Gas Laser and Semiconductor Lasers

heliumconstruction

The Gas Laser

In laboratory settings, gas lasers (shown right) are often used to eveluate waveguides and other interated optical devices. Essentially, an electric charge is pumped through a gas in a tube as shown to produce a laser output. Gasses used will determine the wavelength and efficiency of the laser. Common choices include Helium, Neon, Argon ion, carbon dioxide, carbon monoxide, Excimer, Nitrogen and Hydrogen. The gas laser was first invented in 1960. Although gas lasers are still frequently used in lab setting sfor testing, they are not practical choices to encorperate into optical integrated circuits. The only practical light sources for optical integrated circuits are semiconductor lasers and light-emitting diodes.

 

The Laser Diode

ladio

The p-n junction laser diode is a strong choice for optical integrated circuits and in fiber-optic communications due to it’s small size, high reliability nd ease of construction. The laser diode is made of a p-type epitaxial growth layer on an n-type substrate. Parallel end faces may functions as mirrors to provide the system with optical feedback.

 

The Tunnel-Injection Laser

The tunnel-injection laser enjoys many of the best features of the p-n junction laser in it’s size, simplicity and low voltage supply. The tunnel-injection laser however does not make use of a junction and is instead made in a single crystal of uniformly-doped semiconductor material. The hole-electron pairs instead are injected into the semiconductor by tunneling and diffusion. If a p-type semiconductor is used, electrons are injected through the insulator by tunneling and if the semiconductor is n-type, then holes are tunneled through the insulator.

The Quantum Well

What is a Quantum Well

Optical Integraded devices are normally built with the consideration that the device size will be large compared to the wavelength of the beams in the system. When however, the device size is reduced to a size of the same order of magnitude as the wavelength of light in the system, unique properties can be observed. The class of device that operates under the unique properties of this arrangement is the “quantum well.”

 

Uses of Quantum Wells

Quantum wells may be integrated to other optical and opt-electronic integrated circuits. Uses of quantum wells include improved lasers, photodiodes, modulators and switches.

 

Building a Quantum Well

A quantum well structure features one or more very thin layers of narrow bandgap semiconductor material, interleaved with layers of wider bandgap semiconductors. The thickness of the layers in a quantum well are typically 100 Angstroms or smaller. Quantum wells with many layers are termed a “Multiple Quantum Well” (MQW) structure and quantum wells with only one layer are termed a “Single Quantum Well (SQW) structure. A typical MQW structure may have around 100 layers. The GaAs-AlAs material system or GaInAsP are common choices for materials in quantum well structures.

quantumwell1

Superlattice Structure

A superlattice structure is a term for a case in whic a multiple quantum well structure is built with barrier wals that are thin enough that electrons are able to tunnel through the structure.

 

The Quantum Well and Quantum Dot

qwell1

The quantum well reduces the separation between an electron and hole in a semiconductor, altering the wavefunction and allowing a strong exciton bonding effect at room temperature. The semiconductor laser results from this process. Wave functions in the well are shown to the right.

When a field is applied across the well, this can result in the tilting of the wells. This can reduce the effective band gap of the material. The process of tilting the wells the alter the band gap is called the Quantum Confined Stark Effect.

qwell3

 

Quantum wells are generally understood in two dimensions. The conduction band is forced to be closer the valence band. When this is done in three dimensions to create a small box, where this squeezing effect can be emulated in all dimensions, this is termed a Quantum Dot. A Quantum Dot it turns out is highly effective at producing a high level of energy and as a result there is a high probability that it works as a coherent light source (laser). Quantum dots are readily used today, however since the process of fabrication employs the use of defects in a material to create a quantum dot, the coherency of the light produced is not perfect. Quantum dots are used in data centers for light transmission at a distance of meters. Quantum dots remain a low cost and reasonably efficient light transmission source for small distances. One reason for the low cost of quantum dots is that they can be grown on silicon wafers. A quantum well is not easily (highly unreliably, but perhaps not impossible) grown on Silicon wafers. The issue that arises with quantum wells when being grown on silicon wafers is that the size of atoms in the wafers and thereby the lattice constant is not readily compatible.

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

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.

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Semiconductors feature a lattice structure as the two below:

5304

 

Crystal Directions and Planes

The following are three types of crystal directions and planes:

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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.

 

 

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

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

 

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.

 

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.

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.

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.

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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.