Diode
A diode functions 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 blocks it
in the opposite direction.
Applications
Radio demodulation
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 current, 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 the negative
peaks), leaving a signal whose average amplitude is the desired audio signal. The average value is extracted using a simple
filter and fed into a transducer (originally a crystal earpiece, now more likely to be a loudspeaker), which generates sound.
Logic gates
Diodes can be used to construct logic gates: logical and and logical or.
Power conversion
A diode is called a half wave rectifier when it is used to convert
alternating current electricity into direct current, by removing the negative portion of the current.
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 alternating current.
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. Many of the small wind turbines, such as the Lakota from True North Power (example installation (http://wearcam.org/urbine/)) use three double diodes bolted to the same heatsink.
Three-Phase Bridge Rectifier for wind turbine.
Over-Voltage Protection
Diodes are frequently used to conduct dangerously high voltages away from sensitive devices, most commonly by being
reverse-biased (non-conducting) under normal circumstances, and becoming 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 Types below).
Diode technology
The first diodes were vacuum tube devices (also known as thermionic valves), arrangements of electrodes surrounded by a vacuum
within a glass envelope, similar in appearance to incandescent light bulbs. The
arrangement of a filament and plate as a diode was invented in 1904 by John Ambrose Fleming, scientific adviser to the Marconi company, based on an observation by Thomas Edison. Like light bulbs, vacuum tube diodes have a filament through which current is passed, heating the filament. In its heated state it can now emit electrons into
the vacuum. These electrons are electrostatically drawn to a positively charged outer metal plate called the anode, or just the "plate". Electrons do not flow from the plate back toward the filament, even
if the charge on the plate is made negative, because the plate is not heated.
Although vacuum tube diodes are still used for a few specialized applications, most modern diodes are based on semiconductor p-n
junctions. In a p-n diode, conventional current can flow from the p-doped side (the anode) to the n-doped side (the cathode), but not in the opposite
direction. When the diode is reverse-biased, the charge carriers are pulled away from the center of the device, creating a
depletion region.
Analysis
A diode's current-voltage, or I-V, characteristic can be approximated by two
regions of operation. Below a certain difference in potential between the two leads, 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
current 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).
The Shockley ideal diode equation (named after William Bradford Shockley) can be used to approximate the p-n diode's I-V characteristic.
,
where I is the diode current, IS is a scale factor called the saturation current, q is
the charge on an electron (the elementary charge), k is Boltzmann's constant, T is the absolute temperature of the
p-n junction and VD is the voltage across the diode. The term kT/q is the thermal voltage,
sometimes written VT, and is approximately 26 mV at room temperature. n (sometimes omitted) is the
emission coefficient, which varies from about 1 to 2 depending on the fabrication process.
In a normal silicon diode, the drop in potential across a conducting diode is approximately 0.6 to 0.7 volts. The value is different for other diode types - Schottky diodes can be as low as 0.2V and light-emitting diodes (LEDs) can be 1.4V or more.
The voltage drop across an ordinary silicon diode can be used as a simple voltage regulator: a load (such as an incandescent lamp or an electric motor) in series with one or more diodes absorbs the voltage in excess of the "diode drop," while a
second, smaller load (usually a small incandescent lamp), in parallel with the diode(s), receives only the combined voltage drop
of the diodes. This allows for a lamp to be illuminated at roughly constant brightness on the same power supply as (for example)
a variable speed motor, and can also be used to protect small, delicate incandescent lamps placed in series strings from excess
current or voltage. For a 1.5V lamp, two diodes in series provide adequate voltage; for AC or bidirectional DC, a second pair in
reverse parallel is added. This technique is commonly used for lighting model railroad locomotive headlights (using the
locomotive's motor as the "ballast" load), and passenger car lighting (using a concealed 16V lamp as the "ballast" load, as
ordinary resistors do not work well for this purpose).
Diode types
There are several types of semiconductor junction diodes:
- Normal (p-n) diodes: which operate as described above. Usually made of doped silicon or, more rarely, germanium.
- '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.
- Zener diodes (pronounced zEn@r): 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
labelled 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.
- Avalanche diodes: 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, 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.
- Photodiodes: 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 isn't going to turn the photo-diode
on very much. 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
susceptable, 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.
- Schottky diodes: these have a very low forward voltage drop,
usually 0.15 to 0.45 V, which makes them useful in battery-powered and low-voltage circuits. Also in mixer circuits for RF.
- Snap diodes: these can provide very fast voltage transitions.
- Esaki or tunnel diodes: these have a region of operation
showing negative resistance caused by quantum tunneling, thus
allowing amplification of signals and very simple bistable circuits.
- Gunn diodes: 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.
- 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.
- Tube or Valve Diode: This is the simplest kind of vacuum
tube device (referred to as a valve in the UK). Electrons will move from a heated metal surface (cathode) treated with a mixture of barium and strontium oxides into a vacuum (thermionic emission). After leaving the cathode, they can be
attracted to positively charged cool surface (anode). However, electrons are not easily
released from a cold untreated surface when the voltage polarity is reversed and hence any flow is a very small current. For much
of the 20th century they were used in analog signal applications, and as rectifiers in power supplies. Tube diodes were nearly
obsolete by 2001, except as rectifiers in tube guitar and hi-fi amplifiers and in a few specialized high-voltage
applications.
- Gas Discharge Diode: There are two electrodes, not touching, in some kind of gas. One electrode is very sharp. The
other has a smoothly curved finish. If a strong negative potential is applied to the sharp electrode, the electric field near the
sharp edge or point is enough to cause an electrical discharge in the gas, and a current flows. If the reverse potential is
applied, the electrical field strength around the smooth electrode is not enough to start a discharge. (The discharge can only
start easily at the negative end because electrons are much more mobile than positive ions.) These are sometimes used for
high-voltage high-current rectification in power supply applications.
- Varicap or varactor diodes - 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.
Other uses for semiconductor diodes include sensing temperature, and computing analog logarithms.
Related devices
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