UniversityWafer, Inc. supplies semiconductor wafers for university researchers, engineers, and semiconductor manufacturers. We offer silicon, GaAs, InP, SiC, SOI, sapphire and many other substrates in diameters ranging from 1 inch to 12 inch. Whether you need prime, test, dummy, or custom-processed wafers, we support applications including CMOS fabrication, MEMS, photonics, RF devices, quantum research, power electronics and more.
An electrical and electronics engineering graduate student asked:
My thesis involves a fiber-optic temperature sensor. I plan to illuminate a semiconductor at the end of an optical fiber and measure changes in the reflected optical signal as a function of temperature.
The room-temperature band-edge wavelength of GaAs is near 870 nm, while my laser operates at 850 nm. Could GaAs be used at temperatures around 5–10°C? I am also considering GaP with a 650 nm laser. I would also like guidance on substrate thickness.
Both GaAs and GaP reflect some incident light at an air-semiconductor interface because their refractive indices differ substantially from that of air. However, the reflectance is wavelength dependent and also depends on incidence angle, polarization, surface condition, coatings, and the semiconductor's complex refractive index. It should therefore not be assumed that either material reflects a fixed percentage such as 50% at every wavelength.
For a temperature sensor based on the semiconductor absorption edge, the important effect is that the semiconductor band gap changes with temperature. This causes the absorption edge—and therefore the wavelength-dependent reflected or transmitted optical signal—to shift with temperature.
GaAs has a direct band gap of approximately 1.42 eV at room temperature, corresponding to a band-edge wavelength near 870 nm. Its band gap generally increases as temperature decreases, so the absorption edge shifts toward shorter wavelengths at lower temperatures. An 850 nm source is therefore close enough to the GaAs band edge that the exact temperature range, optical geometry, wafer properties, and detection method should be evaluated experimentally for the proposed sensor.
GaP has a substantially larger indirect band gap of approximately 2.26 eV at room temperature. A 650 nm photon has an energy of approximately 1.91 eV, which is below the fundamental room-temperature band-gap energy of GaP. Therefore, a GaP/650 nm system should not be assumed to behave like a GaAs/850 nm band-edge sensor.
Illumination with photon energies above a semiconductor's band gap can generate electron-hole pairs through optical absorption. It does not mean that the material will automatically emit strong light at its band-gap wavelength. Radiative efficiency depends strongly on whether the material has a direct or indirect band gap, as well as defects, doping, carrier recombination, temperature, and device structure.
Substrate thickness can also influence transmission and multiple internal reflections. A thickness such as 250 µm may be mechanically suitable for some experiments, but the optimum thickness should be selected according to the optical configuration and required signal.
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A semiconductor wafer is a thin, flat substrate made from a semiconductor material such as silicon, gallium arsenide (GaAs), silicon carbide (SiC), gallium nitride (GaN), indium phosphide (InP), or other elemental or compound semiconductors.
Silicon is the most widely used semiconductor substrate for conventional integrated circuits. A polished silicon wafer provides the starting surface on which transistors, interconnects, insulating layers, and other microscopic structures can be fabricated.
The wafer itself is not an integrated circuit. Instead, many individual integrated circuits, or dies, can be fabricated simultaneously across a single wafer. After fabrication and testing, the wafer is diced into individual chips, which can then be packaged.
Different semiconductor materials are selected according to the device requirements. Silicon dominates conventional digital electronics, while compound and wide-band-gap semiconductors can offer advantages for applications involving high frequency, optical emission and detection, high voltage, high power, or high-temperature operation.
A semiconductor is a material whose electrical conductivity lies between that of a typical conductor and an insulator and whose carrier concentration and conductivity can be strongly controlled through factors such as doping, temperature, illumination, and electric fields.
Semiconductors can be elemental materials, such as silicon (Si) and germanium (Ge), or compound materials, such as GaAs, InP, GaN, and SiC.
Their controllable electrical and optical properties make semiconductor materials fundamental to diodes, transistors, integrated circuits, LEDs, laser diodes, photodetectors, solar cells, sensors, MEMS, and power-electronic devices.
The band gap is the energy difference between the valence-band maximum and the conduction-band minimum of a semiconductor. Band-gap energy is normally expressed in electron volts (eV).
Electrons in the valence band can be excited into available states in the conduction band when they receive sufficient energy. This process leaves behind holes in the valence band, producing electron-hole pairs that can contribute to electrical conduction.
