Diode is a electrical device made from semiconductor material, usually silicon, which is doped with two impurities. One side is doped with a donor or n-type impurity which releases electrons into the semiconductor lattice. These electrons are not bound and are free to move about. Because there is no net charge in the donor impurity, the n-type semiconductor is electrically neutral. The other side is doped with an acceptor or p-type impurity which imparts free holes into the lattice. A hole is the absence of an electron which acts as a positive charge. The p-type semiconductor is also electrically neutral because the acceptor material adds no net charge. In simple terms, a diode is a device that restricts the direction of flow of charge carriers (electrons in this class) . Essentially, it allows an electric current to flow in one direction, but blocks it in the opposite direction.
The diode has two terminals or electrodes (di-ode), that act like an on-off switch. When the diode is “on”, it acts as a short circuit and passes all current. When it is “off”, it behaves like an open circuit and passes no current. The two terminals are different and are marked as plus and minus. If the polarity of the applied voltage matches that of the diode (forward bias), then the diode turns “on”. When the applied voltage polarity is opposite (reverse bias), it turns “off”.
Function:Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve and early diodes were actually called valves.
When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.
Forward Voltage Drop
The voltage across a semiconductor diode that is carrying current in the forward direction; it is usually approximately constant over the range of currents commonly used. Also known as diode drop; diode voltage; forward voltage drop.
Connecting and Soldering Diodes
Diodes must be connected the correct way round, and circuit diagrams may be labelled ‘a’ or ‘+’ for anode and ‘k’ or ‘-’ for cathode. The cathode is marked by a line painted on the body of the diode.
There is different types of diodes:
Zener diodes – This diode allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, “zener knee voltage” or “zener voltage”.
Light emitting diodes (LED) – This is the most popular kind of diode. When it works in the forward bias condition, the current flows through the junction to produce the light.
Photodiodes – The electrons and holes are generated as light strikes across the p-n junction causing the current to flow. Theses diodes can work as photodetector and are used to generate electricity.
Schottky diode - is a semiconductor diode with a low forward voltage drop and a very fast switching action.
Shockley diode – This is a four layer diode which is also known as PNPN diode. This didoe is similar to thyristor where the gate is disconnected.
Tunnel diodes - The tunnel diode has a region in its voltage current characteristic where the current decreases with increased forward voltage, known as its negative resistance region. This characteristic makes the tunnel diode useful in oscillators and as a microwave amplifier.
Vaccum diodes - This diode is two electrode vacuum tube which can tolerate high inverse voltages.
Varactor diodes – This didoe works in reverse bias condition and restricts the flow of current thorugh the junction. Depending on the amount of biasing, the width of the depletion region keeps varying. This diode comprises of two plates of a capacitor with the depletion region amidst them. The variation in capacitance depends upon the depletion region and this can varied by altering the reverse bias on the diode.
PIN diodes – This diode has intrinsic semiconductor sandwiched between P- type and N- type region. Doping does not occur in this type of diode and thereby the intrinsic semiconductor increases the width of the depletion region. They are used as ohtodiodes and radio frequency switches.
LASER diode – This diode produces laser type of light and are expensive as compared to LED. They are widely used in CD and DVD drives.
Transient voltage supression diodes – This diode is used to protect the electronics that are sensitive against voltage spikes.
Gold doped diodes – These diodes use gold as the dopant and can operate at signal frequencies even if the forward voltage drop increases.
Silicon controlled rectifier - As the name implies this diode can be controlled or triggered to the ON condition due to the application of small voltage. They belong to the family of Tyristors and is used in various fields of DC motor control, generator field regulation, lighting system control and variable frequency drive . This is three terminal device with anode, cathode and third controled lead or gate.
Super barrier diodes – These are also called as the rectifier diodes. This diodes have the property of low reverse leakage current as that of normal p-n junction diode and low forward voltage drop as that of Schottky diode with surge handling ability.
Peltier diodes – This diode is used as heat engine and sensor for thermoelectric cooling.
Gunn diode - is a form of diode used in high-frequency electronics. Its internal construction is unlike other diodes in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions. In the Gunn diode, three regions exist: two of them are heavily N-doped on each terminal, with a thin layer of lightly doped material in between. When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. Conduction will take place as in any conductive material with current being proportional to the applied voltage. Eventually, at higher field values, the conductive properties of the middle layer will be altered, increasing its resistivity, preventing further conduction and current starts to fall. This means a Gunn diode has a region of negative differential resistance.
Crystal diode – These are a type of point contact diodes which are also called as Cat’s whisker diode. This didoe comprises of a thin sharpened metal wire which is pressed against the semiconducting crystal. The metal wire is the anode and the semconducting crystal is the cathode. These diodes are obsolete.
Avalanche diode – This diode conducts in reverse bias condition where the reverse bias volage applied across the p-n junction creates a wave of ionization leading to the flow of large current. These didoes are designed to breakdown at specific reverse voltage in order to avoid any damage.