MOSFETs

MOSFET son las siglas de Metal Oxide Semiconductor Field Effect Transistor. Consiste en un transistor de efecto de campo basado en la estructura MOS. Es el transistor más utilizado en la industria microelectrónica. Prácticamente la totalidad de los circuitos integrados de uso comercial están basados en transistores MOSFET.

Historia
Fue ideado teóricamente por el alemán Julius von Edgar Lilienfeld en 1930, aunque debido a problemas de carácter tecnológico y el desconocimiento acerca de cómo se comportan los electrones sobre la superficie del semiconductor no se pudieron fabricar hasta décadas más tarde. En concreto, para que este tipo de dispositivos pueda funcionar correctamente, la intercara entre el sustrato dopado y el aislante debe ser perfectamente lisa y lo más libre de defectos posible. Esto es algo que sólo se pudo conseguir más tarde, con el desarrollo de la tecnología del silicio


Funcionamiento

Un transistor MOSFET consiste en un sustrato de material semiconductor dopado en el que, mediante técnicas de difusión de dopantes, se crean dos islas de tipo opuesto separadas por un área sobre la cual se hace crecer una capa de dieléctrico culminada por una capa de conductor. Los transistores MOSFET se dividen en dos tipos fundamentales dependiendo de cómo se haya realizado el dopaje:
• Tipo nMOS: Sustrato de tipo p y difusiones de tipo n.
• Tipo pMOS: Sustrato de tipo n y difusiones de tipo p.
Las áreas de difusión se denominan fuente(source) y drenador(drain), y el conductor entre ellos es la puerta(gate).

El transistor MOSFET tiene tres estados de funcionamiento:

Estado de corte
Cuando la tensión de la puerta es idéntica a la del sustrato, el MOSFET está en estado de no conducción: ninguna corriente fluye entre fuente y drenador aunque se aplique una diferencia de potencial entre ambos. También se llama mosfet a los aislados por juntura de dos componentes.

Conducción lineal
Al polarizarse la puerta con una tensión negativa (pMOS) o positiva (nMOS), se crea una región de deplexión en la región que separa la fuente y el drenador. Si esta tensión crece lo suficiente, aparecerán portadores minoritarios (electrones en pMOS, huecos en nMOS) en la región de deplexión que darán lugar a un canal de conducción. El transistor pasa entonces a estado de conducción, de modo que una diferencia de potencial entre fuente y drenador dará lugar a una corriente. El transistor se comporta como una resistencia controlada por la tensión de puerta.

Saturación
Cuando la tensión entre drenador y fuente supera cierto límite, el canal de conducción bajo la puerta sufre un estrangulamiento en las cercanías del drenador y desaparece. La corriente entre fuente y drenador no se interrumpe, ya que es debida al campo eléctrico entre ambos, pero se hace independiente de la diferencia de potencial entre ambos terminales.

Aplicaciones
La forma más habitual de emplear transistores MOSFET es en circuitos de tipo CMOS, consistentes en el uso de transistores pMOS y nMOS complementarios. Véase Tecnología CMOS
Las aplicaciones de MOSFET discretos más comunes son:
• Resistencia controlada por tensión.
• Circuitos de conmutación de potencia (HEXFET, FREDFET, etc).
• Mezcladores de frecuencia, con MOSFET de doble puerta.
Ventajas
La principal aplicación de los MOSFET está en los circuitos integrados, p-mos, n-mos y c-mos, debido a varias ventajas sobre los transistores bipolares:
• Consumo en modo estático muy bajo.
• Tamaño muy inferior al transistor bipolar (actualmente del orden de media micra).
• Gran capacidad de integración debido a su reducido tamaño.
• Funcionamiento por tensión, son controlados por voltaje por lo que tienen una impedencia de entrada muy alta. La intensidad que circula por la puerta es del orden de los nanoamperios.
• Un circuito realizado con MOSFET no necesita resistencias, con el ahorro de superficie que conlleva.
• La velocidad de conmutación es muy alta, siendo del orden de los nanosegundos.
• Cada vez se encuentran más en aplicaciones en los convertidores de alta frecuencias y baja potencia.

BANDGAD SEMICONDUCTORS

Wide bandgap semiconductors are semiconductor materials with electronic band gaps larger than one or two electronvolts (eV). The exact threshold of "wideness" often depends on the application, such as optoelectronic and power devices. Wide bandgap materials are often utilized in applications in which high-temperature operation is important.

