Types of semiconductor diode
Diode Zener diode Schottky diode Tunnel diode
Light-emitting diode Photodiode Varicap Silicon controlled rectifier
Figure- Diode symbols
Figure -Diode packages in same alignment as diode symbol. Thin bar shows the cathode.
Figure -Several types of diodes. The scale is in centimeters.
There are a number of types of junction diodes, which emphasize a different physical aspect of a diode by geometric scaling, choosing the right electrodes, doping level, are just an application of the diode in a special circuit, or are really different devices such as Gunn and laser diode and the MOSFET:
The normal (p-n) diodes, which operate as described above, are generally made of doped silicon or, germanium some times. Before development of the modern silicon power rectifier diodes, cuprous oxide and later selenium was used; its low efficiency gave it a higher forward voltage drop (nearly 1.4-1.7 V per -cell, with multiple cells stacked to increase peak inverse voltage rating in the high voltage rectifiers), and needed a large heat sink, much larger than silicon diode of same current ratings would require. The vast majority of all diodes are the PN diodes found in CMOS integrated circuits, which include 2 diodes per pin and many other internal diodes.
Avalanche diodes
Diodes which conduct in reverse direction when the reverse bias voltage exceeds breakdown voltage. These are electrically similar to Zener diodes, and are mistakenly called Zener diodes, but break down by a different mechanism, avalanche effect. This takes place when reverse electric field across the PN junction causes a wave of ionization, reminiscent of the avalanche, leading to a large current. Avalanche diodes are designed to break down at the reverse voltage without being destroyed. The difference between avalanche diode and the Zener is that the channel length of former exceeds the -mean free path of the electrons, so there are collisions in between them on the way out. The only practical difference is that the 2 types have temperature coefficients of the opposite polarities.
Cat's whisker or crystal diodes
These are the type of point-contact diode. The cat's whisker diode comprises of a thin or sharpened metal wire pressed against the semiconducting crystal, typically galena or a piece of coal. The wire forms anode and the crystal forms cathode. Cat's whisker diodes were also called as crystal diodes and found application in the crystal radio the cat's whisker diodes are usually obsolete, but can be available from a few manufacturers.
Constant current diodes
These are actually a JFET with gate shorted to the source, and the function like a two- terminal current-limiter analog to Zener diode, which is the limiting voltage. They allow a current to pass through them to rise to a certain value, and then level off at the specific value. Also called as CLDs, diode-connected transistors, constant-current diodes, or current- regulating diodes.
Esaki or tunnel diodes
These have a region of operation showing negative resistance created by quantum tunneling, hence allowing amplification of signals and very simple bistable circuits. These diodes are the type most resistant to the nuclear radiation.
Gunn diodes
These are similar to the tunnel diodes in which they are made of materials like GaAs or InP that exhibit a region of the negative differential resistance. With proper biasing, dipole domains form and travel across the diode, allowing the high frequency microwave oscillators to be built.
Light-emitting diodes (LEDs)
In the diode formed from a direct band-gap semiconductor, like gallium arsenide, carriers that cross the junction emit photons when they recombine with majority carrier on other side. Depending on material, wavelengths (or colors) from the infrared to the near ultraviolet can be produced. The forward potential of these diodes depends on wavelength of the emitted photons: 1.2 V corresponds to red, 2.4 V to violet. The first LEDs were red and yellow in color, and higher-frequency diodes have been developed over the time. All the LEDs produce incoherent, narrow-spectrum light; -white LEDs are actually combinations of the three LEDs of a different color, or a blue LED with a yellow scintillator coating. The LEDs can also be used as low-efficiency photodiodes in the signal applications. A LED can be paired with the photodiode or phototransistor in same package, to form an opto-isolator.
Laser diodes
When the LED-like structure is contained in the resonant cavity formed by polishing parallel end faces, a laser can be created. The laser diodes are commonly used in the optical storage devices and for the high speed optical communication.
Peltier diodes
The diodes are used as sensors, heat engines for the thermoelectric cooling. Charge carriers absorb and emit the band gap energies of them as heat.
