Most diodes are semiconductor devices; diode electron tubes, now uncommon, are also available.
A diode has a low resistance to electric current in one direction and a high resistance to it in the reverse direction. This property makes a diode useful as a rectifier, which can convert alternating current (AC) into direct current (DC). The term diode and rectifier will be used interchangeably; however, the term diode usually implies a small signal device with current typically in the milliamp range; and a rectifier, a power device, conducting from1 to 1000 amps or even higher.
When the voltage applied in the reverse direction exceeds a certain value, a semiconductor diode "breaks down and conducts heavily in the direction of normally high resistance. When the reverse voltage at which breakdown occurs remains nearly constant for a wide range of currents, the phenomenon is called avalanching. A diode using this property is called a Zener diode. It can be used to regulate the voltage in a circuit.
Semiconductor diodes can be designed to have a variety of characteristics.
However, some other forms of diode are created by depositing one material onto another e.g. Schottky diodes are made by placing some metal in contact with a semiconductor. In general, whenever we join two different, very pure, materials we're likely to make some sort of diode.
Diodes are referred to as non-linear circuit elements because of the above characteristic curve. For most applications the non-linear region can be avoided and the device can be modeled by piece-wise linear circuit elements. Qualitatively we can just think of an ideal diode has having two regions: a conduction region of zero resistance and an infinite resistance non-conduction region. Figure 4 shows a schematic symbol for a diode and the current-voltage curve for an ideal diode.
Diode is unidirectional, i.e. current flows in only one direction (anode to cathode internally). When a forward voltage is applied, the diode conducts; and when a reverse voltage is applied, there is no conduction.
Forward Voltage Drop , Vf
We create a p-n junction by joining together two pieces of semiconductor, one doped n-type, the other p-type. This causes a depletion zone to form around the junction (the join) between the two materials. This zone controls the behaviour of the diode. The animation shows the general behaviour of a p-n junction.
The negative potential attracts the holes away from the edge of the junction barrier on the P side, while the positive potential attracts the electrons away from the edge of the barrier on the N side. This action increases the barrier width because there are more negative ions on the P side of the junction, and more positive ions on the N side of the junction. This increase in the number of ions prevents current flow across the junction by majority carriers. However, the current flow across the barrier is not quite zero because of the minority carriers crossing the junction.
When the crystal is subjected to an external source of energy (light, heat, etc.), electron-hole pairs are generated. The electron-hole pairs produce minority current carriers. There are minority current carriers in both regions: holes in the N material and electrons in the P material. With reverse bias, the electrons in the P-type material are repelled toward the junction by the negative terminal of the battery. As the electron moves across the junction, it will neutralize a positive ion in the N-type material. Similarly, the holes in the N-type material will be repelled by the positive terminal of the battery toward the junction. As the hole crosses the junction, it will neutralize a negative ion in the P-type material. This movement of minority carriers is called MINORITY CURRENT FLOW, because the holes and electrons involved come from the electron-hole pairs that are generated in the crystal lattice structure, and not from the addition of impurity atoms.
Therefore, when a p-n junction is reverse biased, there will be no current flow because of majority carriers but a very small amount of current because of minority carriers crossing the junction. However, at normal operating temperatures, this small current may be neglected.
In summary, the most important point to remember about the p-n junction diode is its ability to offer very little resistance to current flow in the forward-bias direction but maximum resistance to current flow when reverse biased. A good way of illustrating this point is by plotting a graph of the applied voltage versus the measured current. Figure 20 shows a plot of this voltage-current relationship (characteristic curve) for a typical p-n junction diode.
Figure 20. - p-n junction diode characteristic curve.
Current in the Diode
The behaviour of a diode depends on its polarity in the circuit.
An approximation to the current in the p-n junction region is given by (shown in figure 3a)
where both I0 and VT are temperature
dependent. This equation gives a reasonably accurate prediction of the
current-voltage relationship of the p-n junction itself - especially the
temperature variation - and can be improved somewhat by choosing I0
and VT empirically to fit a particular diode.
Various regions of the curve can be identified:
We can assign a dynamic resistance to the diode in each of the linear
regions: Rf in the forward-biased region and Rr
in the reverse-biased region. These resistances are defined as the inverse
slope of the curve:
Light-emitting diodes (LED) emit light in proportion to the forward current through the diode.
LEDs and photodiodes are often used in optical communication as receiver and transmitter respectively.
The varactor diode symbol is shown below with a diagram representation.
When a reverse voltage is applied to a p-n junction , the holes in the p-region are attracted to the anode and electrons in the n-region are attracted to the cathode terminal creating a region where there is little current. This region, the depletion region, is essentially devoid of carriers and behaves as the dielectric of a capacitor.
The depletion region increases as reverse voltage across it increases; and since capacitance varies inversely as dielectric thickness, the junction capacitance will decrease as the voltage across the p-n junction increases. So by varying the reverse voltage across a p-n junction the junction capacitance can be varied .This is shown in the typical varactor voltage-capacitance curve below.
Notice the nonlinear increase in capacitance as the reverse voltage is decreased. This nonlinearity allows the varactor to be used also as a harmonic generator.
Notice that as the reverse voltage is increased the leakage current remains
essentially constant until the breakdown voltage ( Vz ) is
reached where the current increases dramatically. This breakdown voltage is the
zener voltage for zener diodes.
