Semiconductor Properties, Types, and Applications in Electronics
Intrinsic Semiconductors: Semiconductors whose electrical properties are due to their own nature, that is, the atoms such as composition (electronic configuration) and its crystal structure.
Extrinsic Semiconductors: Semiconductors in which impurities, either givers or acceptors, are added. The number of electrons and holes (charge carriers that are normally equal in intrinsic semiconductors) are different, so we can talk about majority carriers (electrons or holes).
N-type Extrinsic Semiconductor: At ambient temperature, the thermal energy is not sufficient to promote electrons from the extrinsic level to the conduction band. As temperature increases, electrons from the donor level are promoted to the conduction band, leaving ionized impurities. At high temperatures, all impurities are ionized, creating a depletion zone. The conductivity remains constant until the temperature is high enough to promote electrons from the valence band (intrinsic behavior).
P-type Extrinsic Semiconductor: Behavior is similar to n-type, with saturation zones where conductivity is constant. As in n-type, at a certain temperature, all impurities are ionized, and the concentration of carriers is approximately equal to the concentration of dopants introduced.
Charge Carrier Mobility: This magnitude (μ) is defined as the drift velocity (vd) of the carrier per unit of electric field (E).
Mobility can be directly determined by measuring the drift velocity caused by a known electric field. The electric field can be deduced by measuring the voltage and distance.
If p-type: elements from group IIIb of the periodic table, such as boron, aluminum, and gallium, are used. If n-type: elements from group Vb (N, P, As, Sb, Bi) are used.
In a semiconductor, electron (n) and hole (p) concentrations are not independent; if one increases, the other must decrease. This effect is known as the law of mass action. For doping, the equation is:
where ni is the intrinsic carrier concentration.
The Hall Coefficient: This method allows us to determine the type of majority carrier, concentration, and mobility. The Hall coefficient (RH) is:
If the charge carriers are electrons, RH will be negative; if they are holes, RH will be positive. The Hall coefficient is inversely proportional to the density of charge carriers, making it a simple way to determine carrier density.
Increasing temperature decreases mobility and therefore conductivity. The effects of temperature include increased atomic dispersion, increased atomic vibrations, and a weaker effect on the depression of free charge carriers.
Energy Gap: A simple method to determine the energy gap (Eg) is to study optical transmission. Light is shone through a thin semiconductor sheet, and the transmission coefficient is plotted as a function of radiation frequency. If hv < Eg, electrons cannot be promoted from the valence band to the conduction band. If hv > Eg, electrons transition to the conduction band, causing a sudden change in transmission.
Reverse Bias: In n-type material, electrons (majority carriers) are attracted to the positive terminal of the battery, away from the junction. Similarly, holes in p-type material are attracted to the negative terminal. This movement increases the width of the depletion region and prevents majority carrier current flow across the junction. However, a small current due to minority carriers can flow.
Bipolar Junction Transistor (BJT): In an NPN transistor, a current of electrons flows when a negative charge is applied, resulting in a reverse current. This transistor consists of two PN junctions. A signal introduced into the low-resistance circuit appears amplified in the high-resistance output circuit. In a PNP transistor, the current consists of holes, but the behavior is similar (with opposite directions).
Field-Effect Transistor (FET): Works similarly to a BJT but is based on induction from charge phenomena. The field effect can be studied in a silicon crystal doped with acceptor impurities. An insulator (SiO2) is grown on top, followed by metal plates. Applying a voltage induces negative charges at the positive pole, creating an n-type surface channel. Conduction is supported by electrons in this area. The same can be done with an n-type semiconductor to create a p-type surface.
Purification of Semiconductor Crystals (Fractional Solidification): In a binary system of a semiconductor and a small amount of soluble impurity, the base element can be purified through fractional solidification. Consider an alloy with initial impurity concentration C0. If, at an intermediate temperature, we separate the liquid with concentration CL, the remaining solid will have a lower concentration CS < C0. The amount of purified solid is:
Zone Refining: Consider a cylindrical rod of length L and a heating section that creates a molten zone of length a.
Epitaxy: A process in integrated circuit fabrication involving the growth of a thin monocrystalline layer on a substrate, ensuring an ordered structure. This technique controls the impurity level in the semiconductor, defining its characteristics. Using a highly doped wafer as a base and epitaxially growing a surface layer improves mobility and conductivity.
Lithography and Etching: Used to define device areas. The process begins with designing the device at a large scale, then reducing it using a photoreproduction process. A high-resolution emulsion plate (mask) is used to obtain a stable and high-definition pattern. A silicon wafer with a protective SiO2 layer is coated with a photosensitive material. The mask is placed over the wafer, and it’s illuminated. The mask lets light through, impressing the pattern on the photosensitive material. After processing, the wafer is immersed in hydrofluoric acid, which etches the SiO2, exposing the desired areas.
Thermal Diffusion: This procedure introduces acceptor or donor impurities into a silicon wafer. For example, phosphorus (donor) is used because its diffusion rate and mobility in silicon are higher than other impurities. Boron (acceptor) is also used due to its high diffusion speed. The process involves three stages: The wafer is placed in a furnace under vacuum, and a protective gas is introduced. Then, in a pre-deposition step, the impurity atoms are deposited. Finally, the wafer is heated, allowing the atoms to diffuse inward. Depending on temperature and time, this can convert the entire wafer or just a specific area.
Metallization: Consists of covering the material by depositing a metal layer (usually aluminum) on the wafer. Metallization is usually performed in a vacuum chamber. The metal is volatilized from a refractory filament and deposited onto the wafer surface.