation of semiconductor diode operation
A semiconductor diode's current-voltage, or I-V, characteristic curve is ascribed
to the behavior of the so-called Depletion Layer or Depletion Zone which exists
at the p-n junction between the differing semiconductors. When a p-n junction
is first created, conduction band (mobile) electrons from the N-doped region
diffuse into the P-doped region where there is a large population of holes (places
for electrons in which no electron is present) with which the electrons "recombine".
When a mobile electron recombines with a hole, the hole vanishes and the electron
is no longer mobile. Thus, two charges carriers have vanished. The region around
the p-n junction becomes depleted of charge carriers and thus behaves as an
insulator. However, the Depletion width cannot grow without limit. For each
electron-hole pair that recombines, a positively-charged dopant ion is left
behind in the N-doped region, and a negatively charged dopant ion is left behind
in the P-doped region. As recombination proceeds and more ions are created,
an increasing electric field develops through the depletion zone which acts
to slow and then finally stop recombination. At this point, there is a 'built-in'
potential across the depletion zone. If an external voltage is placed across
the diode with the same polarity as the built-in potential, the depletion zone
continues to act as an insulator preventing a significant electric current.
However, if the polarity of the external voltage opposes the built-in potential,
recombination can once again proceed resulting in substantial electric current
through the p-n junction. For silicon diodes, the built-in potential is approximately
0.6 V. Thus, if an external current is passed through the diode, about 0.6 V
will be developed across the diode such that the P-doped region is positive
with respect to the N-doped region and the diode is said to be 'turned on'.
A diode's I-V, characteristic can be approximated by two regions of operation.
Below a certain difference in potential between the two leads, the Depletion
Layer has significant width, and the diode can be thought of as an open (non-conductive)
circuit. As the potential difference is increased, at some stage the diode will
become conductive and allow charges to flow, at which point it can be thought
of as a connection with zero (or at least very low) resistance. More precisely,
the transfer function is logarithmic, but so sharp that it looks like a corner
(see also signal processing).
Types of semiconductor diode
There are several types of semiconductor junction diodes:
Gold Doped Diodes
Transient Voltage Suppression
Light Emitting Diodes
Normal (p-n) diodes
which operate as described above. Usually made of doped silicon or, more rarely,
germanium. Before the development of modern silicon power rectifier diodes,
cuprous oxide and later selenium was used; its low efficiency gave it a much
higher forward voltage drop (typically 1.4-1.7V per "cell," with multiple
cells stacked to increase the peak inverse voltage rating in high voltage rectifiers),
and required a large heat sink (often an extension of the diode's metal substrate),
much larger than a silicon diode of the same current ratings would require.
'Gold doped' diodes
The gold causes 'minority carrier suppression.' This lowers the effective capacitance
of the diode, allowing it to operate at signal frequencies. A typical example
is the 1N914. Germanium and Schottky diodes are also fast like this, as are
bipolar transistors 'degenerated' to act as diodes. Power supply diodes are
made with the expectation of working at a maximum of 2.5 x 400 Hz (sometimes
called 'French power' by Americans), and so are not useful above a kilohertz.
Diodes that can be made to conduct backwards. This effect, called Zener breakdown,
occurs at a precisely defined voltage, allowing the diode to be used as a precision
voltage reference. Some devices labeled as high-voltage Zener diodes are actually
avalanche diodes (see below). Two (equivalent) Zeners in series and in reverse
order, in the same package, constitute a transient absorber (or Transorb, a
registered trademark). They are named for Dr. Clarence Melvin Zener of Southern
Illinois University, inventor of the device.
