6.4 p-n junctions

P-n junctions are the building blocks of semiconductor devices. They form when p-type and n-type semiconductors meet, creating a boundary with unique electrical properties. Understanding p-n junctions is key to grasping how diodes, transistors, and solar cells work.

This topic covers the formation, energy band diagrams, biasing, capacitance, and breakdown mechanisms of p-n junctions. It also explores their applications in rectification, LEDs, solar cells, and photodetectors, highlighting their importance in modern electronics.

Formation of p-n junctions

• P-n junctions are fundamental building blocks of semiconductor devices and play a crucial role in the functioning of diodes, transistors, and solar cells
• The formation of p-n junctions involves the joining of two differently doped semiconductor materials, creating a boundary between p-type and n-type regions
• Understanding the process of doping and the characteristics of p-type and n-type semiconductors is essential for comprehending the behavior of p-n junctions

Doping of semiconductors

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• Doping introduces impurities into intrinsic semiconductors to modify their electrical properties
• Dopant atoms can be either donors (n-type) or acceptors (p-type)
  • Donor atoms (phosphorus, arsenic) have extra valence electrons and increase the concentration of free electrons
  • Acceptor atoms (boron, gallium) create holes by accepting electrons from the valence band
• The concentration of dopants determines the majority charge carriers and the conductivity of the semiconductor

p-type vs n-type semiconductors

• P-type semiconductors have an excess of holes as majority charge carriers
  • Holes are created by acceptor dopants that accept electrons from the valence band
  • The Fermi level in p-type semiconductors is closer to the valence band
• N-type semiconductors have an excess of electrons as majority charge carriers
  • Electrons are provided by donor dopants that contribute extra electrons to the conduction band
  • The Fermi level in n-type semiconductors is closer to the conduction band

Contact between p-type and n-type regions

• When p-type and n-type semiconductors are brought into contact, a p-n junction is formed
• Initially, there is a concentration gradient of electrons and holes across the junction
• Diffusion of majority carriers occurs, with electrons flowing from the n-type to the p-type region and holes flowing in the opposite direction
• As the carriers diffuse, they leave behind exposed ionized dopant atoms, creating a space charge region or depletion region near the junction

Energy band diagram of p-n junctions

• The energy band diagram provides a visual representation of the energy levels and the behavior of charge carriers in a p-n junction
• It illustrates the built-in potential barrier, depletion region, space charge distribution, and electric field within the junction
• Understanding the energy band diagram is crucial for analyzing the electrical characteristics and operation of p-n junctions

Built-in potential barrier

• When a p-n junction is formed, the diffusion of majority carriers creates a built-in potential barrier ($V_{bi}$) across the junction
• The built-in potential opposes further diffusion of majority carriers and establishes an equilibrium condition
• The magnitude of the built-in potential depends on the doping concentrations and the semiconductor material properties

Depletion region

• The depletion region is a region near the p-n junction where the majority carriers have diffused away, leaving behind exposed ionized dopant atoms
• It is also known as the space charge region due to the presence of fixed charges from the ionized dopant atoms
• The width of the depletion region depends on the doping concentrations and the applied voltage across the junction

Space charge distribution

• The space charge distribution describes the spatial variation of the fixed charges in the depletion region
• In the p-type region, the space charge consists of negatively charged acceptor ions
• In the n-type region, the space charge consists of positively charged donor ions
• The space charge distribution gives rise to an electric field within the depletion region

Electric field in depletion region

• The presence of the space charge in the depletion region creates an electric field across the junction
• The electric field points from the n-type region to the p-type region, opposing the diffusion of majority carriers
• The magnitude of the electric field is highest at the junction and decreases linearly towards the edges of the depletion region
• The electric field plays a crucial role in the transport of charge carriers across the junction

Biasing of p-n junctions

• Biasing refers to the application of an external voltage across a p-n junction to control its electrical behavior
• The type of bias (forward or reverse) determines the direction of current flow and the junction's operating characteristics
• Understanding the biasing of p-n junctions is essential for designing and analyzing semiconductor devices

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
  • It reduces the built-in potential barrier and allows majority carriers to flow easily across the junction
  • The depletion region width decreases, and a significant forward current flows through the junction
• Reverse bias occurs when a negative voltage is applied to the p-type region and a positive voltage to the n-type region
  • It increases the built-in potential barrier and prevents the flow of majority carriers across the junction
  • The depletion region width increases, and only a small leakage current (reverse saturation current) flows