Motivation Driving Utilization in Devices

Solid state lighting could reduce the amount of energy required to provide lighting as compared with incandescent lights, which are associated with a light output of less than 20 lumens per watt. The efficiency of light emitting diodes is on the order of 160 lumens per watt. Wide bandgap semiconductors can be used to create light throughout the visible spectrum.
Wide bandgap semiconductors can also be used in RF signal processing. Silicon-based power transistors are reaching limits of operating frequency, breakdown voltage, and power density. Wide bandgap materials can be used in high-temperature and power switching applications.

Materials

There are many III-V and II-VI compound semiconductors with high bandgaps. The only high bandgap materials in group IV are diamond and silicon carbide (SiC).
Aluminium nitride (AlN) can be used to fabricate ultraviolet LEDs with wavelengths down to 200-250 nm.
Gallium nitride (GaN) is used to make blue LEDs and lasers.

Material Properties

Wide bandgap materials are defined as semiconductors with bandgaps greater than 1.7 eV.

Bandgap

The magnitude of the coulombic potential determines the bandgap of a material, and the size of atoms and electronegativities are two factors that determine the bandgap. Materials with small atoms and strong, electronegative atomic bonds are associated with wide bandgaps. Smaller lattice spacing results in a higher perturbing potential of neighbors.
Elements high on the periodic table are more likely to be wide bandgap materials. With regard to III-V compounds, nitrides are associated with the largest bandgaps, and, in the II-VI family, oxides are generally considered to be insulators.
Bandgaps can often be engineered by alloying, and Vegard's Law states that there is a linear relation between lattice constant and composition of a solid solution at constant temperature.
The position of the conduction band minima versus maxima in the band diagram determine whether a bandgap is direct or indirect. Most wide bandgap materials are associated with a direct bandgap, with SiC and GaP as exceptions.

Optical Properties

The minimum photon energy that is needed to excite an electron into the conduction band is associated with the bandgap of a material. When electron-hole pairs undergo recombination, photons are generated with energies that correspond to the magnitude of the bandgap.
A phonon is required in the process of absorption or emission in the case of an indirect bandgap. There must be a direct bandgap in applications of optical devices.

Breakdown Field

Impact ionization is often attributed to be the cause of breakdown. At the point of breakdown, electrons in a semiconductor are associated with sufficient kinetic energy to produce carriers when they collide with lattice atoms.
Wide bandgap semiconductors are associated with a high breakdown voltage. This is due to a larger electric field required to generate carriers through impact mechanism.
At high electric fields, drift velocity saturates due to scattering from optical phonons. A higher optical phonon energy results in fewer optical phonons at a particular temperature, and there are therefore fewer scattering centers, and electrons in wide bandgap semiconductors can achieve high peak velocities.
The drift velocity, reaches a peak at an intermediate electric field and undergoes a small drop at higher fields. Intervalley scattering is an additional scattering mechanism at large electric fields, and it is due to a shift of carriers from the lowest valley of the conduction band to the upper valleys, where the lower band curvature raises the effective mass of the electrons and lowers mobility. The drop in drift velocity at high electric fields due to intervalley scattering is small in comparison to high saturation velocity that results from low optical phonon scattering. There is therefore an overall higher saturation velocity.

Saturation Velocity

High effective masses of charge carriers are a result of low band curvatures, which correspond to low mobility. Fast response times of devices with wide bandgap semiconductors is due to the high carrier drift velocity at large electric fields, or saturation velocity.

Bandgap Discontinuity

When wide bandgap semiconductors are used in heterojunctions, band discontinuities formed at equilibrium can be a design feature, although the discontinuity can result in complications when creating ohmic contacts.
Polarization
Wurtzite and zincblende structures characterize most wide bandgap semiconductors. Wurtzite phases allow spontaneous polarization in the (0001) direction. A result of the spontaneous polarization and piezoelectricity is that the polar surfaces of the materials are associated with higher sheet carrier density than the bulk.The polar face produces a strong electric field, which creates high interface charge densities.

Thermal Properties

Melting temperatures, thermal expansion coefficients, and thermal conductivity can be considered to be secondary properties that are essential in processing, and these properties are related to the bonding in wide bandgap materials. Strong bonds result in higher melting temperatures and lower thermal expansion coefficients. A high Debye temperature results in a high thermal conductivity. With such thermal properties, heat is easily removed.

