7.2 Extrinsic semiconductors
6 min read•august 21, 2024
Extrinsic semiconductors are materials with deliberately added impurities that alter their electrical properties. By introducing donor or acceptor atoms, these semiconductors can be tailored for specific electronic applications, forming the basis of modern devices.
This topic explores the types of extrinsic semiconductors, doping processes, and their impact on carrier concentrations and energy levels. We'll examine how doping affects electrical and optical properties, and discuss key applications and characterization techniques for these materials.
Types of extrinsic semiconductors
- Extrinsic semiconductors form a crucial component of modern electronic devices by altering the electrical properties of intrinsic semiconductors
- Doping process introduces impurities into the crystal lattice modifies the band structure and carrier concentrations
- Condensed matter physics principles explain the behavior of charge carriers in these modified semiconductor materials
N-type semiconductors
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- Created by doping intrinsic semiconductors with donor impurities (, arsenic)
- Donor atoms contribute extra electrons to the conduction band increases
- Majority charge carriers electrons while minority carriers holes
- shifts closer to the conduction band enhances
P-type semiconductors
- Formed by introducing acceptor impurities (, gallium) into intrinsic semiconductors
- Acceptor atoms create holes in the valence band increases
- Majority charge carriers holes while minority carriers electrons
- Fermi level moves closer to the valence band improves hole-based conduction
Doping process
- Doping alters the electronic structure of semiconductors enables tailoring of electrical properties
- Precise control of dopant concentration and distribution critical for device performance
- Condensed matter physics principles guide the selection of appropriate dopants and doping methods
Substitutional doping
- Dopant atoms replace host atoms in the crystal lattice maintains overall structure
- Commonly used for silicon doping with elements from groups III and V
- Dopant size and electronic configuration must be compatible with host lattice
- Activation energy required to ionize dopants depends on the energy level difference
Interstitial doping
- Dopant atoms occupy spaces between host atoms in the crystal lattice
- Often used for doping compound semiconductors (GaAs, InP)
- Can introduce strain in the lattice affects electronic and optical properties
- Diffusion coefficients of interstitial dopants typically higher than substitutional dopants
Donor and acceptor levels
- Impurity energy levels introduced by dopants modify the band structure of semiconductors
- Understanding these levels crucial for predicting and controlling semiconductor behavior
- Condensed matter physics theories explain the formation and effects of these energy levels
Energy band diagrams
- Donor levels appear just below the conduction band in n-type semiconductors
- Acceptor levels form slightly above the valence band in p-type semiconductors
- remains largely unchanged but carrier concentrations significantly altered
- Impurity bands can form at high doping concentrations modify electronic properties
Fermi level shifts
- Doping causes the Fermi level to move from mid-gap position in intrinsic semiconductors
- N-type doping shifts Fermi level closer to conduction band increases electron concentration
- P-type doping moves Fermi level towards valence band enhances hole concentration
- Fermi-Dirac statistics describe the occupation probability of energy states
Carrier concentration
- Doping dramatically increases the concentration of majority carriers in semiconductors
- Carrier concentration directly impacts electrical conductivity and device performance
- Condensed matter physics models predict carrier behavior under various conditions
Temperature dependence
- Carrier concentration varies with temperature due to thermal of dopants
- Low temperatures freeze-out region where dopants are not fully ionized
- Intermediate temperatures saturation region with stable carrier concentration
- High temperatures intrinsic region where thermally generated carriers dominate
Doping concentration effects
- Increasing dopant concentration raises carrier concentration up to a limit
- Heavy doping can lead to impurity band formation alters electronic properties
- Degenerate doping occurs when Fermi level enters conduction or valence band
- Doping compensation can reduce effective carrier concentration in unintentional doping
Electrical properties
- Extrinsic semiconductors exhibit distinct electrical characteristics compared to intrinsic materials
- Understanding these properties essential for designing and optimizing electronic devices
- Condensed matter physics principles explain the observed electrical behaviors
Conductivity vs temperature
- Conductivity generally increases with temperature in extrinsic semiconductors
- Low temperature region dominated by ionization of dopants
- Intermediate temperature region shows relatively stable conductivity
- High temperature region intrinsic conduction becomes significant
- Mobility decreases with temperature due to increased phonon scattering
Hall effect in extrinsics
- Hall effect measurements determine carrier type, concentration, and mobility
- Hall coefficient inversely proportional to carrier concentration
- Sign of Hall coefficient indicates majority carrier type (negative for n-type, positive for p-type)
- Hall mobility provides insight into scattering mechanisms and crystal quality
Optical properties
- Doping influences the optical characteristics of semiconductors
- Understanding these changes crucial for optoelectronic device design
- Condensed matter physics theories explain the observed optical phenomena
Absorption spectrum changes
- Doping introduces new absorption features related to impurity energy levels
