Semiconductor Materials

Semiconductor Materials & Characteristics

• Si (silicon), Ge (germanium), and GaAs (gallium arsenide) are key semiconductor materials
• The fundamental principles of semiconductors have remained consistent over time, with advancements primarily in construction, characteristics, and applications
• Miniaturization has led to systems on wafers thousands of times smaller than earlier elements
• Jack Kilby invented the first integrated circuit (IC) at Texas Instruments in 1958
• Gordon E. Moore predicted in 1965 that the transistor count in an IC chip would double every two years, which has proven accurate
• Semiconductor material quality, network design, manufacturing, and innovation limit further miniaturization
• Semiconductors are elements with conductivity between conductors and insulators
• Semiconductor materials are classified as single-crystal (e.g., germanium and silicon) or compound (e.g., gallium arsenide)
• Germanium was initially favored due to its ease of refinement, but silicon later became preferred for its temperature stability and abundance

Silicon and Alternatives

• Silicon transistor was introduced in 1954 and remains a material of choice due to advances in manufacturing and design
• GaAs transistors, developed in the early 1970s, offer speeds up to five times that of silicon
• GaAs use increased with demand for speed in VLSI circuits
• Germanium is still used in specific applications due to its characteristics and low cost
• Semiconductor choices depend on atomic structure and crystalline formation
• Atoms consist of electrons, protons, and neutrons; electrons orbit the nucleus
• Silicon, germanium, gallium, and arsenic are common semiconductors, with four valence electrons save for gallium with 3 and arsenic with 5
• Valence indicates the potential to remove electrons; tetravalent, trivalent, and pentavalent refer to the number of valence electrons

Covalent Bonding

• Covalent bonding is the bond from sharing electrons, forms between atoms in pure silicon or germanium crystals
• GaAs is a compound semiconductor which shares electrons, with each gallium or arsenic atom surrounded by the complementary type
• Valence electrons can break covalent bonds via kinetic energy from external causes and become "free" electrons
• Free electrons are sensitive to electric fields
• Intrinsic semiconductor materials have very low impurity levels
• Intrinsic carriers describe free electrons due to external causes
• Germanium has the highest number of intrinsic carriers, GaAs the lowest
• Relative mobility measures free carrier movement, higher in GaAs than in Si
• Impurity levels are now very low at 1 part in 10 billion
• Doping (adding impurities) transforms semiconductors from poor to good conductors

Temperature

• Conductors have a positive temperature coefficient (resistance increases with heat)
• Semiconductors have a negative temperature coefficient(conductivity increases with heat)
• There are specific electron energy levels within each atom's shells
• Electrons farther from the nucleus have higher energy
• Electrons that leave their parent hold a higher energy state
• Energy gaps must be overcome for valence electrons to become free carriers
• An electron in silicon needs more energy than one in germanium to become a free carrier
• Higher energy gap makes silicon less effected by temperature then germanium

Semiconductors and Light

• Wider energy gaps allow energy to release in the form of light waves
• GaAs is used in LEDs because of its large gap
• Electron Volt (eV) quantifies the voltage for one energy transfer
• Doping significantly alters semiconductor characteristics by just adding 1 single part in 10 million
• Extrinsic Materials are semiconductors that have been subjected to a doping process
• Donor atoms have been diffused on a structure, creating n-type material, because they have 5 valence electrons
• N-type is electrically neutral, meaning there is the same of protons and electrons
• The Donor level is less than that of the intrinsic material, allowing a large number of carriers at the conduction level, and increasing the conductivity of the material significantly

P-Type Material and Carriers

• Impurity atoms in the p-type material have only three valence electrons which is boron on a base of silicon, and the diffused impurities with 3 are called acceptor Atoms
• P-type material means that the end result is electrically neutral
• In the intrinsic state the hole is known as a vacancy
• Electrons transfer energy, and a vacancy is created to move
• Conventional use in text direction is of hole flow
• In the n-type material the electron is known as the majority carrier and the minority carrier is the hole
• In the p-type Material the role is the majority carrier, in the former is the minority
• Each element has a + sign and there is also one by the donor ion
• N and P type materials both represent basic building blocks for semi-conductors that can be joined to create diodes

Diode Functions

• Combining N and P create Semiconductor diodes with limitless applications
• At a given point in the diode (joining of N and P) there are carriers in the electrons that region combine
• Positive and negative ions are called the depletion region, where there Is there a depletion of free carriers in the region
• Leads creates a 2-terminal device that results in having no, forward or reverse bias.
• Term bias = a voltage creating a response
• No external voltage results in the no-bias condition
• No bias is simply is simply do it with two leads isolated on a bench
• Polarity voltage has defined polarities for diode
• Positive means the voltage is the same, reverse = negative voltage
• Under no bias, negative can pass into P Material
• Layering with a larger minority layer can cause overstatement
• Vectors are canceled and not the same, but at the same in the diode
• Abscence of an external bias a net flow of charge means its at zero

Reverse Bias Condition

• Positive terminal gets n-type; when negative its p-type then reverse-biased happens
• Uncovered Positive ions result in a depletion region by attracting the force
• Widening of the depletion region results in a greater barrier preventing majority control
• Saturation current is the term as the current doesn't change as much between the minority and minority.
• Few microamps describe the constant state or nA but there is some power involved/used
• Directed is against the arrow, for example it is negative
• Forward bias means establishing a positive force which requires the use of pressure

Biasing Diodes

• Pressure electrons is the meaning of the term force for forward bias conditions
• Depletion regions with recombination ions cause current
• Current is exponential
• Scales are measured in milliamps
• Volatage is often within or less than 1 Volt
• Forward diode has less than 1
• Is(eVD/nVT – 1) defines semiconductor diode
• Is is the reverse saturation current

Equation for Calculating Voltage

• Volt-bias describes voltage forward bias
• 1 and 2 are ranges of the ideality
• VT defines thermal voltage
• kTK/q describes thermal voltage as well
• Voltage can be defined for other functions
• Each element and diode is exponential but complicated to analyze.

Graphs in Circuit Analysis

• Graphs are used to view and predict voltage drops
• Graph is useful, howeverscales may change
• Curves may be commericially availible
• Equation means it all should be exponential

Equation Analysis

• -15 and negative voltage of vd creates diodes
• 0 mA means its at 0V
• Ohms law gives an understanding of the relation to voltage current and diode
• Ideal is infinite and open because of location
• Shape and curvature needs to be less than 1 Ohm in forward

Diode Types

• Diodes exist are are used as open and closed switches in forward
• Various resistance exist
• One device is a static resistor
• Applying DCV to a diode helps its resitance
• Dc resistance levels are great
• Lower current to a diode means its greater the DC resistance that point

Dynamic Resistance

• The shape if a diode depends on its own characteristics
• Applying ac changes the situation if there is no signal being applied
• Tangent point or a tangent line on the curve defines the dynamic resistence for equation

Formula

rd ≡ ΔVD/ΔID

• slope defines ac
• Q point defines AC
• Lower Q the greater
• Power is low when generating something
• Graph and calculous allow this

Questions cover common semiconductor materials, the history of integrated circuits, and the properties of semiconductors. Topics include material purity, carrier mobility, and the number of free carriers in intrinsic materials.