Diode

                           Diode
Diode is a two-terminal electronic component that conducts current primarily in one direction. it has low (ideally zero) resistance in one direction, and high (ideally infinite) Resistance in the other. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate. A semiconductor diode, the most commonly used type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals.Semiconductor diodes were the first semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other materials such as gallium arsenide and germanium are also used.

 Main function: 

               The most common function of a diode is to allow an electric current to pass in one direction (called the diode's forwarddirection), while blocking it in the opposite direction (the reversedirection). As such, the diode can be viewed as an electronic version of a check valve. This unidirectional behavior is called rectification, and is used to convert alternating current (ac) to direct current (dc). Forms of rectifiers, diodes can be used for such tasks as extracting modulation from radio signals in radio receivers.

                         semiconductor diode. A diode made of semiconductorcomponents, usually silicon. The cathode, which is negatively charged and has an excess of electrons, is placed adjacent to the anode, which has an inherently positive charge, carrying an excess of holes.

  

     Use of semiconductor :

         Diodes regulate the flow of voltage in a circuit

The simplest semiconductor component—the diode—performs a variety of useful functions related to its core purpose of managing the direction of the flow of electrical current. Diodes allow current to flow through them in one direction only.

Semiconductor Diode – Internal construction

Semiconductor diode consists of two, differently doped semiconductor crystals – “p” and “n” types. Together, they form so-called “p-n junction”, where the “n” layer(with electron donor dopants) has an excess amount of electrons, which are the majority carriers there (we have more electrons (-) than electron holes (+)). However, in the “p” layer (electron acceptor dopants) the majority carriers are electron holes (+) rather than electrons (-), so we have more holes “to fill”, than electrons available. The electron hole is a vacancy created by the electron “travelling” from its initial place to some other location in that crystal. In reality, there is no such thing as “a hole”, but that lack of electron kind of makes it a positively charged particle, which attracts negative electrons to form a pair again (holes can move too).
After they combine, a proportional distribution of electrons begins. Electrons, which previously lacked in “p” layer are transferred there from “n” layer, where were too many of them. So, “n” layer is a good friend for “p” layer, right? 🙂 And this is where so-called depletion region is formed, which prevents the flow of the electric current (thermodynamic equilibrium).

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 2. P-N junction in state of thermodynamic equilibrium
To allow the flow of the electric current through the “p-n junction” (electric valve on), external positive electric voltage must be applied to “push” and help large group of electrons and holes to meet together (forward bias of the diode). After they are “pushed” through the depletion region with enough force (V_F = 0,7V) diode starts conducting current, so it starts to flow through it.

. 3. P-N junction forward-biased (electric valve on)

To make sure, that the electric current won’t flow (electric valve off), it is needed to apply external negative voltage to the semiconductor diode (reverse bias) to make depletion region even larger (illustration below).

Fig. 4. P-N junction reverse-biased (electric valve off)

With passing time, technological requirements were increasing what resulted in development of new types of diodes. When a semiconductor is combined with the corresponding metal, we acquire MS junction (Metal-Semiconductor), which also possesses rectifyingproperties (current conduction in one direction) – it is used for example in fast Schottky diodes.

Types of Semiconductor Diodes

• Rectifier diode – alternating current rectification,
• Zener diode – stabilization of voltage and current in electronic systems,
• Light Emitting Diode (LED) – emits light in the infrared or visible light spectrum,
• Variable capacitance diode – its capacity depends on the voltage applied to it in the reverse bias,
• Switching diode – used in pulse electronic systems that require very fast switching times,
• Tunnel diode – specially designed diode characterized by the negative dynamic resistance region,
• Photodiode – diode that works as photodetector – it reacts to light radiation (visible, infrared or ultraviolet),
• Gunn Diode – component used in high-frequency electronics.