Diode (1) Definition and principle

1. Definition of diode

A diode is an electronic component that has two terminals (anode and cathode) that allows current to flow in one direction and stop it in the other. This characteristic allows the diode to play the role of rectification, protection, switching, etc. in the circuit.

A diode, also known as a diode, is an electronic device with two terminals (cathode terminals, hence the name "diode") with asymmetric conductance; This two-pole allows the current to be conducted, in principle, in one direction only in one direction, with low resistance (ideally zero), high current, and high resistance in the other direction. Nowadays, semiconducting materials are used for the second pole. Due to the characteristics of diodes, it is often used as a rectifier in power engineering (turning alternating current into direct current); It is often used as a geophone in electronic engineering (detecting echo waves from amplitude modulated waves); It is often used as a logic gate for logic circuits in the logic design of computer hardware.

In 1874, Germany physicist Karl · Braun discovered the rectification ability of crystals at the Karlsruhe Institute of Technology. Therefore, the first generation of diodes, the "cat's whisker diode", developed in 1906, was made of mineral crystals such as galena. Early diodes also included vacuum tubes, which had two electrodes, an anode and a thermal cathode, and the voltage added between the electrodes could make the hot electrons reach the anode from the cathode, thus having the effect of rectification. After the discovery of semiconductor properties, semiconductor diodes became the world's first semiconductor devices. Most of today's diodes are produced using silicon, but other semiconductor materials such as germanium are sometimes used. The most common structure is that a junction chip with semiconductor properties is connected to two electrical terminals via a PN junction.

2. What is a PN junction?

Intrinsic semiconductors

Intrinsic semiconductors, such as high-purity silicon, are theoretically important, but their conductivity is really poor due to their very low concentrations of free electrons and holes (about 10¹⁰ cm⁻³), with bulk resistivity of about 10⁶Ω·cm. This makes it less practical in practical applications unless there is a way to improve its conductivity.

Doping of impurity semiconductors

Through doping, the conductivity of semiconductor materials can be greatly improved. There are two main types of doping:

1. N-type semiconductors (negative semiconductors):

• 掺入5价杂质元素：如磷（P）、砷（As）。
• These impurity atoms form covalent bonds with silicon atoms, and the excess electrons become free electrons, thus increasing electrical conductivity.
• The majority of carriers are free electrons and a minority are holes.

P-type semiconductors (positive semiconductors):

• Doped with 3-valent impurity elements: such as boron (B), gallium (Ga).
• These impurity atoms form covalent bonds with silicon atoms, and the missing electrons form holes, which increases the conductivity.
• Most carriers are holes and a few carriers are free electrons.

3. The working principle of the diode

Diodes are primarily made of semiconductor material (usually silicon or germanium) with a PN junction (formed by bonding P-type semiconductors and N-type semiconductors).

How it works can be understood from the following two aspects:

1. Forward Bias:

• When the anode (P-zone) of the diode is connected to a positive voltage and the cathode (N-zone) is connected to a negative voltage, it is called forward bias.
• At this point, the barrier voltage of the PN junction decreases, allowing the carriers (electrons and holes) to cross the junction region, causing current to flow through the diode.
• The higher the forward voltage, the greater the current through the diode.

  

Reverse Bias:

• When the anode (P-zone) of the diode is connected to a negative voltage and the cathode (N-zone) is connected to a positive voltage, it is called reverse bias.
• At this point, the barrier voltage of the PN junction increases, preventing the carriers from crossing the junction region, and almost no current flows through the diode (only a very small reverse saturation current).
• An increase in the reverse voltage to a certain point (exceeding the breakdown voltage) can cause a diode breakdown, at which point the current increases dramatically, but this state usually damages the diode.

  

The diode has two terminals, an anode and a cathode, and the current can only flow in one direction. That is, current can flow from the anode to the cathode, but not from the cathode to the anode. The application of this unidirectional characteristic of the diode, often referred to as the "rectification" function, can convert alternating current into pulsating direct current, for example, the modulation of radio signals by a radio receiver is done by rectification. Because of its forward-flow and reverse blocking characteristics, the diode can be conceived as an electronic version of the check valve. In practice, however, diodes do not exhibit such perfect switching, but rather exhibit more complex nonlinear electronic signatures – depending on the specific type of diode technology. In general, the diode will only operate if the barrier voltage is exceeded in the forward direction (this state is known as forward bias).

The change in voltage drop across a forward-biased diode is only marginally related to current and is a function of temperature. This feature can therefore be used for temperature sensors or voltage references. The nonlinear current-voltage characteristics of semiconductor diodes can be changed according to the selection of different semiconductor materials and the doping of different impurities to form impurity semiconductors.

In addition to being used as a switching method, the modified diode has many other functions, such as regulating the voltage (Zener diode), limiting high voltage to protect the circuit (avalanche diode), radio tuning (varactor diode), generating RF oscillations (tunnel diode, Geng's diode, IMPATT diode) and generating light (light-emitting diode). Among semiconductor diodes, there are PN junction effects that use the bonding surfaces of two types of semiconductors, P-type and N-type, and there are also types that use the Schottky effect generated by the bonding of metals and semiconductors to achieve rectification. In the case of a PN junction diode, the P-type side is the anode, and the N-type side is the cathode.