atomic structure of semiconductors
Silicon, which covers 28% of the Earth's surface, is considered a common material with an atomic number of 14. The image shows the atomic structure of silicon: silicon has four electrons in its outer rings, called valence electrons. In some cases, these valence electrons can bond with the valence electrons of other atoms. When this happens, covalent bonds are formed, resulting in a crystalline lattice structure.
Pure silicon is a good insulator, meaning it cannot conduct electricity. However, by adding impurities to the pure silicon structure, the conductivity of this semiconductor can be altered.
During doping, impurities are added to a pure semiconductor material, making it extrinsic (which is intrinsic relative to a pure semiconductor). Some of the most common types of impurities are trivalent (three valence electrons), such as boron and gallium, and pentavalent (five valence electrons), such as arsenic and antimony.
Suppose a pure silicon semiconductor is doped with pentavalent arsenic. The four valence electrons of arsenic will form covalent bonds with the four valence electrons of silicon, while one electron from the arsenic remains free. In this example, the arsenic essentially "donates" a free electron to the silicon structure.
All pentavalent impurities "donate" free electrons and are called donor impurities. As a result, in the structure formed, conduction occurs through electrons, and the crystal is called an N-type crystal.
Trivalent impurities can only form covalent bonds using three electrons; therefore, one electron is needed to complete the crystal lattice structure, and a hole is retained to replace the lost electron. Because this hole can accept an electron, the trivalent impurity is called an acceptor impurity. In the resulting structure, electricity is conducted through the hole, and the crystal is called a p-type crystal.
pn junction architecture
In P-type semiconductors, group III elements of the periodic table are added as dopants, while in N-type semiconductors, group V elements are used as dopants. In P-type semiconductors, the majority carriers are holes, and the minority carriers are electrons.
A pn junction is an interface composed of two different types of doped semiconductors. In the p-region, there is a trivalent dopant element with one more gap (more positive charge) than the semiconductor forming the junction, while in the n-region, there is a pentavalent dopant element with more electrons (more negative charge).
A "junction" refers to the region where two dopants (P and N) are connected. Therefore, if the P-type and N-type layers are very close together, a vacancy region is created at the junction. This occurs when holes in the P-region tend to approach the N-layer and electrons in the N-region tend to move towards the P-region. Thus, the vacancy region appears to be free of charge because adjacent charges cancel each other out.
As a result, an electric field is generated in the junction from right to left (from positive to negative), and a potential difference delta V is generated in the opposite direction of the electric field.
The P-region of a semiconductor is also called the acceptor region because it accepts electrons into its region, while the N-region is also called the donor region because it supplies electrons to the P-region.
Assume the p-n junction is electronically connected to the battery. If the P-region of the junction is connected to the positive terminal of the battery and the N-region is connected to the negative terminal, then the pn junction is connected in a direct polarization manner.
In this scenario, current will flow through the pn junction because the negative charges in the N region are attracted to the positive terminal of the battery. As this occurs, they reach the positive terminal after overcoming the potential difference in the junction. Simultaneously, holes are neutralized by electrons.
Furthermore, if the pn junction is reverse-connected, it should be connected to the P region, which is connected to the negative terminal of the battery, and the N region, which is connected to the positive terminal. During this process, no current will flow within the junction because the vacated region will expand, preventing any charge from passing through.
A diode is a passive nonlinear electronic component with two terminals. Its function is to allow current to pass through in only one direction, i.e., direct polarization, and to prevent current from passing through when the polarization is reversed.
Transistors and their types
The first working prototype of the transistor was developed at Bell Telephone Laboratories in 1947. The invention of the transistor is indeed one of the most relevant discoveries in the field of electronics, both digital and analog.
We will focus on bipolar transistors obtained by properly connecting two pn junctions, also known as bipolar junction transistors (BJTs). More precisely, a BJT consists of three doped semiconductor regions, like this:
The two lateral regions, called the emitter and collector, belong to the same type (P or N).
• The central region, called the base, is the opposite of the type of the lateral region.
As shown in Figure 4, there are two bipolar transistor configurations: PNP and NPN. There will be two potential barriers that produce two opposite crossover points.
In order to achieve a transistor effect instead of two simple opposing diodes, the following manufacturing limitations must be met:
• The emitter and collector regions must be heavily doped, thus having abundant free charges, while the central region must be weakly doped, thus having very poor free charges.
• The base must be very narrow and its thickness comparable to the potential barrier. This way, the time from emitter to collector in most chargers will be very short, and very little charge can combine at the base.
The main difference between NPN and PNP transistors is:
In a PNP transistor, the majority of charge carriers are holes, while in an NPN transistor, electrons are the majority of charge carriers.
When current flows through the base of the transistor, the NPN transistor turns on. In an NPN transistor, current flows from the collector to the emitter.
A PNP transistor is turned on when there is no current at its base. In a PNP transistor, current flows from the emitter to the collector.
Transistors can operate in three modes:
• Amplifier (also known as active mode). In this mode, the emitter-base junction is forward biased, while the collector-base junction is reverse biased. This causes current to flow from the emitter to the collector, and its value is proportional to the base current.
• Off-mode. Both emitter-base and collector-base junctions are reverse biased. No current flows except for a negligible quiescent current; the transistor is off.
• Saturation mode. Both the emitter-base and collector-base junctions are forward biased. All current flows from the emitter to the collector, and the transistor is turned on.
As mentioned above, transistors are typically used as current amplifiers or as switches to enable/disable the loads connected to them.