Photodiodes
All semiconductors are subject to the optical charge carrier generation. This is characteristically an undesired effect, so most semiconductors are packaged in the light blocking material. Photodiodes are intended to sense light (photodetector), so they are packaged in the materials which allow light to pass, and are generally PIN (the type of diode most sensitive to light).
The photodiode can also be used in the solar cells, in photometry, or in the optical communications. Multiple photodiodes can be be packaged in a single device, either as a linear array or as the two-dimensional array. These arrays should not be confused with the charge-coupled devices.
Point-contact diodes
These work same as the junction semiconductor diodes described above, but their construction is quite easy. 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 semiconductor. Some of the metals migrates into semiconductor to make a small region of p-type semiconductor close the contact. The long-popular 1N34 germanium version is used still in radio receivers as a detector and rarely in specialized analog electronics.
PIN diodes
A PIN diode has a central un-doped, or the intrinsic, layer, forming a p-type structure. They are used as the radio frequency switches and attenuators. They are used as large volume ionizing radiation detectors and as the photodetectors. PIN diodes are also used in the power electronics, as their central layer can withstand high voltages. In addition, the PIN structure can be found in several power semiconductor devices, like power MOSFETs, IGBTs, and thyristors.
Schottky diodes
The Schottky diodes are constructed from a metal to the semiconductor contact. They possess a lower forward voltage drop than the p-n junction diodes. Their forward voltage drop at the forward currents of about 1 mA is in range 0.15 V to 0.45 V, which makes them useful in the voltage clamping applications and prevention of transistor saturation. They can be used as low loss rectifiers also, although their reverse leakage current is usually higher than that of the other diodes. Schottky diodes are majority carrier devices and so do not suffer from the minority carrier storage problems which slow down many other diodes - thus they have a faster -reverse recovery than the p-n junction diodes. They also tend to have much lower junction capacitance than the p-n diodes which provides for the high switching speeds and their use in high-speed circuitry and RF devices like switched-mode power supply, mixers and detectors.
Super barrier diodes
Super barrier diodes are rectifier diodes which incorporate low forward voltage drop of the Schottky diode with surge-handling capability and low reverse leakage current of the normal Pn junction diode.
Gold-doped diodes
As a dopant, gold or platinum acts as the recombination centers, which help a fast recombination of the minority carriers. This allows diode to operate at the signal frequencies, at expense of a higher forward voltage drop. The gold doped diodes are faster than rest p-n diodes (but not as fast as Schottky diodes). They also posses less reverse-current leakage than the Schottky diodes (but not as good as the rest p-n diodes). A example of it is 1N914.
Snap-off or Step recovery diodes
The term step recovery relates to the form of reverse recovery characteristic of these devices. After a forward current has been passing in the SRD and current is interrupted or reversed, the reverse conduction will cease abruptly (as in the step waveform).
SRDs can thus provide very fast voltage transitions by the very sudden disappearance of charge carriers.
Transient voltage suppression diode (TVS)
These are avalanche diodes designed specifically to protect other semiconductor devices from high-voltage transients. Their PN junctions have a very larger cross-sectional area than those in the normal diode, letting them conduct large currents to ground without sustaining damage.
Varicap or varactor diodes
These are generally used as voltage-controlled capacitors. These are significant in PLL (phase- locked loop) and FLL (frequency-locked loop) circuits, allowing the tuning circuits, such as those in the television receivers, to lock quickly, replacing older designs which took long time to warm up and lock. A PLL is faster than the FLL, but prone to integer harmonic locking (if one attempts to lock the broadband signal). They enabled tunable oscillators in early discrete tuning of radios also, where a cheap and stable, but fixed-frequency, crystal oscillator provided the reference frequency for the voltage-controlled oscillator.
Zener diodes
Diodes which can be made to conduct backwards. This effect, called as Zener breakdown, occurs at precisely defined voltage, allowing diode to be used as a accuracy voltage reference. In the practical voltage reference circuits Zener and switching diodes are connected in the series and opposite directions to balance temperature coefficient close to zero. Some devices labeled as high-voltage Zener diodes are actually the avalanche diodes. Two (equivalent) Zeners in the series and in reverse order, in same package, constitute a transient absorber (or the Transorb, a registered trademark).
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