2. Semiconductor components

Semiconductors such as diodes or transistors are the silicon-based components that have developed modern electronics to the point of transforming our entire society.

In this unit we will study the fundamentals of electronic components, their operation and typical electronical diagrams.



Insulating materials such as plastic or wood do not allow the passage of electronical current. Conductive materials such as copper or aluminum allow the passage of electronical current very easily. On the other hand, semiconductor materials such as silicon or germanium can behave as insulators or conductors depending on the voltage they receive. This behavior can be exploited to make circuits that have very fast, electronically controlled semiconductor switches.

In order for semiconductors to conduct current, it is necessary to alloy them with traces of elements that provide positive charges (Boron, Indium, etc.). or negative charges (Phosphorus, Arsenic, etc.) So that an already alloyed semiconductor can be called P type (positive) or N type (negative).

The diode

It is the simplest electronic component that can be made from semiconductor materials and has two terminals. Internally it is formed by the union of a silicon block of P type with a silicon block of N type. This junction allows current to flow in one direction, but does not allow it to flow in the opposite direction.

Símbolo del diodo semiconductor.

In the previous image you can see the symbol of the diode, an arrow in the direction in which it allows the passage of current and the name of its two terminals. The diode only conducts when the anode has positive voltage and the cathode has negative voltage.

The following image is a photograph with various types of diodes.

Fotografía de varios diodos semiconductores.

Diodes have multiple applications. For example, rectify alternating current, regulate voltages or emit light (LED diodes).

Schematic of a polarized LED diode with a resistor that reduces the current so that it does not burn out.

Esquema de un diodo led polarizado con una resistencia.

Diagram of a rectifier diode that converts the alternating voltage of the electronical network into direct voltage.

Esquema de un diodo rectificador.

The transistor

The transistor is an electronic component with three terminals that allows the passage of electronic current between two terminals according to the voltage received by the third terminal. It is like a voltage controlled switch. The first silicon transistor was commercialized in 1954.

Transistor de potencia.

Power transistor

Transistores de señal.

Signal transistor.

Símbolo del transistor bipolar.
Símbolo del transistor MOSFET.

Transistor states

Depending on the control voltage that the transistor receives through the base or through the gate, it can be in three different states.

Cutoff: the transistor does not conduct current, it behaves like an open switch.

Saturation: the transistor conducts as much current as possible and behaves like a closed switch.

The two previous states are used in digital circuits such as a computer, TV, smartphone, etc.

Linear zone: the transistor only conducts part of the current and behaves like a resistor.

This behavior is used in analog circuits such as sound amplifiers.

Typical circuits

amplifier transistor. This circuit works like a light amplifier. When the resistor LDR lights up, the current through it increases. That current reaches the base of the transistor and is amplified by the transistor through the collector, turning on the connected lamp. This is an analog circuit because the transistor works in a linear zone, behaving like a resistor controlled by the base current.

Símbolo del transistor MOSFET.

Amplifier transistor.

digital transistor. This circuit is a NOR logic gate formed from transistors. Thanks to the parallel of the two collectors, the output only has high voltage when the two inputs are at low voltage. These logic gates are the basis of digital circuits and computers.

Símbolo del transistor MOSFET.

Digital transistor.

LDR resistors

LDRs (Light Dependent Resistors) are, as their name suggests, sensors that detect light. Their resistance is reduced when the lighting is greater, increasing the current they conduct the more light they receive.

Symbol and photograph of an LDR resistor.

Símbolo de una resistencia LDR.
Fotografía de una resistencia LDR.

Integrated circuits

An integrated circuit is a small silicon tablet, also called a chip, that contains a multitude of electronic components inside.

Fotografía de una resistencia LDR.

LM555 integrated circuit.

Fotografía de una resistencia LDR.

DAC08 integrated circuit.

With the development of technology, each year the size of the components is reduced, being able to group more and more transistors in a single integrated circuit. In the early 1960s the aerospace industry began purchasing circuits that integrated up to 100 transistors on a single chip. This lowered the prices of production and encouraged the development of technology. At the beginning of 1980, chips with 100 thousand transistors could already be bought, in 2000 100 million transistors and in 2020 100 billion transistors on a single chip. This exponential growth in the number of transistors integrated into a chip that doubles every year and a half is known as Moore's law and has allowed the development of the digital society that we all know, with a multitude of smart devices, memories, cameras, drones, etc. . based on these powerful integrated circuits.


  1. What types of materials are there depending on how they conduct electricity? Write two examples of each.
  2. Why are semiconductors so useful?
  3. What does it take to make a semiconductor conduct electric current?
  4. How is a semiconductor diode built?
  5. Draw the symbol for a semiconductor diode and name its terminals.
  6. When does a diode conduct current?
  7. Draw two electronical schematics with diodes.
  8. What applications do diodes have?
  9. What is a transistor? How many terminals does it have?
  10. What states can a transistor have?
  11. What transistor states are used in analog circuits? And in the digital ones?
  12. Draw the symbol of a bipolar transistor and a MOSFET with the names of their legs.
  13. Draw a circuit with a transistor working as a light amplifier.
  14. Draw a NOR logic gate with transistors.
  15. What is an LDR and what do those acronyms stand for?
  16. What is an integrated circuit or chip?
  17. When did integrated circuits start being made and how many transistors did they have?
  18. Draw a graph with the number of transistors that a chip contains. On the X axis, place the years and on the Y axis the number of transistors in exponential scale (10, 100, 1000, 10 thousand, etc.)
  19. What is Moore's law?