Heterojunction Bipolar Transistor (HBT)
The heterojunction bipolar transistor (HBT) is an improvement of the bipolar junction transistor (BJT) that can handle signals of very high frequencies up to several hundred GHz. It is common in modern ultrafast circuits, mostly radio-frequency (RF) systems, as well as applications requiring a high power efficiency, such as power amplifiers in cellular phones. The idea of employing a heterojunction is as old as the conventional BJT, dating back to a patent from 1951
The principal difference between the BJT and HBT is in the use of differing semiconductor materials for the emitter and base regions, creating a heterojunction. The effect is to limit the injection of holes from the base into the emitter region, since the potential barrier in the valence band is higher than in the conduction band. Unlike BJT technology, this allows a high doping density to be used in the base, reducing the base resistance while maintaining gain. The efficiency of the device is measured by the Kroemer factor, after Herbert Kroemer who received a Nobel Prize for his work in this field in 2000 at the University of California, Santa Barbara.
Materials used for the substrate include silicon, gallium arsenide, and indium phosphide, while silicon / silicon-germanium alloys, aluminium gallium arsenide / gallium arsenide, and indium phosphide / indium gallium arsenide are used for the epitaxial layers. Wide-bandgap semiconductors are especially promising, eg. gallium nitride and indium gallium nitride.
In SiGe graded heterostructure transistors, the amount of germanium in the base is graded, making the bandgap narrower at the collector than at the emitter. That tapering of the bandgap leads to a field-assisted transport in the base, which speeds transport through the base and increases frequency response.

Fabrication
Due to the need to manufacture HBT devices with extremely high-doped thin base layers, molecular beam epitaxy is principally employed. In addition to base, emitter and collector layers, highly doped layers are deposited on either side of collector and emitter to facilitate an ohmic contact, which are placed on the contact layers after exposure by photolithography and etching. The contact layer underneath the collector is, named subcollector, is an active part of the transistor.
Other techniques are used depending on the material system. IBM and others use UHV CVD for SiGe; other techniques used include MOVPE for III-V systems

Metalorganic Chemical Vapor Deposition
This process is used to manufacture compound semiconductor devices, which consist of thin films of gallium arsenide, indium phosphide and other alloys of the group III and V elements of the Periodic Table. Compound semiconductors are used in a vast array of electronic and photonic devices, such as in solid-state lasers, light-emitting diodes, space solar cells, and high-speed transistors. These are critically needed components in both optical and wireless telecommunications systems.

Compound Semiconductor devices are used for the solar panels and the RF transmitters and receivers in communications satelites (pictured is a DirecTV satelite by Hughes Electronics).
In the metalorganic chemical vapor deposition (MOCVD) process, volatile precursors, e.g., trimethylindium, trimethylgallium and phosphine, are fed to the reactor in hydrogen carrier gas. When these molecules flow over the hot substrate, they decompose and deposit a thin film, e.g., InGaP. By depositing a compound that is lattice matched to the substrate (e.g., GaAs (001)), an epitaxial single crystal is grown. A device is produced by varying the composition and doping in the layers, while maintaining lattice matching at all times.

Heterojunction bipolar transistor (HBT) consisting of an n-type GaAs collector, p+ GaAs base, only 20 nm thick, and an InGaP emitter. These devices amplify RF signals by 20 times at a frequency of 1.9 GHz.
An example of a device grown by MOCVD is the heterojunction bipolar transistor (HBT) pictured above. This device is used extensively in digital cellular telephones and in high-speed communication networks. The critical layers in this device are only 10 to 20 nm thick. Thus, our research is at the cutting edge of Nanoscience and Technology.
During MOCVD, a series of surface reactions occur as shown in the diagram below. These include adsorption and desorption of the precursor molecules, surface diffusion, nucleation and growth, and desorption of reaction products. In our laboratory, we characterize these surface reactions, and in particular, identify the sites on the semiconductor surface that mediate them. By understanding the atomic-scale processes that govern thin film growth, we make it possible to build new and more powerful devices.



At UCLA, the surface chemistry of MOCVD is revealed with state-of-the-art instruments, including reflectance difference spectroscopy, infrared spectroscopy, electron diffraction, x-ray photoemission, scanning tunneling microscopy, and ab initio molecular cluster calculations. Click on Facilities to learn more about our capabilities. In addition, you can check out some of the surfaces and surface sites that we have characterized by going to the STM Gallery and Molecular Clusters links, or by viewing our Publications.
Careers
Students working in this field learn all the skills necessary for rewarding careers in the microelectronics, communications, and high-tech materials industries. Our graduates are in great demand, and have landed exciting jobs with fast growing companies. You can see where our graduates have gone by clicking on Recent Graduates.

