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  • mbenkerumass 7:13 pm on February 7, 2020 Permalink | Reply
    Tags: Semiconductors,   

    The Bipolar Transistor, Modes of Operation 

    The transistor is a multifunction semiconductor device that, when used with other circuit elements has the ability to produce a current gain, voltage gain and signal-power gain. The transistor is referred to as a passive device, while the diode is passive. The three basic types of transistor technologies are the bipolar transistor, the metal-oxide-semiconductor field effect transistor (MOSFET) and the junction field effect transistor (JFET). The bipolar transistor most often functions as a voltage-controlled current source.

    The Bipolar Junction Transistor

    The BJT has three separately doped regions and two pn-junctions, which are close enough to interact between each other. The BJT can either be constructed as an NPN or PNP transistor, which stands for the arrangement of positive and negatively doped regions.

    pnpnpn

    The main connections of a BJT transistor are referred to as the collector, base and emitter. Generally, the emitter side is doped to a higher level than the collector. The result of this is that when a supplied a voltage, the electrons will flow in the direction from the emitter to the collector. The direction of current then will be from the collector to the emitter.

    npnpnp2pnp1

    BJT Modes of Operation

    There exist three modes of operation for the BJT transistor. In reference to the diagram below, when the Base-Emitter voltage is zero or reverse biased, the majority of carrier electrons from the emitter will not be injected into the base. This mode where all currents in the transistor are zero is referred to as cut-off. When the Base Emitter voltage is positive (forward biasing), an emitter current is generated. As the Base Emitter voltage increases, the collector current will continue to increase until a certain point at which both the Base Emitter and Base Collector junctions become forward biased. This mode is called saturation.

    npppnn

     
  • mbenkerumass 5:15 pm on January 22, 2020 Permalink | Reply
    Tags: , Semiconductors   

    ECE530 Advanced Electronics and Optoelectronics 1/21/2020 Class Notes (1st lecture) 

     

    Textbook:        High speed electronics and optoelectronics: devices and circuits, by Sheila Prasad, Hemann Schumacher, and Anand Gopinath, Cambridge university press, 2009

    Learning objective:    Principles of advanced electronics and optoelectronics are illustrated by showing their applications in advanced radar, wired/wireless communications, and electronic sensing. Key electronics/photonics devices including high speed transistors, diodes, lasers, high frequency modulators, photodetectors, amplifiers and passive circuitries are discussed.

    Outcome:       Following the completion of this course students will be able to

    1. Perform quantitative analysis of electronic and photonic systems using the basic principles covered in this course that include: wave propagation through dielectric media and optical waveguides, high frequency electronic circuits, generation and detection of light from semiconductor devices including semiconductor lasers, light emitting diodes and photodetectors and the modulation of light through the electro-optic
    2. Articulate state-of-the-art electronics and photonics technology and future trends
    3. Apply the theory of operation of electronic and photonic devices
    4. Articulate the performance and design trade-offs amongst RF, Digital, and Photonic solutions in EW architecture

    COURSE OUTLINE

    • Review of semiconductor materials and physics
      1. Semiconductor materials/crystal structure (1 week)
      2. Carrier transport/recombination/generation (2 week)
      3. Heterostructures (1 week)
    • Electronic devices
      1. High speed FET (2 week)
      2. High speed HBT (1 week)
    • Optoelectronics
      1. Optical sources (2 week)
      2. Photodetector (2 week)

     

     

    Review of Quantum Mechanics

    A course in devices would not be complete without device physics. The foundations of semiconductor devices are… Quantum Mechanics! A good resource for review in Quantum Mechanics, aside from the course textbook is the Quantum Physics course provided by MIT OpenCourseWare. This includes video lecutres, assignments, exams and more for three whole semesters’ worth of Quantum Mechanics. Quantum Physics is also important for studying the subject of Photonics and Quantum Electronics deeper and is necessary to become an expert in a related field. More review of Quantum Mechanics is to come soon.

    Quantum Physics I
    Video lectures: https://archive.org/details/MIT8.04S16/
    Syllabus: https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2016/syllabus/

    Quantum Physics II
    https://ocw.mit.edu/courses/physics/8-05-quantum-physics-ii-fall-2013/

    Quantum Physics III
    https://ocw.mit.edu/courses/physics/8-06-quantum-physics-iii-spring-2016/

    Other courses available are found here: https://ocw.mit.edu/courses/find-by-topic/

     

    T-CAD, RSoft

    This course features the use of Rsoft and T-CAD, Silvaco for device modeling, doping and bandgap engineering problems.

