Semiconductor Fundamentals: Intrinsic, N-Type, and P-Type
Semiconductor Fundamentals
A semiconductor is a material element whose electrical conductivity lies between that of an insulator and a conductor.
The best-known semiconductors are silicon (Si) and germanium (Ge). Because the behavior of silicon is more stable than germanium against external factors that can alter its normal response, silicon (Si) is the most commonly used semiconductor element in the manufacture of solid-state electronic components. We will generally refer to silicon, keeping in mind that the process for germanium is quite similar.
Like all atoms, the silicon atom has many positive charges in its nucleus and electrons orbiting around it. In the case of silicon, this number is 14. Semiconductor behavior is related to its ability to give rise to a current, that is, a movement of electrons. As you know, an electron feels more connected to the nucleus the closer it is. Thus, the electrons with less attractive force to the nucleus, which are found in the outer orbits, can be released from it. These electrons can be freed by injecting a little energy. Our focus lies on these electrons, so instead of using the complete model of the silicon atom (Figure 1), we use the simplified representation (Figure 2), which highlights the area of interest.
The shaded area in Figure 2 represents a simplified version of the shaded area in Figure 1.
As can be seen in the figure, there are four electrons that can potentially be released from the nucleus’s attractive force.
Intrinsic Semiconductors
When silicon is formed by atoms of the type described in the previous section, it is said to be in pure form, or more commonly, it is an intrinsic semiconductor.
A bar of pure silicon is formed by a group of atoms linked together according to a specific geometric structure known as a lattice.
If we inject energy from outside under these conditions, some of the electrons in the outer shells will no longer be bound and can move. Logically, if an electron is removed from the atom, the atom is no longer complete; we say that it is positively charged because it has one less negative charge, or that a hole has appeared. We then associate the hole with a positive charge or the site previously occupied by the electron.
The atom will always tend to be in its normal state, with all its charges, so in our case, it will try to attract an electron from another atom to fill the hole.
Any external energy injection occurs as a continuous process where we can observe two things:
- Electrons are released and move from one atom to another along the bar of silicon semiconductor material.
- Appearance and disappearance of holes in the various atoms of the semiconductor.
It is thus clear that the only real movement that exists within a semiconductor is that of electrons. However, the holes appear and disappear, representing “positive charges” at different points in the semiconductor, making them seem to move, resulting in a stream of positive charges. This apparent movement of holes is not real; the holes do not move, it just seems like they do.
Nevertheless, to facilitate the study of semiconductors, we will talk about hole current (positive charges) as it is more convenient, and the results are the same as the actual electron current.
Semiconductor Doping
Applying a voltage to the silicon crystal, the positive terminal of the battery will try to attract the negative electrons and the holes, thus promoting the emergence of a current through the circuit.
Direction of movement of an electron and a hole in the silicon.
However, this tendency that appears is of very little value because few electrons can be dislodged from the bonds between silicon atoms. To increase the value of this current, we have two possibilities:
- Applying a higher voltage.
- Introducing electrons or holes from outside the semiconductor.
The first solution is not feasible because, even with a significant increase in the applied voltage, the resulting current is insufficient. The chosen solution is the second.
In the latter case, we say that the semiconductor is “doped.”
Doping is the process of replacing some silicon atoms with atoms of other elements. These latter elements are known as impurities. Depending on the type of impurity used to dope the pure or intrinsic semiconductor, there are two kinds of semiconductors:
- P-type semiconductor
- N-type semiconductor
N-Type Semiconductor
If in a crystal lattice of silicon (silicon atoms linked together)…
Covalent bonding of germanium atoms; note that each atom shares each of its electrons with four other atoms.
…we replace one of its atoms (which we know has 4 electrons in its outer layer) with an atom of another element that contains five electrons in its outer shell, four of those electrons will be used to link with the rest of the lattice atoms, and the fifth will be free.
N-type doped semiconductor
This network of silicon “doped” with this kind of impurity is called “N-type silicon.”
In this situation, there is a greater number of electrons than holes. Thus, the holes are called “minority carriers,” and the electrons are called “majority carriers.”
The most commonly used N-type impurities in the doping process are arsenic, antimony, and phosphorus.
