Science Fair Project Encyclopedia
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.
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.
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.
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) use three double diodes bolted to the same heatsink.
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).
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.
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 semiconducting junction is first created, electrons from the N-doped region diffuse across into the P-doped region where they "recombine," falling into holes. Recombined electrons and holes are immobile, and any region lacking mobile charge carriers behaves as an insulator. The Depletion width doesn't grow without limits. For each hole-electron pair which cancels out, a positively-charged dopant atom (ion) is left behind in the P-doped region, and a negative ion is left behind in the N-doped region. The entire carrier populations of the opposite-doped semiconductors do not recombine, since a significant e-field appears between the populations of opposite-charged ions which remain behind. This permanent e-field has a polarity which pushes the two kinds of charge carriers back into their respective doped regions. However, an externally applied voltage can defeat this e-field, shrinking the Depletion Layer until carriers are easily able to tunnel across the insulating gap, and so the diode "turns 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).
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).
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. 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.
- Zener diodes (pronounced ): 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 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.
- 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, free carriers are created, and a low resistance path appears. 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 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.
Other uses for semiconductor diodes include sensing temperature, and computing analog logarithms.
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