     

    Semiconductors

    Silicon wafers (~4″) go for about $20. GaN wafers on the other hand go for about ~$1k. The price differential may be one of the few things silicon has going for it. In other cases, consider that Silicon does not work at high speeds. For high speed semiconductor devices, III-V semiconductors are preferred and will work better. One other interesting downside of Silicon is that it is unable to emit light.

    5301

    The Hybrid HBT is a better option for transistor technologies at higher frequencies. The HBT features higher electron mobility.

    5302

    This course will also feature study of Ternary and Quarternary Compounds. Quarternary compounds are used for quantum well lasers. Ternary Compounds feature one variable x where quarternary compounds feature two variables x,y. Another important ternary compound not listed in InGaAs.

     

    Material Growth

    We will also discuss the process for fabrication and material growth. Material growth and fabrication is a process that generally requires a high level of technological know-how and thus only few countries manage to perform this operation. Currently, it is even possible to grow one atomic layer (Angstrom) at a time. Two methods of material growth are MOCVD (Nobel Prize awarded for this discovery) and MBE (should also get a Nobel Prize soon). MOCVD is particularly best for producing many at the same time, while MBE is better used for research production.

     

    Bell Laboratories

    As an aside, consider the company that had existed before breaking up, Bell Laboratories. At Bell Laboratories, given a monopoly it was able to fund scientists and researchers to conduct free-range scientific research and discovery. Today, researchers at Universities (primarily) need to provide evidence of advancement ever 6 months or so. A program such as Bell Laboratories allowed for aimless research to be conducted, which ended up being far more successful than could have been imagined.

     

    Types of Solids

    Consider the three types of solids, Crystalline, Polycrystalline and Amorphous. Crystalline structures feature atoms that are aligned periodically and produce a unique shape (example: Quartz). Polycrystalline solids include ceramic, saphire, even possibly other metals such as steel. Interesting point about saphire – saphire is used in PCVD as a film. Amorphous solids include liquidated solids, glass and other liquids.

    5303

    Semiconductors feature a lattice structure as the two below:

    5304

     

    Crystal Directions and Planes

    The following are three types of crystal directions and planes:

    5305

    It is of note that etching in crystals must be done in only certain directions according to the ‘grain’ of the crystal. In order to split a waver along a grain, make a notch at one point on the side and apply a pressure to the wafer.

     

    Atomic Bonding
    It is important to review the effect of covalent bonding between valence electrons.

    5307

    Next class, the topic of wave equations will be covered.

     

     

     
  • mbenkerumass 7:00 am on January 11, 2020 Permalink | Reply
    Tags: Semiconductors,   

    BJT vs. FET 

    Transistors are important components that are used in a variety of applications. Some types can be used for switching, some for amplification or both. Other transistors perform exclusive tasks, such as the phototransistor, which responds to light by producing a current.

    The main premise of a transistor is that by feeding a transistor a source voltage or current (depending on the type), the transistor allows for the passage of electrons. This process is accomplished through pnp or npn semiconductor structures. The following diagrams provide a general example of the function of a transistor:

    t4t2

    transistor

    t3

     

     

    Bipolar Junction Transistors (BJT) are controlled using a biasing current at the base pin. This means that they will also consume more current than other transistors such as the FET. One advantage of BJT transistors is that they offer greater output gain than an FET. However, BJT can be much larger in size than FET and for this reason, they are less popular, despite being easier to manufacture.

    bjt

    Field Effect Transistors (FET) are voltage-controlled. For this reason they essentially draw no current and therefore do not pose a substantial load to a circuit. FETs are not as useful for gain as BJT, however if the intent is not for amplification then this is not a problem. FETs can be manufactured very small and this is important in manufacturing integrated circuits that use many transistors. FETs and especially the MOSFET subtype are more expensive to manufacture, but remain more popular than the BJT.

    fet

    Some FET transistor types are even constructed on the nano-scale. The FinFET for example is about 10 nm, currently used by Intel, Samsung and others.

    FinFET size

    (2) http://www.learningaboutelectronics.com/Articles/Types-of-transistors.php

     
  • mbenkerumass 6:00 am on January 4, 2020 Permalink | Reply
    Tags: , Semiconductors   

    The P-N Junction 

    A P-N junction is created in a single semiconductor crystal by doping one side as a p-type and one as an n-type. The region where the two types converge is known as the p-n junction.