Band gap is therefore an energy property of the semiconductor's electronic band structure. It is not the name for the movement of electrons or holes through the material.
In general, a smaller band gap produces a higher intrinsic carrier concentration at a given temperature, while a larger band gap produces a lower intrinsic carrier concentration. However, the conductivity of an actual semiconductor also depends strongly on doping, carrier mobility, temperature, defects, and material quality.
Learn more about band-gap energy in semiconductor materials .
In an intrinsic semiconductor, thermal or optical excitation can promote electrons from the valence band into the conduction band, creating electron-hole pairs.
When an electric field is applied, conduction-band electrons drift in the direction opposite to the electric field, while holes behave as positive charge carriers and drift in the direction of the electric field. Both types of carriers therefore contribute to electrical current.
The availability and concentration of charge carriers depend on factors including doping, temperature, illumination, defects, and the semiconductor's band structure.
In an intrinsic semiconductor, the equilibrium electron and hole concentrations are equal. In a doped, or extrinsic semiconductor, one carrier type generally becomes the majority carrier.
An intrinsic semiconductor is an ideally pure or sufficiently uncompensated semiconductor in which electrical behavior is governed primarily by thermally generated electron-hole pairs rather than intentionally introduced dopants.
In thermal equilibrium, intrinsic material has equal electron and hole concentrations:
n = p = ni
An extrinsic semiconductor is a semiconductor whose electrical properties have been intentionally modified through doping. Adding donor or acceptor impurities changes the concentration of free charge carriers.
Extrinsic does not mean that the semiconductor is something other than silicon. Silicon itself can be intrinsic, n-type, or p-type depending on its impurity and dopant concentrations.
N-type silicon is commonly produced by introducing donor dopants such as phosphorus, arsenic, or antimony. These dopants provide additional electrons, making electrons the majority carriers.
P-type silicon is commonly produced using an acceptor dopant such as boron. Boron has three valence electrons and can accept an electron in the silicon lattice, increasing the hole concentration. Holes are therefore the majority carriers in ordinary p-type silicon.
Once silicon is intentionally doped to become n-type or p-type, it is classified as an extrinsic semiconductor, not an intrinsic semiconductor.
Semiconductor fabrication involves a sequence of highly controlled processes used to create microscopic electronic structures on and within a wafer. The exact sequence varies considerably depending on whether the final device is a CMOS integrated circuit, MEMS sensor, photodiode, power device, solar cell, or another semiconductor device.
Common fabrication processes include:
Wafer Preparation: Silicon wafers are grown, sliced, shaped, lapped or ground as required, chemically treated, polished, and cleaned to produce the required substrate surface.
Thermal Oxidation: When required, silicon can be thermally oxidized to form a controlled layer of silicon dioxide (SiO2). Thermal oxide can serve as an electrical insulator, surface passivation layer, masking layer, or part of a device structure.
Photolithography: A photosensitive resist is deposited and selectively exposed using a mask or other lithographic system. Development creates a patterned resist that defines areas for subsequent processing.
Etching: Wet or dry etching can selectively remove exposed material to form patterned structures in semiconductor, dielectric, or metal layers.
Doping: Dopants can be introduced by processes such as ion implantation, diffusion, or during epitaxial growth. Donor and acceptor dopants create controlled n-type and p-type regions.
Thin-Film Deposition: Conductors, dielectrics, semiconductors, and other films can be deposited using techniques such as chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), epitaxy, and related methods.
Annealing and Thermal Processing: Thermal treatments may be used to activate implanted dopants, repair crystal damage, modify interfaces, densify films, or drive diffusion. The purpose of an anneal depends on the fabrication step.
Planarization: Advanced semiconductor fabrication often uses chemical-mechanical polishing (CMP) to reduce surface topography and prepare the wafer for additional device layers.
Metallization and Interconnects: Conductive layers are patterned to form electrical contacts and interconnections between device components.
Many of these processes are repeated to build complex multilayer structures. After wafer fabrication, individual dies can be electrically tested, separated by dicing, and packaged to form usable semiconductor devices.
Semiconductor wafers provide the starting substrates for a broad range of technologies, including:
A semiconductor wafer is a thin substrate made from a semiconductor material and used as the foundation for fabricating electronic, photonic, sensing, MEMS, and power devices. Silicon is the most widely used semiconductor wafer material, but many applications use compound or wide-band-gap semiconductors.