As the reverse voltage increases the diode can avalanche-breakdown (zener
breakdown). This causes an increase in current in the reverse direction.
Transient Voltage Supressor
Transient Voltage Suppressors (TVS) are semiconductor devices designed to
provide protection against voltage and current transients. The silicon TVS is
designed to operate in the avalanche mode and uses a large junction area to
absorb large transient currents. Operation in the avalanche mode insures a low
impedance; also the TVS is characterized by a fast response time. The TVS is
available as unipolar or bipolar (that is it can suppress transients in one
direction or in both directions).
The breakdown voltage Vbr is the point the TVS device enters avalanche, a high conductance region. Vrwm, also referred to as the working voltage. Ir is the maximum current measured at the working voltage. The maximum peak pulse current for a TVS is Ipp. The maximum clamping voltage Vc is the maximum voltage across the TVS when it is subjected to Ipp.
The Silicon Controlled Rectifier (SCR) is simply a conventional rectifier controlled by a gate signal. The main circuit is a rectifier, however the application of a forward voltage is not enough for conduction. A gate signal controls the rectifier conduction.
The rectifier circuit (anode-cathode) has a low forward resistance and a high
reverse resistance. It is controlled from an off state (high resistance) to the
on state (low resistance) by a signal applied to the third terminal, the gate.
Once it is turned on it remains on even after removal of the gate signal, as
long as a minimum current, the holding current Ih is
maintained in the main or rectifier circuit. To turn off an SCR the
anode-cathode current must be reduced to less than the holding current Ih.
Notice the reverse characteristics are the same as discussed previously for the rectifier or diode, having a breakover voltage with its attending avalanche current; and a leakage current for voltages less than the breakover voltage. However, in the forward direction with open gate, the SCR remains essentially in an off condition up until the forward breakover voltage is reached. At that point the curve snaps back to a typical forward rectifier characteristic. The application of a small forward gate voltage switches the SCR onto its standard diode forward characteristic for voltages less than the forward breakover voltage.
Obviously, the SCR can also be switched by exceeding the forward breakover voltage, however this is usually considered a design limitation and switching is normally controlled with a gate voltage. Most SCR applications are in power switching, phase control, chopper, and inverter circuits.
The diac is a bidirectional trigger diode which is designed
specifically to trigger a triac or SCR. Basically the diac does not conduct
(except for a small leakage current) until the breakover voltage is reached. At
that point the diac goes into avalanche conduction also at that point the device
exhibits a negative resistance characteristic, and the voltage drop across the
diac snaps back, typically about 5 volts, creating a breakover current
sufficient to trigger a triac or SCR.
Although most diacs have symmetric switching voltages, asymmetric diacs are available.
The triac is a three terminal semiconductor for controlling current in either direction. Below is the schematic symbol for the triac. Notice the symbol looks like two SCRs in parallel ( opposite direction) with one trigger or gate terminal.The main or power terminals are designated as MT1 and MT2 . (See the schematic representation below) When the voltage on the MT2 is positive with regard to MT1 and a positive gate voltage is applied, the left SCR conducts. When the voltage is reversed and a negative voltage is applied to the gate, the right SCR conducts. Minimum holding current, Ih, must be maintained in order to keep a triac conducting.
A triac operates in the same way as the SCR however it operates in both a forward and reverse direction. To get a quick understanding of its operation refer to its characteristic curve below and compare this to the SCR characteristic curve. It can be triggered into conduction by either a PLUS (+) or MINUS (-) gate signal.
Obviously a triac can also be triggered by exceeding the breakover voltage. This is not normally employed in triac operation. The breakover voltage is usually considered a design limitation. One other major limitation, as with the SCR, is dV/dt, which is the rate of rise of voltage with respect to time. A triac can be switched into conduction by a large dV/dt. Typical applications are in phase control, inverter design, AC switching, relay replacement, etc.
a heavily doped PN-junction diode that is characterized by a negative-resistance region in the forward-bias direction; in the negative-resistance region, an increase in applied voltage causes a decrease in forward current and vice versa. Tunnel diodes are used in ultrahigh-frequency and microwave oscillator and amplifier circuits. Also, ESAKI DIODE.
A tunnel diode is a semiconductor with a negative resistance region that results in very fast switching speeds, up to 5 GHz. The operation depends upon a quantum mechanic principle known as "tunneling" wherein the intrinsic voltage barrier (0.3 Volt for Germanium junctions) is reduced due to doping levels which enhance tunneling. Refering to the curves below, superimposing the tunneling characteristic upon a conventional P-N junction, we have:
Resulting in a composite characteristic which is the tunnel diode characteristic curve.
The negative resistance region (between points A and B) is the important characteristic for the tunnel diode. In this region, as the voltage is increased, the current decreases; just the opposite of a conventional diode. The most important specifications for the tunnel diode are the Peak Voltage (Vp), Peak Current (Ip) , Valley Voltage (Vv), and Valley Current (Iv).
A Back diode is a tunnel diode with a suppressed Ip and so approximates a conventional diode characteristic See the comparison below:
The reverse breakdown for tunnel diodes is very low, typically 200mV, and the TD conducts very heavily at the reverse breakdown voltage. Referring to the BD curve the back diode conducts to a lesser degree in a forward direction . It is the operation between these two points that makes the back diode important. Forward conduction begins at 300 mV (for germanium) and a voltage swing of only 500mV is required for full range operation.