    The extra electrons that were added to the n-type semiconductor move towards the p-type junction side while the holes added through p-type doping are positioned closer to the n-type junction.

    pnj2

    As electrons leave the n-type region, it becomes positively charged. This process is called diffusion. The depletion region is the area between the p and n-type sides. The state of equilibrium in the p-n junction is the state of the depletion region without any external electrical potential applied. As mentioned before in a previous paper, the Fermi level is the average between the conduction band and the valence band. By altering the levels of electron holes and electrons in the p-type and n-type sections, holes drift toward the the n-type side and electrons move towards the p-type side, which causes both sections to be closer to the Fermi level in their regions of the material.

    pnj

    When voltage is applied to the pn junction, electrons and electron holes from either side tend towards equilibrium. If the positive potential is applied to the p-type and it is more positive than the n-type area, holes will travel towards the negative voltage. Through diffusion, electrons or electron holes may jump through the depletion layer. For the reason however that electron holes (positive charge) may only move in the direction of the n-type region and electrons (negative charge) may only move in the opposite direction. The direction of electron flow, due to their negative charge is opposite the conventional direction of current flow. Since electrons are only moving from the n-type region to the p-type region, it can be understood that current will only move in the direction going from the side of the p-type region towards the n-type region.pnj1

     

     
  • mbenkerumass 9:00 am on January 2, 2020 Permalink | Reply
    Tags: Semiconductors   

    P-Type and N-Type Semiconductors 

    N-Type Semiconductors are created when doping a semiconductor with impurities that adds extra valence electrons to the outermost shell to share free electrons with neighboring atoms. Phosphorous, arsenic and antimony are examples of atoms with five valence electrons, also known as pentavalent impurities, adding an extra electron for each doped atom. This does not mean however that an N-type semiconductor is negatively changed, because there will exist a balancing positive charge in the nucleus of the doped atom. An N-type semiconductor is a better conductor than intrinsic semiconductor materials.

    P-Type Semiconductors are formed by adding group 3 elements, known as trivalent impurity atoms such as boron, aluminum and indium to the semiconductor structure. These atoms have only three electrons in the outermost shell, producing an extra electron hole, which attracts neighboring electrons.

    p-type and n-type semiconductors

    To recap, N-type semiconductors:

    • possess pentavalent elements as impurity atoms to add a donor electron to the material.
    • do not have a negative charge since atom nucleus charge offsets added electrons, meaning they are electrically neutral.

    P-type semiconductors:

    • possess trivalent impurity elements as impurity atoms to add an electron hole.
    • are also electrically neutral.

     

     
  • mbenkerumass 9:00 am on December 28, 2019 Permalink | Reply
    Tags: Semiconductors   

    Fermi Level in Semiconductor Materials 

    The Fermi level in a semiconductor is the probability that energy levels in a valence band and conduction band in the atoms are occupied. At absolute zero temperature, a semiconductor acts as a perfect insulator. As the temperature increases, free electrons are made available.  An intrinsic semiconductor is a pure crystal with no impurities or defect atoms. In an intrinsic semiconductor, the probability of occupation of energy levels in either the conduction band or the valence band are equal. The Fermi level of an intrinsic semiconductor lies between the valence band and the conduction band. This area between both bands is known as the forbidden band.

    fermi

    Where KB is the Boltzmann constant (1.3806503 × 10-23 m2 kg s-2 K-1), T is the absolute temperature of the intrinsic semiconductor, Nv is the density of states in the valence band, the hole concentration in the valence band is:

    hole

    Where Nc is the density of states in the conduction band, the electron concentration in the conduction band is calculated:

    electron

    The Fermi level for an intrinsic semiconductor is given as the average of the conduction band level and the valence band level.

    fermilev

     

    Electron Doping

    A intrinsic semiconductor may be altered by adding controlled amounts of specific atoms, called dopants to the crystal. This alters the number of electrons in the conduction bands or electron holes in the valence bands.

     
  • mbenkerumass 9:00 am on December 26, 2019 Permalink | Reply
    Tags: , Semiconductors, Solid State Physics   

    Crystal Structures 

    Crystal Structures

    Crystalline structures are noted by their regular, predictable and periodic arrangement of atoms or molecules. The  arrangement of atoms and molecules for crystal structures is called a lattice. Crystalline materials include many metals, chemical salts and semiconductors.

    copper

    Solid crystals are classified by the cohesive forces that hold the lattice together and the shape or arrangement of the atoms in the material. Different arrangements include a simple cubic crystal, a face-centered cubic structure and a body-centered cubic structure.

    cubic

    In metals, each atom contributes at least one loosely bound electron to build an electron gas of nearly free electrons that move throughout the lattice structure. When an electric field E is applied to a metal, a current flows in the direction of the field. The flow of charges is described in terms of a current density J, or current per unit cross-sectional area. The current density is proportional to the applied electric field by a factor of the electrical conductivity σ of the material.

