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History of electronics: Transistors

Updated: May 7, 2021

The transistor is a semiconductor device with three terminals for electrical connection. The name comes from the English transfer resistor which has numerous applications depending on some characteristics.

A semiconductor device is an electronic component, usually composed of Silicon or Germanium, which has as its main characteristic the intermediate electrical conductivity between conductive materials (Copper, Silver, etc.) and insulating materials (Glass, plastic, etc.), besides being sensitive to environmental conditions such as temperature and electrical state (for example, polarization).

This type of technology was already known since the beginning of the 20th century with the invention of the thermionic triode with a vacuum tube. However, the triode has limitations because it is fragile (due to the body being made of glass) and had a high energy consumption. With this, alternatives were created such as relays and valves that improved the performance in relation to the previous components, but still with several limitations.

The watershed was in the post-World War II period when the greatest need was in the communications and information technology sector. At that time, experiments began resulting in the amplification of signal power, later being developed what we know as a transistor and became popular in the 1960s.

Transistores Types

There are several types of transistors, mainly those associated with integrated circuits and chips. However, there are also those of practical use and the main ones are:

  • BJT (Bipolar junction transistor)

There are two types of BJT transistors, the PNP and the NPN. The P region (Positive) silicon is doped by an element with 3 electrons in the valence layer (usually Boron) and in the N region (Negative), it is doped by an element with 5 electrons in the valence layer (usually phosphorus). It has three terminals: Collector (C), Emitter (E) and Base (B).

This type of transistor has two applications: .

Switching: The transistor works as an open circuit and short circuit. For the open circuit condition, the transistor enters the cut-off region, where no current is allowed to pass through the collector and exit the transmitter. Only when there is current passing through the base, a high voltage appears between it and the emitter, breaking the potential barrier created between them, which enters a saturation state (saturation occurs when the collector receives a high current), acting as a short circuit.

This applies to NPN type, if it is PNP type, saturation occurs when there is no current in the base and cut when there is current.

At UFRJ Nautilus, we will have two examples of the use of switching, one of them is in the activation of a pneumatic valve in the torpedo and the other is in the activation of an electromagnet that drives the ball release system. In both cases, the base receives a digital signal from an ATMega chip.

Amplifier: Between the cut and saturation region, there is the amplification region, also known as active mode that has as main characteristic the current gain, which has the current relation of the base / current of the collector. To achieve this application, it is necessary the addition of resistors in the circuit so that the voltage between the collector and the emitter is lower than the saturation.

  • UJT (Unijunction Transistor)

The terminals are Transmitter, Base 1 and Base 2. The silicon plate is doped with N type impurity (explained above). In addition, there is a PN junction formed at the border between the Emitter terminal and the Silicon plate.

Just like the BJT, it has some applications:

Oscillator circuits: A voltage is applied to the circuit, the capacitor, connected to the emitter, is charged. When it reaches the trigger value of the emitter, the capacitor discharges into the transistor, exiting through Base 1. Then the whole cycle is repeated.

Sawtooth signal generator: it is a circuit derived from the oscillator, being more connected to the supply current ratio and capacitance of the capacitor. In this case, the current must necessarily be constant, for the capacitor to be charged linearly and with faster discharge tending to be instantaneous. The circuit is basically the same assembly.

  • FET (Field Effect Transistor)

It works through an electric field at the junction. Its main characteristic is a high impedance, which in some cases can replace a current transformer. This type has 3 terminals: Door (G), Source (S) and Drain (D).

That’s two types:

MOSFET: Metal Oxide Semiconductor Field Effect. It is composed of a channel of semiconductor material (silicon or germanium) with P or N type doping and the polycrystalline silicon terminals.

JFET: Joint FET uses materials that carry load perpendicularly and in direct contact with its channel so that the passage of electric current can be controlled and can be doped with type P or type N.

FET has the same applications as BJT, however, in relation to amplification, FET amplifies the voltage signal and not the current, due to its operation being through an electric field. It is more common to use MOSFET because it has a higher gain.

FET type transistors also have the versatility to be polarized in several different ways depending on the application.

  • Darlington Transistor

Combination of two or more bipolar transistors on a chip. It was the beginning of the creation of what would become integrated circuits. It is useful in relation to the space occupied by the transistors and it is also possible to achieve a greater current gain. It is very useful for circuits where a high current control with low frequency is desired.

From this composition, the first ICs (Integrated Circuits) with numerous applications such as computer chips, microcontrollers, among others, emerged.


  • Two thousand 45 nanometer transistors fit the thickness of a hair shaft.

  • A 45 nanometer transistor can change state (on and off) about 300 billion times per second.

  • The chips of the most modern processors can count up to 30 billion transistors.

Written by Lucas Alexandre


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