In our previously published articles, we have discussed about MOSFET and Darlington Transistor. These three articles including this one are important for Arduino users with a lesser idea about these semi-conductors. Our target is to distribute the basic knowledge so that they decide and purchase the correct type of transistor.
The discovery of the BJT transistor made it possible to effectively replace the tubes in the 1950s and thus improve the miniaturization and reliability of electronic circuits.
A bipolar junction transistor is a semiconductor-based electronic device of the transistor family. Its operating principle is based on two PN junctions, one live and one reverse. The polarization of the reverse PN junction by a weak electric current (sometimes called the transistor effect) makes it possible to “control” a much larger current, following the principle of current amplification. It should be noted that no electron is actually “created”: the appropriate polarization allows a reservoir of free electrons to circulate differently.
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A PN junction refers to an area of the crystal where doping varies abruptly from p doping to n doping. When the p-doped region is brought into contact with the n region, electrons and holes spontaneously diffuse on either side of the junction, creating a depletion zone, or space charge zone, where the concentration of free carriers is almost zero. While a doped semiconductor is a good conductor, the junction hardly lets current through. The width of the depletion zone varies with the voltage applied on either side of the junction. The smaller this area, the lower the resistance of the junction. The physics of PN junctions has great practical uses in the creation of semiconductor devices. The current rectifier diode as well as most other types of diodes thus contains a PN junction. Photovoltaic cells also consist of a large-area PN junction in which the electron-hole pairs created by light are separated by the electric field from the junction. Finally, a type of transistor, the bipolar transistor, is made by putting two PN junctions in opposite directions – PNP or NPN transistor.
Photo is of the initial BJT transistor (Bell Labs). It looks like a sewing machine but it is easier to understand how the thing is constructed.
Transistor catalogues have a large number of models. Bipolar transistors can be NPN or PNP. These are two complementary types, that is to say, the direction of currents and voltages for the PNP is the complement of those of the NPN. NPN transistors generally have better characteristics than PNPs (in terms of bandwidth), they are the most used.
Transistors for amplifying small signals dissipate only a few tens or hundreds of milliwatts. Medium-power transistors support a few watts; power transistors, used for example in power audio amplifiers or stabilized power supplies, can withstand, provided they are placed on a suitable heat sink, more than 100 W.
Which BJT Transistors We Commonly Use with Arduino
When we want to drive a load that needs high values of current or voltage (than Arduino can drive) and a control is required then we use a BJT transistor.
For most circuits either a BJT or MOSFET can be used. The decision depends on the load current we need to switch. For example, for 5050 LED Strip with Arduino, we would prefer MOSFET but BJT transistors will nicely work. These are commonly used BJT transistors:
- 2N3904 (NPN)
- 2N2222 (NPN)
- 2N4401 (NPN)
- BC549C (NPN)
- BC547 (NPN)
- BC557 (PNP)
- 2N3906 (PNP)
TIP 120 (it is a Darlington pair) is not bad in certain situations because FQP30N06L (MOSFET) is costlier and 2N3904, 2N3906 fail to serve the purpose.
