This article describes the fundamental properties of semiconductors, including electrical conductivity, energy band structure, and responses to temperature, light, and mechanical stress.
Substances with resistivity values between conductors and insulators are called Semiconductors. At absolute zero, semiconductors behave as perfect insulators. At room temperature, the density of electrons in the conduction band is not as high as in metals, which results in lower electrical conductivity compared to metals. However, semiconductors do not have as low electrical conductivity as insulators and high conductivity as metal.
The resistivity of semiconductors can vary widely depending on the material and its level of doping. Generally, it ranges from about 10-3 ohm-cm to 103 ohm-cm. Common examples of semiconductors include silicon, germanium, and gallium arsenide. These materials are extensively used in electronic components like transistors, diodes, and integrated circuits due to their ability to conduct electricity under certain conditions. Silicon, in particular, is the most widely used semiconductor in electronic devices due to its abundance and excellent semiconductor properties.
| S.no | Substance | Nature | Resistivity |
|---|---|---|---|
| 1 | Copper | Conductor | 1.72 x 10-6 Ωcm |
| 2 | Germanium | Semiconductor | 63 Ωcm |
| 3 | Glass | Insulator | 9 x 1013 Ωcm |
| 4 | Nichrome | Resistance Material | 10-2 Ωcm |
Properties of Semiconductors
Resistivity of a Semiconductor
Resistivity in semiconductors is a critical parameter that indicates how much the material opposes the flow of electric current. Unlike metals, the resistivity of semiconductors is not constant and can be significantly altered by changes in temperature, the presence of impurities (doping), and exposure to light.
- Temperature Dependence: As temperature increases, the resistivity of semiconductors typically decreases. This is because higher temperatures provide more energy to the charge carriers (electrons and holes), increasing their mobility.
- Doping: Adding impurities to a semiconductor (a process known as doping) can dramatically change its resistivity. N-type doping adds extra electrons, while P-type doping creates more holes, effectively lowering the resistivity by increasing the number of charge carriers.
- Light Exposure: When semiconductors are exposed to light, they can generate electron-hole pairs, which increase the conductivity and thus decrease the resistivity.
The ability to control the resistivity of semiconductors through these methods is fundamental to creating electronic components like transistors and diodes, which require precise control over their electrical properties.
| S.no | Substance | Nature | Resistivity |
|---|---|---|---|
| 1 | Copper | Conductor | 1.72 x 10-6 Ωcm |
| 2 | Germanium | Semiconductor | 63 Ωcm |
| 3 | Glass | Insulator | 9 x 1013 Ωcm |
| 4 | Nichrome | Resistance Material | 10-2 Ωcm |
The resistivity of a semiconductor is lower than that of an insulator but higher than that of a conductor. The table below shows the resistivity of various materials.
The table above shows that the resistivity of the semiconductor Germanium is significantly higher than that of the conductor Copper yet much lower than that of the insulator Glass. Furthermore, the resistivity of Germanium is considerably higher than that of Nichrome, which is one of the materials with the highest resistivity.
Negative Temperature Co-efficient
The negative temperature coefficient of a semiconductor refers to the characteristic where its electrical resistance decreases as the temperature increases. This occurs because higher temperatures provide more energy to the semiconductor’s atoms, causing more electrons to break free from their atomic bonds and increase the number of free charge carriers (electrons and holes). Consequently, as the temperature rises, the increased charge carriers enhance the conductivity of the semiconductor, thus reducing its resistance. This behavior is in contrast to most metals, where resistance typically increases with temperature.
Doping Response
Doping significantly affects semiconductors’ current-conducting properties. By introducing small amounts of impurities into a semiconductor, its electrical conductivity can be greatly enhanced. This process involves adding either electron-rich (n-type) or electron-deficient (p-type) dopants.
In n-type doping, dopants donate extra electrons, increasing the number of negative charge carriers in the semiconductor. Conversely, in p-type doping, dopants create holes by accepting electrons, increasing the number of positive charge carriers. These added carriers facilitate the flow of electric current through the semiconductor, thus enhancing its conductivity. This manipulation of conductivity through doping is fundamental to the operation of various electronic devices like transistors and diodes.
Energy Band Structure
The conductivity of semiconductors is largely governed by their band structure. They have a valence band filled with electrons and a conduction band that is typically empty. The energy gap between these bands, known as the bandgap, is crucial. It is small enough to allow electrons to move to the conduction band under certain conditions like increased temperature or light exposure yet large enough to prevent free electron flow at low temperatures.
Photoconductivity of Semiconductor
Photoconductivity is a property of semiconductors where their conductivity increases when exposed to light. This happens because light photons absorbed by the semiconductor material can excite electrons to higher energy states, creating electron-hole pairs. These additional charge carriers (free electrons and holes) enhance the material’s electrical conductivity.
When the light source is removed, the electrons typically return to their original energy states, recombining with holes, and the semiconductor’s conductivity decreases back to its normal level. This light-sensitive property of semiconductors is utilized in devices such as photodetectors, solar cells, and light-sensitive resistors.
Hall Effect
Semiconductors exhibit the Hall effect, where a voltage is generated across an electrical conductor, transverse to the electric current in the conductor, and an applied magnetic field perpendicular to the current. This effect is used to determine the type of charge carrier (positive or negative) and measure magnetic fields.
Volatility
Some semiconductors also exhibit volatility where their physical properties can be permanently changed by applying an electric field. This feature is utilized in the development of non-volatile memory chips.
Quantum Aspects
The quantum aspect of semiconductors focuses on how quantum mechanics governs the behavior of electrons within these materials. This is primarily observed in the discrete energy levels and band structures that define how electrons can move within the semiconductor.
In quantum terms, semiconductors have a valence band filled with electrons and a conduction band that electrons can move into when they gain enough energy. The gap between these bands, known as the bandgap, is crucial because it determines the semiconductor’s electrical properties. Quantum mechanics explains that this bandgap results from the wave-like nature of electrons and their interactions within the semiconductor lattice. Moreover, quantum effects in semiconductors are also exploited in devices like quantum dots and semiconductor lasers.
Electroluminescence
Electroluminescence in semiconductors refers to the phenomenon where a semiconductor emits light when an electric current is passed through it. This occurs when electrons and holes recombine within the semiconductor material. As electrons fall from a higher energy level (conduction band) to a lower one (valence band), they release energy as photons, which is observed as light.
This process is the basis for light-emitting diodes (LEDs) and other electroluminescent devices. Different semiconductor materials and impurities can produce light of various colors.
Piezoresistivity
Piezoresistivity in semiconductors refers to the change in electrical resistivity caused by mechanical stress or strain. This property is particularly pronounced in semiconductor materials compared to metals. When applied to a semiconductor, stress can deform the lattice structure, affecting the band structure and mobility of the charge carriers (electrons and holes). This change in carrier mobility alters the material’s resistivity. The piezoresistive effect is widely used in developing pressure sensors and strain gauges.
Conclusion:
In conclusion, semiconductors’ properties, such as adjustable electrical conductivity, sensitivity to temperature and light, and responsiveness to doping, make them indispensable in modern electronics. These characteristics enable the precise control for developing a wide array of electronic components, from transistors and sensors to cutting-edge quantum and optoelectronic devices.