1.1 Introduction to Semiconductors
Semiconductor devices are fundamental components of electronic circuits, and they are made from semiconductor materials. Semiconductor materials are defined as substances with electrical conductivity Between conductors and insulators. In addition to having conductivity between that of conductors and insulators, semiconductors also possess the following properties:
1,A rise in temperature can significantly enhance the conductivity of semiconductors. For example, the resistivity of pure silicon (Si) doubles when the temperature increases from 30°C to 20°C.
2,Trace amounts of impurities (their presence and concentration) can drastically alter the conductivity of semiconductors. For instance, if one impurity atom (such as a +3 or +5 valence element) is introduced per million silicon atoms, the resistivity at room temperature (27°C; why is room temperature 27°C? Because absolute temperature is an integer, T=273+t=273+t, and the closest value for T is 300 K, hence t is 27°C) decreases from 214,000 Ω·cm to 0.2 Ω·cm.
3,Light exposure can significantly improve the conductivity of semiconductors. For example, a cadmium sulfide (CdS) film deposited on an insulating substrate has a resistance of several megohms (MΩ) in the absence of light, but under illumination, the resistance drops to several tens of kilohms (kΩ).
4,Additionally, magnetic and electric fields can also markedly alter the conductivity of semiconductors.
Therefore, semiconductors are materials with conductivity Between conductors and insulators, and their intrinsic properties are highly susceptible to significant changes due to external factors such as light, heat, magnetism, electric fields, and trace impurity concentrations.
Given these advantageous properties, semiconductors can be effectively utilized. In particular, the subsequent discussions on diodes, transistors, and field-effect transistors will demonstrate how the property of trace impurities significantly altering semiconductor conductivity is harnessed.
1.2 Intrinsic Semiconductors
How do we introduce trace impurities into semiconductors? Can we directly add impurities to natural quartz (whose main component is Si)? We cannot use natural silicon directly because it contains various impurities, which make its conductivity uncontrollable. To serve as the fundamental material for all semiconductors, the primary goal is to achieve controllable conductivity.
Therefore, we need to purify natural silicon into a pure silicon crystal structure. This pure semiconductor crystal structure is referred to as an intrinsic semiconductor.
Characteristics of intrinsic semiconductors: (Intrinsic semiconductors are pure crystal structures)
1,Purity, meaning no impurities.
2,Crystal structure, representing stability. The atoms are bound to each other, preventing free movement, which results in even lower conductivity compared to natural silicon.
1.2.1 Crystal Structure of Intrinsic Semiconductors
In chemistry, we learned that the outermost electrons of two adjacent silicon (Si) atoms in a crystal become shared electrons, forming covalent bonds. However, not all outermost electrons of each Si atom remain strictly within their own covalent bonds. The reason for this is that the material exists in an environment with temperature. In addition to ordered motion, the outermost electrons also undergo thermal motion-random movement-due to the influence of temperature. Occasionally, an electron may possess higher energy than other atoms, allowing it to break free from the covalent bond and become a free electron. Even with a small amount of energy, the outermost electrons of a conductor can generate directional motion.
Intrinsic semiconductors are free of impurities. When an electron breaks free from a covalent bond, it leaves behind a vacancy known as a hole. In intrinsic semiconductors, the number of free electrons is equal to the number of holes, and they are generated in pairs. The crystal structure, holes, and free electrons are illustrated in the figure below:
1.2.1 Crystal Structure of Intrinsic Semiconductors (Continued)
If an external electric field is applied across an intrinsic semiconductor:
1,Free electrons move directionally, forming an electron current.
2,Due to the presence of holes, valence electrons move in a specific direction to fill these holes, causing the holes to also undergo directional movement (since free electrons and holes are generated in pairs). This movement of holes forms a hole current. As free electrons and holes carry opposite charges and move in opposite directions, the total current in an intrinsic semiconductor is the sum of these two currents.
The above phenomena demonstrate that both holes and free electrons act as particles carrying electric charge (such particles are called charge carriers). Thus, both are charge carriers. This distinguishes intrinsic semiconductors from conductors: in conductors, there is only one type of charge carrier, whereas in intrinsic semiconductors, there are two types of charge carriers.