HEMT

Los HEMT son Transistores tipo FET, en que se reemplaza el canal de conducción por una juntura en la que se unen dos materiales semiconductores con diferentes brechas entre las bandas de conducción y de valencia, lo que produce una capa muy delgada en la cual el nivel de Fermi esta un poco por sobre la banda de conducción, por otro lado los portadores quedan confinados a una capa tan angosta que se los puede describir como un gas de electrones de dos dimensiones. Por estas dos razones los portadores de carga adquieren una muy alta movilidad y una alta velocidad de saturación, habilitándolos para reaccionar a campos que varían a muy altas frecuencias, como también reduce muy significativamente el efecto de dispersión que los átomos de dopaje producen sobre los portadores de carga rediciendo en gran medida el ruido que este dispositivo emite.
Normalmente los dos materiales semiconductores tiene la misma estructura cristalina permitiendo un adecuado calce entre estas, esto con el objeto de evitar que los portadores queden atrapados en las discontinuidades que se podrían producir. Reduciendo su rendimiento.
Existe un tipo de HEMT en los cuales esto no se cumple, los pseudomorphic HEMT (PHEMT), en ellos se pone una capa extremadamente delgada de de unos de los materiales, tanto que esta se deforma para calzar con el otro material. Con esto se logran brechas de energía mucho más altas permitiendo un mejor rendimiento del transistor.
Otra forma de lograr lo anterior es la inserción de una capa muy delgada de adaptación entre los dos materiales de forma que esta se la encargada de unir las dos estructuras cristalinas, esto presenta una ventaje cuando la capa de adaptaciones esta construida con AlInAs, en este material la concentración de In es graduada de forma de calzar las estructuras cristalinas, entonces se tiene que una alta concentración de In produce alta ganancia y una baja concentración produce bajo ruido

Military communications
Historically, the first military communications had the form of sending/receiving simple signals (often hidden or encoded to be unrecognizable for the enemy). Respectively, the first distinctive tactics of military communications were called Signals, while units specializing in those tactics received the Signal Corps name. Later Signals and Signaller became a highly-distinct military occupation dealing rather with general communications methods (similar to those in civil use) than with weapons.
Present-day militaries of an informational society conduct very intense and complicated communicating activities on a daily basis, using modern high-tech telecommunications and computing methods. Only a small part of these activities is immediately related to the combat actions. That's why some prefer the term "military communications".
In 1934 the USSR invented the first military based equipment inside an automotive vehicle.

MESFET

MESFET stands for Metal Semiconductor Field Effect Transistor. It is quite similar to a JFET in construction and terminology. The difference is that instead of using a p-n junction for a gate, a Schottky (metal-semiconductor) junction is used. MESFETs are usually constructed in compound semiconductor technologies lacking high quality surface passivation such as GaAs, InP, or SiC, and are faster but more expensive than silicon-based JFETs or MOSFETs. Production MESFETs are operated up to approximately 45 GHz[1], and are commonly used for microwave frequency communications and radar. From a digital circuit design perspective, it is increasingly difficult to use MESFETs as the basis for digital integrated circuits as the scale of integration goes up, compared to CMOS silicon based fabrication.

The MESFET differs from the common insulated gate FET in that there is no insulator under the gate over the active switching region. This implies that the MESFET gate should, in transistor mode, be biased such that one does not have a forward conducting metal semiconductor diode instead of a reversed biased depletion zone controlling the underlying channel. While this restriction inhibits certain circuit possibilities, MESFET analog and digital devices work reasonably well if kept within the confines of design limits. The most critical aspect of the design is the gate metal extent over the switching region. Generally the narrower the gate modulated carrier channel the better the frequency handling abilities, overall. Spacing of the source and drain with respect to the gate, and the lateral extent of the gate are important though somewhat less critical design parameters. MESFET current handling ability improves as the gate is elongated laterally, keeping the active region constant, however is limited by phase shift along the gate due to the transmission line effect. As a result most production MESFETs use a built up top layer of low resistance metal on the gate, often producing a mushroom-like profile in cross section.

Bipolar junction transistor

A bipolar (junction) transistor (BJT) is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes. Charge flow in a BJT is due to bidirectional diffusion of charge carriers across a junction between two regions of different charge concentrations. This mode of operation is contrasted with unipolar transistors, such as field-effect transistors, in which only one carrier type is involved in charge flow due to drift. By design, most of the BJT collector current is due to the flow of charges injected from a high-concentration emitter into the base where they are minority carriers that diffuse toward the collector, and so BJTs are classified as minority-carrier devices