It is clear that if a doped semiconductor has a voltage applied to its terminals, the possibility of a current occurring in the circuit is higher than if the same voltage were applied to an intrinsic or pure semiconductor.
P-Type Semiconductor
If in a crystal lattice of silicon (silicon atoms linked together)…
Covalent bonding of germanium atoms; note that each atom shares each of its four electrons with other atoms.
…we replace one of its atoms (which we know has 4 electrons in its outer layer) with an atom of another element that has three electrons in its outer layer, these three electrons will fill the gaps left by the electrons of the silicon atom. However, since there are four gaps, one will remain unfilled. Thus, replacing one atom with another causes the appearance of holes in the silicon crystal. So now the “majority carriers” are the holes, and the electrons are the “minority carriers.”
This network of silicon doped with this kind of impurity is called “P-type silicon.”
Doped P-type semiconductor
Comments
Doped semiconductors are represented within diagrams, indicating the type of majority carriers.
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N-type semiconductor | P-type semiconductor |
The doping level of a semiconductor is not always the same; it can be “lightly doped,” “heavily doped,” etc.
The (+) sign is used to indicate that a semiconductor is heavily doped.
N-type heavily doped semiconductor. P-type heavily doped semiconductor.
All solid-state electronic components that we will see later (transistors, diodes, thyristors) are essentially sets of semiconductors arranged in different ways.
Bias
If we apply an external voltage to this junction that is opposite to the internal potential barrier, the barrier’s width will decrease. The higher the externally applied voltage, the lower the internal barrier, and we can even make the barrier disappear completely.
At this point, the electrons (majority carriers) in the N region 
can move to the P region. Similarly, the holes in the P region will want to “pass” to the N region.
a) Non-polarized
b) Weak forward bias; the depletion region is reduced but not eliminated.
c) By increasing the bias, the depletion region and its associated internal potential barrier have been eliminated.
* In practice, a diode is made from a single piece of silicon, introducing different types of impurities into two regions, creating P-type material in one and N-type material in the other. This process is performed at high temperatures. |
The external voltage that cancels the potential barrier and leaves the junction ready for the passage of the respective majority carriers is called the threshold voltage. It is represented by Vu, and its practical values are:
- For Si: Vu = 0.4 – 0.5 Volts
- For Ge: Vu = 0.05 – 0.06 Volts
In this situation, by applying an increased external voltage, the electrons will be attracted to the positive pole of the battery and the holes to the negative pole. There is no difficulty in crossing the junction, and therefore a majority current will flow through the circuit. From here, any increase in voltage causes an increase in current.
The set of voltages that create a current proportional to the diode are called bias voltages or operating voltages. Their typical values are:
- For Si: 0.5 to 0.8 Volts
- For Ge: 0.06 to 0.15 Volts
Current flow in a forward-biased diode.
It might seem that there will come a time when, by increasing the external voltage and thus the current in the junction, the process must stop. This is because, from a certain value of the applied external voltage, electrons are neutralized by a greater number of holes inside the diode, and fewer can reach the external circuit. This means that the increase is absorbed by the diode itself. The voltage at which the current through the diode remains constant (it increases slightly in practice) is called the saturation voltage.
Typical values are:
- For Si: Vsat = 0.8 to 0.9 Volts
- For Ge: Vsat = 0.15 to 0.2 Volts
Any attempt to cause an increase in current beyond this point may result in the destruction of the diode.
Reverse Bias
If the externally applied voltage to the diode has the same sign as the internal potential barrier, the diode is said to be reverse biased. The positive terminal of the battery attracts electrons from the N material away from the junction, while the negative terminal attracts positive charges from the P material, also separating them from the junction. This creates a region in the junction with no charge, forming a current called “reverse saturation current” or “leakage current.” Its value is negligible, on the order of nA (nanoamperes).
The width of the depletion layer increases with reverse bias.
a) No reverse bias.
b) By applying a reverse bias, the width of the depletion layer increases.
As the reverse voltage increases, a point is reached where the diode loses its ability to block, and a large reverse current flows. This voltage is called the “breakdown voltage.” Normally, in this situation, the diode is destroyed.