6.5 Semiconductor devices

Semiconductor devices are the building blocks of modern electronics, utilizing the unique properties of materials like silicon and gallium arsenide. These devices, including transistors and diodes, form the foundation of integrated circuits that power our digital world.

This topic explores various semiconductor devices, their underlying physics, and fabrication techniques. From basic p-n junctions to advanced quantum well structures, we'll examine how these devices work and their applications in electronics and optoelectronics.

Types of semiconductor devices

• Semiconductor devices are electronic components that exploit the electrical properties of semiconductor materials (silicon, germanium, gallium arsenide)
• They form the foundation of modern electronics, enabling the development of transistors, diodes, and integrated circuits
• Semiconductor devices have revolutionized various fields, including computing, telecommunications, and consumer electronics

Semiconductor materials

Elemental semiconductors

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• Elemental semiconductors consist of a single element from group IV of the periodic table (silicon, germanium)
• Silicon is the most widely used elemental semiconductor due to its abundance, low cost, and favorable electrical properties
• Germanium was used in early semiconductor devices but has been largely replaced by silicon

Compound semiconductors

• Compound semiconductors are formed by combining elements from groups III and V (gallium arsenide, indium phosphide) or groups II and VI (cadmium telluride, zinc selenide) of the periodic table
• They offer unique properties such as higher electron mobility, wider bandgaps, and better optical performance compared to elemental semiconductors
• Compound semiconductors find applications in high-frequency electronics, optoelectronics, and photovoltaics

Energy bands in semiconductors

Valence and conduction bands

• In semiconductors, the valence band is the highest occupied energy band at absolute zero temperature, while the conduction band is the lowest unoccupied energy band
• Electrons in the valence band are bound to the atoms and do not contribute to electrical conduction
• Electrons in the conduction band are free to move and participate in electrical conduction

Band gap and Fermi level

• The band gap is the energy difference between the top of the valence band and the bottom of the conduction band
• The Fermi level represents the energy level at which the probability of an electron occupying a state is 50%
• In intrinsic semiconductors, the Fermi level lies approximately in the middle of the band gap
• Doping can shift the Fermi level towards the conduction band (n-type) or valence band (p-type)

Charge carriers in semiconductors

Electrons and holes

• Electrons are negatively charged particles that can move freely in the conduction band and contribute to electrical conduction
• Holes are the absence of electrons in the valence band and behave as positively charged particles
• Both electrons and holes can participate in electrical conduction in semiconductors

Intrinsic vs extrinsic semiconductors

• Intrinsic semiconductors are pure semiconductors without any intentional doping
• In intrinsic semiconductors, the number of electrons in the conduction band equals the number of holes in the valence band
• Extrinsic semiconductors are doped with impurities to increase the concentration of either electrons (n-type) or holes (p-type)
• Doping allows for the control of electrical properties and the creation of semiconductor devices

Doping of semiconductors

n-type doping

• n-type doping involves introducing impurities (dopants) with an excess of valence electrons (phosphorus, arsenic) into the semiconductor material
• The extra electrons from the dopants occupy energy levels near the conduction band, making it easier for them to be excited into the conduction band
• n-type semiconductors have a higher concentration of electrons compared to holes

p-type doping

• p-type doping involves introducing impurities (dopants) with a deficiency of valence electrons (boron, gallium) into the semiconductor material
• The missing electrons create holes in the valence band, which can accept electrons from neighboring atoms
• p-type semiconductors have a higher concentration of holes compared to electrons

p-n junctions

Built-in potential

• When a p-type and an n-type semiconductor are brought into contact, a p-n junction is formed
• Due to the concentration gradient, electrons diffuse from the n-type region to the p-type region, and holes diffuse in the opposite direction
• This diffusion creates a built-in potential ($V_{bi}$) across the junction, which opposes further diffusion of charge carriers

Depletion region

• The diffusion of charge carriers leaves behind immobile ionized dopant atoms near the p-n junction, creating a region depleted of free charge carriers called the depletion region
• The depletion region has a high resistance and acts as a barrier to the flow of charge carriers
• The width of the depletion region depends on the doping concentrations and the applied voltage

Forward vs reverse bias

• Forward bias occurs when a positive voltage is applied to the p-type region and a negative voltage to the n-type region, reducing the built-in potential and allowing current to flow through the p-n junction
• Reverse bias occurs when a negative voltage is applied to the p-type region and a positive voltage to the n-type region, increasing the built-in potential and preventing current flow
• The behavior of a p-n junction under forward and reverse bias forms the basis for the operation of diodes and other semiconductor devices

Diodes

Ideal vs real diodes

• An ideal diode allows current to flow in the forward direction (forward bias) with zero resistance and blocks current flow in the reverse direction (reverse bias) with infinite resistance
• Real diodes have a small forward voltage drop ($V_f$) in the forward bias condition, typically 0.6-0.7 V for silicon diodes and 0.2-0.3 V for germanium diodes
• Real diodes also have a small leakage current in the reverse bias condition, known as the reverse saturation current ($I_s$)

Current-voltage characteristics

• The current-voltage (I-V) characteristics of a diode describe its behavior under different bias conditions
• In the forward bias region, the current increases exponentially with the applied voltage according to the Shockley diode equation: $I = I_s(e^{qV/kT} - 1)$
• In the reverse bias region, the current remains close to the reverse saturation current until the reverse breakdown voltage is reached, at which point the current increases rapidly

Diode applications

• Rectification: Diodes are used to convert alternating current (AC) to direct current (DC) by allowing current to flow only in one direction (half-wave rectification, full-wave rectification)
• Voltage regulation: Zener diodes are used to maintain a constant voltage across a load by operating in the reverse breakdown region
• Overvoltage protection: Diodes can be used to protect circuits from voltage spikes by limiting the voltage across sensitive components
• Logic gates: Diodes are used in the implementation of logic gates (AND, OR) in diode-resistor logic (DRL) and diode-transistor logic (DTL)