During semiconductor fabrication, structures are formed on or within the wafer using processes such as photolithography, etching, thin-film deposition, oxidation, epitaxy, ion implantation, diffusion, annealing, and chemical-mechanical polishing (CMP). The exact sequence depends on the device being manufactured.
A semiconductor wafer is not itself an integrated circuit. Instead, many individual devices or integrated-circuit dies can be fabricated across a wafer. After processing and testing, the wafer may be diced into individual dies for packaging.
Silicon (Si) is the dominant substrate for conventional integrated circuits because of its mature manufacturing infrastructure, useful electrical properties, and ability to form a high-quality native silicon dioxide (SiO2).
Other important semiconductor materials and substrate platforms include:
Sapphire (Al2O3) is also widely used in semiconductor manufacturing, particularly as a substrate for epitaxial growth of materials such as GaN. However, sapphire itself is an electrical insulator rather than a conventional semiconductor.
A semiconductor is a material whose electrical behavior can be controlled much more readily than that of a typical metal or electrical insulator.
Semiconductor conductivity can be modified through doping, temperature, illumination, electric fields, material composition, and device structure. This controllability is what makes semiconductors useful for electronic and photonic devices.
Semiconductors are materials; diodes and transistors are devices made using semiconductor materials. A semiconductor should therefore not be defined as a device that simply allows current to flow in one direction.
Semiconductor materials may be elemental, such as silicon and germanium, or compounds, such as GaAs, InP, GaN, and SiC.
Semiconductor-grade silicon begins with highly purified silicon feedstock. Single-crystal silicon ingots are commonly grown using either the Czochralski (CZ) or Float-Zone (FZ) process.
The ingot is then processed into wafers through steps that can include:
Device fabrication takes place after the substrate wafer has been prepared. Depending on the device, fabrication may then involve:
These processes can be repeated many times to construct transistors, interconnects, dielectric layers, sensors, or other structures across the wafer.
Silicon wafers are manufactured in multiple diameters. Common research and industrial sizes include 25.4 mm (1 inch), 50.8 mm (2 inch), 76.2 mm (3 inch), 100 mm, 125 mm, 150 mm, 200 mm, and 300 mm.
300 mm (12 inch) silicon wafers are widely used in advanced high-volume semiconductor manufacturing. Smaller diameters remain important for research, legacy fabrication lines, MEMS, sensors, power devices, and specialized applications.
Wafer thickness is not determined by diameter alone. Standard thicknesses are used for handling and processing, while wafers can also be thinned substantially for specialized devices after fabrication.
Temperature affects several semiconductor properties simultaneously, including carrier concentration, carrier mobility, band-gap energy, resistivity, leakage current, and device performance.
In an intrinsic semiconductor, increasing temperature provides more thermal energy for exciting electrons from the valence band into the conduction band. Each excitation creates a conduction-band electron and a corresponding hole in the valence band.
As a result, the intrinsic carrier concentration increases strongly with temperature. This generally causes the electrical conductivity of intrinsic semiconductor material to increase as temperature rises.
Carrier mobility often moves in the opposite direction. At sufficiently high temperatures, increased lattice vibrations produce stronger phonon scattering, which generally reduces electron and hole mobility.
Semiconductor conductivity can be written approximately as:
σ = q(nμn + pμp)
where q is the elementary charge, n and p are the electron and hole concentrations, and μn and μp are their respective mobilities.
Therefore, it is not scientifically correct to say simply that increasing temperature always increases or always decreases semiconductor conductivity. The result depends on the semiconductor, doping level, temperature range, carrier concentration, and carrier mobility.
A doped semiconductor can exhibit different temperature regimes:
The temperatures at which these regimes occur depend on the material, dopant species, dopant concentration, and semiconductor band structure.
An extrinsic semiconductor is a semiconductor whose electrical properties have been intentionally modified by introducing dopant atoms.
Extrinsic does not mean that the material is something other than silicon. Silicon itself becomes extrinsic silicon when intentionally doped with donor or acceptor impurities.
Doping is the controlled introduction of electrically active impurities into a semiconductor to modify its carrier concentration and electrical properties.
In silicon, group-V elements such as phosphorus, arsenic, and antimony can act as donor dopants. They introduce donor states close to the conduction-band edge and can provide electrons to the conduction band.