Diodes that conduct in the reverse direction when the reverse bias voltage exceeds
the breakdown voltage. These are electrically very similar to Zener diodes,
and are often mistakenly called Zener diodes, but break down by a different
mechanism, the Avalanche Effect. This occurs when the reverse electric field
across the p-n junction causes a wave of ionization, reminiscent of an avalanche,
leading to a large current. Avalanche diodes are designed to break down at a
well-defined reverse voltage without being destroyed. The difference between
the avalanche diode (which has a reverse breakdown above about 6.2 V) and the
Zener is that the channel length of the former exceeds the 'mean free path'
of the electrons, so there are collisions between them on the way out. The only
practical difference is that the two types have temperature coefficients of
opposite polarities. Practical voltage reference circuits feature Zener and
switching diodes connected in series and opposite directions to balance the
temperature coefficient to near zero.
Transient voltage suppression (TVS) diodes
These are avalanche diodes designed specifically to protect other semiconductor
devices from electrostatic discharges. Their p-n junctions have a much larger
cross-sectional area than those of a normal diode, allowing them to conduct
large currents to ground without sustaining damage.
Light-emitting diodes (LEDs)
as the electrons cross the junction they emit photons. In most diodes, these
are reabsorbed, and are at frequencies that can not be seen (usually infrared).
However, with the right materials and geometry, the light becomes visible. The
forward potential of these diodes define their color. Thus different materials
(extrinsic semiconductors) must be used. 1.2 V corresponds to red, 2.4 to violet.
Now, even soft UV diodes are available. The first LED's were red and yellow,
and higher-frequency diodes have been developed over time. Polishing the device
with parallel faces, so as to form a resonant cavity, yields a 'laser diode.'
All LEDs are monochromatic; 'white' LED's are actually combinations of three
LED's of a different color, or a blue LED with a yellow scintillator coating.
The lower the frequency of emission, the greater the efficiency, so to normalize
output when using LED's of different colors it is necessary to increase current
in the higher frequency models. This effect is complicated, somewhat, by the
fact that the human eye is most sensitive in the blue-green.
these have wide, transparent junctions. Photons can push electrons over the
junction, causing a current to flow. Photo diodes can be used as solar cells,
and in photometry. If a photon doesn't have enough energy, it will not overcome
the band gap, and will pass through the junction. LED's can be used as low-efficiency
photodiodes in signal applications. Sometimes a LED is paired with a photodiode
or phototransistor in the same package. This device is called an "opto
isolator." Unlike a transformer, this scheme allows for DC coupling. These
are used to protect hospital patients from shock. Patients with IV's in their
bodies are particularly susceptible, sometimes succumbing to 'carpet shock.'
They are also used to isolate low-current control or signal circuitry from "dirty"
power supply circuits or higher-current motor and machine circuits.
These have a lower forward voltage drop than normal PN junction, because they
are constructed from a metal to semiconductor contact. Their forward voltage
drop at forward currents of about 1 mA is in the range 0.15V to 0.45 V, which
makes them useful in voltage clamping applications and prevention of transistor
saturation. They can also be used as low loss rectifiers although their reverse
leakage current is generally much higher than non Schottky rectifiers. Schottky
diodes are majority carrier devices and so do not suffer from minority carrier
storage problems that slow down most normal diodes. They also tend to have much
lower junction capacitance than PN diodes and this contributes towards their
high switching speed and their suitability in high speed circuits and RF devices
such as mixers and detectors.
Snap-off or 'step recovery' diodes
The term 'step recovery' relates to the form of the reverse recovery characteristic
of these devices. After a forward current has been passing in an SRD and the
current is interruped or reversed, the reverse conduction will cease very abruptly
(as in a step waveform). SRDs can therefore provide very fast voltage transitions
by the very sudden disappearance of the charge carriers.
Esaki or tunnel diodes
T hese have a region of operation showing negative resistance caused by quantum
tunneling, thus allowing amplification of signals and very simple bistable circuits.
These diodes are also the type most resistant to nuclear radiation.
These are similar to tunnel diodes in that they are made of materials such
as GaAs or InP that exhibit a region of negative differential resistance. With
appropriate biasing, dipole domains form and travel across the diode, allowing
high frequency microwave oscillators to be built.