1.2.2 Carrier Concentration in Intrinsic Semiconductors
The phenomenon where a semiconductor generates free electron-hole pairs under thermal excitation is called intrinsic excitation.
During the random motion of free electrons, when they encounter holes, the free electrons and holes simultaneously disappear. This phenomenon is called recombination. The number of free electron-hole pairs generated by intrinsic excitation equals the number of free electron-hole pairs that recombine, achieving a dynamic equilibrium. This means that at a certain temperature, the concentrations of free electrons and holes are the same.
When the ambient temperature rises, thermal motion intensifies, and more free electrons break free from the constraints of valence electrons, leading to an increase in holes. Consequently, the carrier concentration increases, enhancing conductivity. Conversely, when the temperature decreases, the carrier concentration decreases, reducing conductivity. When the temperature drops to absolute zero (0 K), valence electrons lack the energy to break free from covalent bonds, resulting in no conductivity.
In intrinsic semiconductors, conductivity involves the movement of two types of charge carriers. Although the conductivity of intrinsic semiconductors depends on temperature, it remains extremely poor due to their crystalline structure. Despite their poor conductivity, intrinsic semiconductors exhibit strong controllability in their conductive properties.
1.3 Doped Semiconductors
This section will explain why intrinsic semiconductors exhibit such strong controllability in conductivity. Here, we will utilize the following property of semiconductors: trace amounts of impurities can significantly alter their conductivity.
"Doping" refers to the process of introducing appropriate impurity elements into an intrinsic semiconductor. Depending on the type of impurity elements added, doped semiconductors can be classified into N-type semiconductors and P-type semiconductors. By controlling the concentration of the impurity elements, the conductivity of the doped semiconductor can be precisely regulated.
1.3.1 N-Type Semiconductor
"N" stands for Negative, as electrons carry a negative charge and are lightweight. To introduce additional electrons into the crystal structure, pentavalent elements (e.g., phosphorus, P) are typically doped into the semiconductor. Since a phosphorus atom has five valence electrons, after forming covalent bonds with surrounding silicon atoms, one extra electron remains. This electron can easily become a free electron with minimal energy input. The impurity atom, now fixed in the crystal lattice and lacking an electron, becomes a immobile positive ion. This is illustrated in the figure below:
1.3.1 N-Type Semiconductor (Continued)
In an N-type semiconductor, the concentration of free electrons is greater than that of holes. Therefore, free electrons are called majority carriers (multipliers), while holes are called minority carriers (minors). Thus, the conductivity of an N-type semiconductor primarily relies on free electrons. The higher the concentration of doped impurities, the greater the concentration of majority carriers, and the stronger the conductivity.
Let us examine how the concentration of minority carriers changes when the majority carrier concentration increases. The minority carrier concentration decreases because the increased number of free electrons raises the probability of recombination with holes.
When the temperature rises, the number of carriers increases, and the increase in majority carriers is equal to the increase in minority carriers. However, the percentage change in minority carrier concentration is higher than that of majority carriers (due to the different base concentrations of minorities and majors, even though the numerical increase is the same). Therefore, although the concentration of minority carriers is low, they should not be underestimated. Minority carriers are a critical factor affecting the temperature stability of semiconductor devices, and thus their concentration must also be considered.
1.3.2 P-Type Semiconductor
"P" stands for Positive, named after the positively charged holes. To introduce additional holes into the crystal structure, trivalent elements (e.g., boron, B) are typically doped into the semiconductor. When a boron atom forms covalent bonds with surrounding silicon atoms, it creates a vacancy (which is electrically neutral). When a valence electron from a neighboring silicon atom fills this vacancy, the covalent bond generates a hole. The impurity atom then becomes an immobile negative ion. This is illustrated in the figure below:
1.3.2 P-Type Semiconductor (Continued)
Compared to N-type semiconductors, in P-type semiconductors:
Holes are the majority carriers, while free electrons are the minority carriers.
Conductivity primarily relies on holes. The higher the concentration of doped impurities, the greater the concentration of holes, leading to stronger conductivity (as the vacancies in impurity atoms absorb electrons). The minority carrier concentration decreases.
When the temperature rises, the percentage change in free electron concentration is higher than that of hole concentration.
