Analog electronics, Electronics, Electronics components, Semiconductors

Diodes

In this post I will talk about diodes which are semiconductors components which make the current flow in only one direction.

What it is made?

Diode are made for two types of silicon, a silicon material with some boron atoms so that this material lack electrons or gap excess, this material is called type P. Other type of silicon has some phosphorus atoms so that the material has excess electrons, this is the type N. Exist germanium diodes instead of silicon, but the silicon ones are the most used.

Operation

This figure has the schematic diode representation (below) and the low power physical aspect (above).

There are two operation modes: direct polarization and reverse. In direct polarization, the cathode diode is linked to the negative pole of DC source and the anode to the positive pole, this way the current goes to the diode’s anode. This is a circuit with the diode in a direct polarization.

In direct polarization the electrons from N side and the gaps (lack of electrons) from P side close to the junction go to opposite side creating a depletion region in the frontier in P and N sides, this depletion region gives the diode a tension fall of 0.7 V. In germanium diodes the tension fall is 0.3 V.

In reverse polarization the anode is linked with the DC source negative pole and the cathode with the positive pole, in this configuration the electrons from negative pole go to the diode’s P side while the electrons from diode’s N side are attracted by the positive pole of DC source.

The result is the expansion of depletion region and circulate only a very small current called reverse saturation current represented by I_{s}, is a so slow current that in some cases it is considered that there is no current in reverse polarization.

Here is the characteristic curve of diode, in the right side is the direct polarization and the left side is the reverse polarization.

When the reverse tension increases too much, there is a point where increases too much the current in the reverse direction, the diode enter in the called zener region which stays in the work zone on the left of the graphic.

This is the equation to the forward bias curve (in direct polarization):

I_{d}=I_{s}(e^{v_{D}/nV_{T}}-1)

I_{d} is the direct current, I_{s} is the reverse saturation current, v_{D} is the temperature in Kelvin, n is the ideality factor and V_{T} is the thermal voltage and is calculated in the following way:

V_{T}=k\frac{T}{q}

k is the Boltzmann constant and have 1,38\times 10^{-23} J/K, T is the temperature in Kelvin and q is the electron charge which value 1,6\times 10^{-19} C.

Applications

Diodes have many applications, one of them is the construction of digital logic doors.

Other very used application is rectification of CA signals, that is, transform a CA signal in CC.

Others applications examples are:

  • Protection;
  • Warranty polarity;
  • The logic door OR can be used like backup supply system;
  • Establish a voltage level independent of current.

 

 

 

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About Pedro Ney Stroski

84 thoughts on “Diodes

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