Electrons are therefore the majority carriers in ordinary n-type silicon .
Group-III elements such as boron can act as acceptor dopants in silicon. Boron has three valence electrons, not five. Substitutional boron introduces an acceptor state and can increase the concentration of holes.
Holes are therefore the majority carriers in ordinary p-type silicon.
Doping does not simply replace the semiconductor's fundamental band gap with a new “dopant band gap.” Donor and acceptor impurities introduce energy states associated with the dopants, while the semiconductor retains its underlying valence and conduction bands. At very high doping concentrations, additional effects such as band-gap narrowing and degeneracy may occur.
A semiconductor can contain large concentrations of electrons, holes, and ionized dopants while remaining approximately electrically neutral overall.
In n-type material, ionized donor atoms carry positive charge after donating electrons. In p-type material, ionized acceptors carry negative charge after accepting electrons. Charge neutrality relates the concentrations of electrons, holes, ionized donors, and ionized acceptors.
N-type material is not automatically more conductive than p-type material. Conductivity depends on both carrier concentration and carrier mobility:
σ = q(nμn + pμp)
An intrinsic semiconductor is an ideally pure or sufficiently uncompensated semiconductor in which the equilibrium carrier concentrations are determined primarily by thermal generation rather than intentional doping.
At thermal equilibrium:
n = p = ni
where ni is the intrinsic carrier concentration.
An intrinsic semiconductor does not suddenly “become a semiconductor” only after it is heated. It is already a semiconductor; temperature changes the number of thermally generated carriers and therefore its electrical behavior.
Thermal energy can excite electrons from the valence band into the conduction band, leaving holes in the valence band.
When an electric field is applied, electrons and holes drift in opposite directions. Electrons, which carry negative charge, drift opposite to the electric field. Holes behave as positive charge carriers and drift in the direction of the electric field. Both carrier motions contribute to conventional electrical current.
This carrier motion is not called a band gap.
The band gap is the energy difference between the valence-band maximum and the conduction-band minimum.
The band-gap energy Eg is the energy difference between the valence-band maximum and the conduction-band minimum.
It represents the fundamental energy separation relevant to exciting electrons from valence-band states into conduction-band states.
Band gaps can be characterized using several techniques depending on the material. Common approaches include optical absorption, reflectance, photoluminescence, spectroscopic ellipsometry, and temperature-dependent electrical measurements.
Atomic force microscopy (AFM) is primarily a surface-topography and force-measurement technique and should not be described as the standard method for measuring semiconductor band-gap energy.
The intrinsic carrier concentration depends strongly on band-gap energy and temperature. A commonly used relationship has the form:
ni = √(NCNV) exp(−Eg/2kBT)
where NC and NV are the effective densities of states, Eg is the band gap, kB is Boltzmann's constant, and T is absolute temperature.
This relationship helps explain why, all else being equal, a smaller band gap generally produces a higher intrinsic carrier concentration at a given temperature.
Actual wafer conductivity cannot be predicted from band gap alone, however. Doping, carrier mobility, temperature, defects, compensation, and material quality must also be considered.
A hole represents an unoccupied electronic state in the valence band that can behave as a positive charge carrier.
Hole transport does not mean that a physical positively charged particle is moving from atom to atom. Instead, neighboring valence electrons can successively occupy available states, making the vacancy appear to move through the crystal.
Under an applied electric field, holes drift in the direction of the field while electrons drift in the opposite direction.
Semiconductor optical absorption depends on photon energy and the material's electronic band structure.
When a photon has sufficient energy for an allowed interband transition, it can be absorbed and excite an electron from the valence band into the conduction band, producing an electron-hole pair.
For a simple direct-band-gap semiconductor, strong interband absorption begins near the band-gap energy. In an indirect-band-gap semiconductor such as silicon, momentum conservation means that phonons participate in near-edge optical transitions.
Photons with energy substantially greater than the band gap can initially create carriers with excess energy. These carriers typically relax toward the band edges through interactions with the lattice, converting much of that excess energy into phonons (heat).
The generated electron-hole pairs can then be separated and collected in devices such as solar cells and photodetectors, or they may recombine radiatively or nonradiatively depending on the semiconductor and device structure.
No single semiconductor material is optimal for every application. Material selection depends on properties such as band gap, band-gap type, carrier mobility, breakdown field, thermal conductivity, lattice constant, optical properties, substrate quality, and fabrication requirements.