- Free carrier absorption increases in heavily doped semiconductors
- Band gap narrowing occurs at high doping concentrations
- Urbach tail formation in the absorption spectrum due to band edge fluctuations
Photoluminescence in extrinsics
- Doping introduces new radiative recombination pathways
- Donor-acceptor pair recombination produces characteristic emission lines
- Band-to-impurity transitions result in broader emission peaks
- Non-radiative recombination through defects can quench luminescence
Applications of extrinsic semiconductors
- Extrinsic semiconductors form the basis of numerous electronic and optoelectronic devices
- Understanding the physics of these materials crucial for advancing technology
- Condensed matter physics principles guide the development of new semiconductor applications
Transistors and diodes
- Bipolar junction (BJTs) use both n-type and p-type regions for amplification
- Field-effect transistors (FETs) control current flow through doped semiconductor channels
- P-n junction diodes fundamental building blocks for rectification and switching
- Zener diodes utilize heavily doped p-n junctions for voltage regulation
Solar cells and LEDs
- employ p-n junctions to convert light into electrical energy
- Doping optimization crucial for maximizing solar cell efficiency
- Light-emitting diodes (LEDs) use radiative recombination in doped semiconductors
- Quantum well structures in LEDs enhance emission efficiency and allow color tuning
Characterization techniques
- Various methods used to analyze the properties of extrinsic semiconductors
- Characterization techniques provide insights into doping effects and material quality
- Condensed matter physics principles underlie these measurement techniques
Hall measurements
- Determine carrier type, concentration, and mobility in extrinsic semiconductors
- Van der Pauw technique allows measurements on arbitrary sample shapes
- Temperature-dependent Hall measurements reveal activation energies of dopants
- Magnetoresistance effects can complicate Hall measurements in certain materials
DLTS and photoconductivity
- Deep Level Transient Spectroscopy (DLTS) identifies deep-level defects in semiconductors
- DLTS reveals activation energies and capture cross-sections of traps
- Photoconductivity measurements assess carrier generation and recombination processes
- Spectral dependence of photoconductivity provides information on impurity levels
Extrinsic vs intrinsic semiconductors
- Extrinsic semiconductors exhibit distinct properties compared to their intrinsic counterparts
- Understanding these differences crucial for selecting appropriate materials for specific applications
- Condensed matter physics theories explain the observed variations in behavior
Carrier concentration comparison
- Extrinsic semiconductors have much higher carrier concentrations than intrinsic materials
- Carrier concentration in extrinsics controlled by doping level rather than band gap
- Intrinsic carrier concentration depends exponentially on temperature and band gap
- Extrinsic semiconductors maintain stable carrier concentrations over wider temperature ranges
Temperature sensitivity differences
- Intrinsic semiconductors highly sensitive to temperature changes
- Extrinsic semiconductors show more stable electrical properties with temperature
- Intrinsic conduction dominates at high temperatures in both types
- Low-temperature behavior significantly different due to dopant freeze-out in extrinsics
Compensation in semiconductors
- Compensation occurs when both donors and acceptors present in a semiconductor
- Understanding compensation effects crucial for controlling semiconductor properties
- Condensed matter physics models describe the interplay between different dopant types
Donor-acceptor compensation
- Donors and acceptors can partially neutralize each other's effects
- Net carrier concentration determined by the difference between donor and acceptor concentrations
- Compensation can occur intentionally or unintentionally during growth or processing
- Partially compensated semiconductors can exhibit unique properties (semi-insulating GaAs)
Effects on carrier concentration
- Compensation reduces the effective carrier concentration below the total dopant concentration
- Can lead to high-resistivity materials even with significant doping levels
- Temperature dependence of carrier concentration more complex in compensated semiconductors
- Compensation affects the Fermi level position and the onset of degeneracy
Key Terms to Review (18)
Band Gap: The band gap is the energy difference between the top of the valence band and the bottom of the conduction band in a solid material. This energy gap determines how well a material can conduct electricity and plays a crucial role in the electrical and optical properties of semiconductors, insulators, and conductors. The size of the band gap influences whether a material behaves as a conductor, semiconductor, or insulator, and it is pivotal in understanding electronic devices such as diodes and transistors.
Boron: Boron is a chemical element with the symbol 'B' and atomic number 5, known for its role as a dopant in semiconductors. When introduced into silicon, boron acts as a p-type dopant, creating 'holes' or positive charge carriers that enhance electrical conductivity. This property makes boron an essential element for the creation of extrinsic semiconductors, which are crucial for modern electronic devices.
Conductivity: Conductivity is the ability of a material to conduct electric current, which is determined by the presence and mobility of charge carriers, such as electrons or holes. In metals, conductivity arises from free electrons that can move easily through the lattice structure, while in semiconductors, conductivity can change significantly with temperature and impurity levels. Understanding conductivity is essential for analyzing the electronic properties of various materials, including metals and semiconductors.