    J = σ*E

    The electrons in the lattice material experience a force F = -e*E due to the field and become accelerated. The velocity of electrons in the lattice is known as the drift velocity.

     

    Bonding and the formation of Semiconductors

    In atomic structures, different types of molecules have a varying number of electrons in the outer atomic rings or shells (valence electrons). Ionic bonding is performed by electrons present in the outermost shell, easily forming a positive ion by releasing the outer electron (net positive charge) or enter the outermost shell of another atom to make it a negative ion (net negative charge). Metallic bonding uses a loosely bound electron in an outermost shell to contribute to the crystal as a whole, creating a metallic crystal.

    periodic table

    The method of bonding for Ge, C and Si can be quite different however, since they have four valence electrons in the outermost shell. These four electrons can be shared with four neighboring molecules. The bonding force that results from this phenomenon is covalent bonding. In this formation however, electrons belonging to the same bond do not have a definite position in any one atom, meaning they may move between atoms that are bonded. Compound semiconductors such as GaAs (Gallium Arsenide), AlAs (Aluminum Arsenide) and InP (Indium Phosphide) have mixed bonding including both covalent and ionic bonding. These bonding characteristics and the ability for electrons to both move throughout atoms in the structure and to form ionic bonds are the basis for the use of semiconductor materials.

     
  • mbenkerumass 9:00 am on December 18, 2019 Permalink | Reply
    Tags: , Semiconductors   

    Quantum Theory of Solids 

    Classical mechanics have long been proven to be useful for predicting the motion of large objects. Newton’s laws however prove to be highly inaccurate for measurements involving electrons and high frequency electromagnetic waves. Semiconductor physics, for example requires that a new model be adopted. The quantum mechanical model proves to be appropriate in these cases. Quantum mechanics allows for the calculation of the response of an electron in a crystallized structure to an external source such as an electric field, for instance. The movement of an electron in a lattice will differ from it’s movement in free space and quantum mechanics is used to relate classical Newtonian mechanics to such circumstances.

    crystal1

    The photoelectric effect is one example of a circumstance that is not describable using classical mechanics. Planck devised a theory of energy quanta in a formula that states that the energy E is equal to the frequency of the radiation multiplied by h, Planck’s constant (h = 6.625 x 10^(-34) J*s). Einstein later interpreted this theory to conclude that a photon is a particle-like pack of energy, also modeled by the same equation, E = hv. With sufficient energy can remove an electron for the surface of a material. The minimum energy required to remove an electron is called the work function of the material. Excess photon energy is is converted to kinetic energy in the moved electron.

    photoelectriceffect

    Hertz discovered the photoelectric effect in 1887. He found that polished plates irrradiated may emit electrons. This was termed the photoelectric effect. It was found that there was a minimum frequency threshold required to produce a current. The minimum frequency threshold was a function of the type of metal and configuration of atoms at the surface. The magnitude of the current emitted is proportional to the light intensity. The energy of the photo-electrons (electrons emitted by photons) was independent of the intensity of light, however the energy emitted increased linearly with the frequency of light.

    Einstein in 1905 explained that light is composed of quanta (photons) with energy E = h*ν, where h is Planck’s constant and ν is the frequency. The work function specifies how much energy is needed to release electrons from a metal. The energy of the electron then is equal to the energy of the photon minus the work function. The remainder energy of the photon is transmitted as kinetic energy. An experimental verification of Einstein’s prediction came 10 years later.

    u444444444444444

    The following is an example problem for photoelectric effect calculations:

    photoexample

     

    The wave-particle duality principle was presented by de Broglie to suggest that, since waves exhibit particle-like behavior, particles also should show wave-like properties. The momentum of a photon was then proposed to be equal to Planck’s constant devided by the wavelength. The ultimate conclusion to de Broglie’s hypothesis was that in some cases, electromagnetic waves behave as photons or particles and sometimes particles behave as waves. This is an important principle used in quantum mechanics.

    waveparticleduality

    The Heisenberg Uncertainty Principle states that it is impossible to simultaneously describe with absolute accuracy the momentum and the position of a particle. This may also include angular position and angular momentum. The principle also states that it is impossible to describe with absolute accuracy the energy of a particle and the instant of time that the particle is energized. Rather than determining the exact position of an electron for instance, a probability density function is developed to determine the likelihood that an electron is in a particular location or has a certain amount of energy.

     
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