There are other types of diodes, which all share the basic function of allowing
electrical current to flow in only one direction, but with different methods
of construction. Some examples are Point Contact Diode, Varicap or varactor
diodes, Current-limiting field effect diodes.
Point Contact Diode
This works the same as the junction semiconductor diodes described above, but
its construction is simpler. A block of n-type semiconductor is built, and a
conducting sharp-point contact made with some group-3 metal is placed in contact
with the semiconductor. Some metal migrates into the semiconductor to make a
small region of p-type semiconductor near the contact. The long-popular 1N34
germanium version is still used in radio receivers as a detector and occasionally
in specialized analog electronics.
Varicap or varactor diodes
These are used as voltage-controlled capacitors. These were important in PLL
(phase-locked loop) and FLL (frequency-locked loop) circuits, allowing tuning
circuits, such as those in television receivers, to lock quickly, replacing
older designs that took a long time to warm up and lock. A PLL is faster than
a FLL, but prone to integer harmonic locking (if one attempts to lock to a broadband
signal). They also enabled tunable oscillators in early discrete tuning of radios,
where a cheap and stable, but fixed-frequency, crystal oscillator provided the
reference frequency for a voltage-controlled oscillator.
Current-limiting field-effect diodes
These are actually a JFET with the gate shorted to the source, and function
like a two-terminal current-limiting analog to the Zener diode; they allow a
current through them to rise to a certain value, and then level off at a specific
value. Also called CLDs, constant-current diodes, or current-regulating diodes.
Other uses for semiconductor diodes include sensing temperature, and computing
The first use for the diode was the demodulation of amplitude modulated (AM)
radio broadcasts. The history of this discovery is treated in depth in the radio
article. In summary, an AM signal consists of alternating positive and negative
peaks of voltage, whose amplitude or 'envelope' is proportional to the original
audio signal, but whose average value is zero. The diode rectifies the AM signal
(i.e. it eliminates peaks of one polarity), leaving a signal whose average amplitude
is the desired audio signal. The average value is extracted using a simple filter
and fed into an audio transducer (originally a crystal earpiece, now more likely
to be a loudspeaker), which generates sound.
A half wave rectifier can be constructed from a single diode where it is used
to convert alternating current electricity into direct current, by removing
either the negative or positive portion of the AC input waveform.
A special arrangement of four diodes that will transform an alternating current
into a direct current, using both positive and negative excursions of a single
phase alternating current, is known as a diode bridge, single-phase bridge rectifier,
or simply a full wave rectifier.
With a split (center-tapped) alternating current supply it is possible to obtain
full wave rectification with only two diodes. Often diodes come in pairs, as
double diodes in the same housing.
When it is desired to rectify three phase power, one could rectify each of
the three phases with the arrangement of four diodes used in single phase, which
would require a total of 12 diodes. However, due to redundancy, only six diodes
are needed to make a three phase full wave rectifier. Most devices that generate
alternating current (such devices are called alternators) generate three phase
Disassembled automobile alternator, showing the six diodes that comprise a full-wave
three phase bridge rectifier.
Disassembled automobile alternator, showing the six diodes that comprise a full-wave
three phase bridge rectifier.
For example, an automobile alternator has six diodes inside it to function
as a full wave rectifier for battery charge applications
Diodes are frequently used to conduct damaging high voltages away from sensitive
electronic devices. They are usually reverse-biased (non-conducting) under normal
circumstances, and become forward-biased (conducting) when the voltage rises
above its normal value. For example, diodes are used in stepper motor and relay
circuits to de-energize coils rapidly without the damaging voltage spikes that
would otherwise occur. Many integrated circuits also incorporate diodes on the
connection pins to prevent external voltages from damaging their sensitive transistors.
Specialized diodes are used to protect from over-voltages at higher power (see
Diode types above).
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