Einstein Relation: The Einstein relation is a fundamental equation in condensed matter physics that links the diffusion coefficient of charge carriers to their mobility. This relationship is essential for understanding how carriers move through materials, particularly in extrinsic semiconductors where impurities significantly affect electrical properties. It highlights the interplay between thermal energy and the response of charge carriers to electric fields.
Electron concentration: Electron concentration refers to the number of free electrons available for conduction in a given volume of a semiconductor material. This term is particularly relevant in understanding extrinsic semiconductors, where doping introduces additional charge carriers, enhancing the material's conductivity. The electron concentration plays a vital role in determining the electrical properties of semiconductors and is crucial for the functioning of electronic devices.
Energy Bands: Energy bands refer to the ranges of energy levels that electrons can occupy in a solid material, distinguishing between allowed and forbidden energy states. These bands arise due to the interaction of atoms in a solid, leading to the formation of bands where electrons can move freely, as opposed to energy gaps where no states are available. Understanding energy bands is crucial for comprehending electrical properties, the behavior of semiconductors, and various electronic phenomena.
Fermi Level: The Fermi level is the highest energy level occupied by electrons in a solid at absolute zero temperature. It plays a crucial role in determining the electrical properties of materials, as it helps to distinguish between conductors, insulators, and semiconductors, and influences how energy bands are filled with electrons.
Free carrier conduction: Free carrier conduction refers to the movement of charge carriers, such as electrons and holes, in a material that allows electrical current to flow. This phenomenon is particularly significant in extrinsic semiconductors, where the introduction of impurities creates additional charge carriers that contribute to conductivity, enabling devices like diodes and transistors to function effectively.
Hole concentration: Hole concentration refers to the density of holes, or the absence of electrons, in a semiconductor material. This concept is critical for understanding extrinsic semiconductors, where impurities are introduced to enhance electrical conductivity by increasing the number of holes available for conduction. The hole concentration directly influences the electrical properties of the semiconductor and plays a key role in determining its behavior in electronic devices.
Ionization: Ionization is the process by which an atom or molecule acquires a positive or negative charge by gaining or losing electrons. This fundamental process plays a critical role in the behavior of extrinsic semiconductors, where the introduction of impurities can lead to the creation of free charge carriers that enhance electrical conductivity. Understanding ionization helps explain how dopants interact with the semiconductor material to create n-type and p-type semiconductors, influencing their electronic properties.
Mass action law: The mass action law states that the concentration of charge carriers in a semiconductor is determined by the equilibrium between the generation and recombination of electron-hole pairs. This principle is critical for understanding extrinsic semiconductors, where doping introduces additional charge carriers, altering the balance of electrons and holes and influencing the material's electrical properties.
N-type semiconductor: An n-type semiconductor is a type of extrinsic semiconductor that has been doped with elements that have more valence electrons than the semiconductor material itself, typically adding electrons to the conduction band. This doping introduces extra electrons, which act as charge carriers, allowing for increased conductivity. The presence of these additional electrons plays a crucial role in forming p-n junctions, which are fundamental to many electronic devices.
P-type semiconductor: A p-type semiconductor is a type of extrinsic semiconductor that is doped with acceptor impurities, resulting in an abundance of holes, or positive charge carriers. This leads to a higher concentration of holes than electrons, creating a material that conducts electricity primarily through the movement of these holes. The presence of holes makes p-type semiconductors essential in forming junctions with n-type semiconductors, which play a crucial role in many electronic devices.
Phosphorus: Phosphorus is a non-metal element with the atomic number 15, known for its role as a dopant in semiconductors, particularly in the context of extrinsic semiconductors. It introduces additional electrons into the material, which enhances electrical conductivity and allows for the creation of n-type semiconductors, where electrons serve as the primary charge carriers.
Solar cells: Solar cells are semiconductor devices that convert light energy directly into electrical energy through the photovoltaic effect. They are essential components in solar panels and are primarily made from silicon, which can be manipulated to create both intrinsic and extrinsic semiconductors to enhance their efficiency in converting sunlight into electricity.
Temperature Coefficient: The temperature coefficient is a numerical value that represents how much a material's property changes with temperature. In the context of extrinsic semiconductors, it indicates how the conductivity or resistivity of the semiconductor material changes as temperature varies. Understanding this coefficient is crucial for predicting semiconductor behavior in different thermal conditions and optimizing their performance in electronic devices.
Thermal Excitation: Thermal excitation refers to the process where particles gain energy due to thermal fluctuations, allowing them to occupy higher energy states. This phenomenon is essential in understanding how materials respond to temperature changes, influencing their electronic properties, magnetic behavior, and conductivity.
Transistors: Transistors are semiconductor devices that can amplify or switch electronic signals and electrical power. They are fundamental building blocks of modern electronic devices, acting as a gate for controlling the flow of current and enabling the operation of complex circuits. Their functionality relies on the properties of semiconductors, which allow them to effectively modulate electrical signals based on input conditions.