Bipolar junction transistors (BJTs)

npn vs pnp transistors

• BJTs are three-terminal devices consisting of three differently doped semiconductor regions: emitter, base, and collector
• npn transistors have a thin p-type base region sandwiched between two n-type regions (emitter and collector)
• pnp transistors have a thin n-type base region sandwiched between two p-type regions (emitter and collector)
• The type of transistor (npn or pnp) determines the direction of current flow and the polarity of the voltages applied to the terminals

Transistor operation and configurations

• BJTs operate by controlling the current flow between the emitter and the collector through a small current injected into the base
• The three main configurations of BJTs are common emitter (CE), common base (CB), and common collector (CC), each with different input and output characteristics
• In the CE configuration, a small change in the base current results in a large change in the collector current, making it suitable for amplification and switching applications

Current gain and amplification

• The current gain ($\beta$) of a BJT is the ratio of the collector current ($I_C$) to the base current ($I_B$): $\beta = I_C/I_B$
• BJTs can achieve high current gains, typically in the range of 50 to 200, enabling them to amplify small input signals
• The voltage gain ($A_V$) of a BJT amplifier is the product of the current gain and the ratio of the collector resistor ($R_C$) to the emitter resistor ($R_E$): $A_V = \beta (R_C/R_E)$

Field-effect transistors (FETs)

JFET vs MOSFET

• FETs are three-terminal devices that control the current flow through a semiconductor channel by applying an electric field
• Junction FETs (JFETs) have a reverse-biased p-n junction to control the channel conductivity, while metal-oxide-semiconductor FETs (MOSFETs) use an insulated gate electrode
• JFETs are depletion-mode devices, meaning they are normally on and require a reverse bias to turn off, while MOSFETs can be either depletion-mode or enhancement-mode (normally off)

Gate, source, and drain

• The three terminals of a FET are the gate, source, and drain
• The gate controls the channel conductivity by applying an electric field, the source is the terminal through which carriers enter the channel, and the drain is the terminal through which carriers leave the channel
• In n-channel FETs, electrons are the majority carriers, while in p-channel FETs, holes are the majority carriers

Transistor operation and characteristics

• FETs operate by modulating the channel conductivity through the application of a gate voltage
• The gate voltage controls the depletion region width in JFETs or the inversion layer thickness in MOSFETs, which in turn affects the channel resistance and the drain current
• The output characteristics of FETs show the relationship between the drain current ($I_D$) and the drain-source voltage ($V_{DS}$) for different gate-source voltages ($V_{GS}$)
• The transfer characteristics of FETs show the relationship between the drain current ($I_D$) and the gate-source voltage ($V_{GS}$) for a fixed drain-source voltage ($V_{DS}$)

Semiconductor device fabrication

Photolithography and etching

• Photolithography is a process used to transfer patterns from a photomask to the surface of a semiconductor wafer
• The wafer is coated with a light-sensitive material called photoresist, which is then exposed to light through the photomask
• The exposed regions of the photoresist are selectively removed (positive photoresist) or retained (negative photoresist) during development
• Etching is used to remove the uncovered regions of the semiconductor material or other layers, transferring the pattern from the photoresist to the wafer

Diffusion and ion implantation

• Diffusion is a process used to introduce dopants into the semiconductor material by exposing the wafer to a high-temperature environment containing the dopant atoms
• The dopant atoms diffuse into the semiconductor material, creating regions with different electrical properties (n-type or p-type)
• Ion implantation is an alternative doping method that involves accelerating dopant ions and directing them towards the wafer surface
• Ion implantation offers better control over the dopant concentration and depth profile compared to diffusion

Metallization and packaging

• Metallization is the process of depositing metal layers on the semiconductor wafer to create electrical connections and contacts
• Metal layers (aluminum, copper) are deposited using physical vapor deposition (PVD) or chemical vapor deposition (CVD) techniques
• The metal layers are patterned using photolithography and etching to form the desired interconnect structures
• Packaging involves encapsulating the semiconductor device in a protective package (plastic, ceramic) and connecting it to external leads or pins for integration into electronic circuits

Advanced semiconductor devices

Thyristors and IGBTs

• Thyristors are four-layer (pnpn) semiconductor devices that exhibit bistable switching characteristics, making them suitable for high-power applications (power control, switching)
• Insulated-gate bipolar transistors (IGBTs) combine the high input impedance and voltage control of MOSFETs with the low on-state resistance and high current capability of BJTs
• IGBTs are widely used in power electronics applications, such as motor drives, inverters, and switch-mode power supplies

Optoelectronic devices

• Optoelectronic devices convert electrical signals to optical signals (light-emitting diodes, laser diodes) or optical signals to electrical signals (photodiodes, solar cells)
• Light-emitting diodes (LEDs) are p-n junction devices that emit light when forward-biased, finding applications in lighting, displays, and optical communication
• Laser diodes are similar to LEDs but produce coherent, monochromatic light through stimulated emission, enabling applications in fiber-optic communication, barcode scanners, and laser pointers

Quantum well and superlattice devices

• Quantum well devices are based on thin layers of semiconductor materials with different bandgaps, creating potential wells that confine charge carriers in one dimension
• Quantum wells exhibit unique electronic and optical properties, such as enhanced carrier mobility and discrete energy levels, making them suitable for high-speed electronics and optoelectronics
• Superlattice devices consist of alternating layers of two different semiconductor materials, creating a periodic potential that affects the electronic and optical properties
• Superlattices find applications in infrared detectors, terahertz devices, and quantum cascade lasers