Current-voltage characteristics

• The current-voltage (I-V) characteristics of a p-n junction describe the relationship between the applied voltage and the resulting current
• In forward bias, the current increases exponentially with the applied voltage, following the ideal diode equation
• In reverse bias, the current remains small and constant (reverse saturation current) until the breakdown voltage is reached
• The I-V characteristics provide valuable information about the junction's rectifying behavior and its suitability for various applications

Ideal diode equation

• The ideal diode equation relates the current flowing through a p-n junction to the applied voltage
• It is given by: $I = I_s(e^{qV/kT} - 1)$, where $I$ is the diode current, $I_s$ is the reverse saturation current, $q$ is the electron charge, $V$ is the applied voltage, $k$ is Boltzmann's constant, and $T$ is the absolute temperature
• The ideal diode equation assumes that the current is solely due to the diffusion of majority carriers and neglects other effects such as recombination and generation

Deviations from ideal behavior

• Real p-n junctions deviate from the ideal diode equation due to various factors
• Series resistance in the semiconductor material and contacts can limit the current flow, especially at high forward bias
• Generation and recombination of carriers in the depletion region can contribute to the current, particularly at low forward bias and in reverse bias
• Surface effects, such as surface leakage and surface recombination, can influence the junction characteristics

Capacitance of p-n junctions

• P-n junctions exhibit capacitive behavior due to the presence of the depletion region and the variation of the space charge with applied voltage
• The capacitance of a p-n junction plays a significant role in the dynamic behavior of semiconductor devices and their high-frequency performance
• Two types of capacitance are associated with p-n junctions: junction capacitance and diffusion capacitance

Junction capacitance

• Junction capacitance, also known as depletion layer capacitance, arises from the variation of the depletion region width with applied voltage
• It is determined by the fixed charges in the depletion region and the permittivity of the semiconductor material
• The junction capacitance is inversely proportional to the square root of the applied reverse bias voltage
• It is an important parameter in high-frequency applications and affects the switching speed of semiconductor devices

Diffusion capacitance

• Diffusion capacitance is associated with the storage of minority carriers in the neutral regions adjacent to the depletion region
• It occurs under forward bias conditions when minority carriers are injected across the junction
• The diffusion capacitance is proportional to the forward bias current and the minority carrier lifetime
• It becomes significant at high forward bias currents and limits the high-frequency performance of bipolar devices

Variation with applied voltage

• The capacitance of a p-n junction varies with the applied voltage
• Under reverse bias, the junction capacitance dominates, and it decreases with increasing reverse voltage due to the widening of the depletion region
• Under forward bias, the diffusion capacitance becomes significant and increases with increasing forward current
• The total capacitance of a p-n junction is the sum of the junction capacitance and the diffusion capacitance
• The voltage dependence of the capacitance is utilized in varactor diodes for tuning and voltage-controlled capacitance applications

Breakdown mechanisms in p-n junctions

• Breakdown in p-n junctions occurs when the applied reverse voltage exceeds a certain critical value, leading to a sudden increase in the reverse current
• Understanding the breakdown mechanisms is crucial for designing devices that can withstand high voltages and for exploiting the breakdown phenomena in specific applications
• The two primary breakdown mechanisms in p-n junctions are Zener breakdown and avalanche breakdown

Zener breakdown

• Zener breakdown occurs in heavily doped p-n junctions with narrow depletion regions
• It is caused by quantum mechanical tunneling of electrons from the valence band of the p-type region to the conduction band of the n-type region
• Zener breakdown is characterized by a sharp increase in the reverse current at a specific reverse voltage called the Zener voltage
• The Zener voltage depends on the doping concentrations and the bandgap of the semiconductor material
• Zener diodes are designed to operate in the Zener breakdown region and are used for voltage regulation and reference voltage applications

Avalanche breakdown

• Avalanche breakdown occurs in lightly doped p-n junctions with wide depletion regions
• It is caused by the acceleration of charge carriers in the high electric field of the depletion region, leading to impact ionization
• When the electric field exceeds a critical value, the accelerated carriers gain sufficient energy to generate electron-hole pairs through collisions with the lattice
• The newly generated carriers are also accelerated, creating an avalanche multiplication of carriers and a rapid increase in the reverse current
• Avalanche breakdown is characterized by a gradual increase in the reverse current at a specific reverse voltage called the breakdown voltage

Breakdown voltage

• The breakdown voltage is the reverse voltage at which the p-n junction undergoes breakdown, either through Zener or avalanche mechanisms
• It depends on the doping concentrations, the semiconductor material properties, and the junction geometry
• The breakdown voltage can be engineered by controlling the doping profiles and using edge termination techniques
• Devices such as avalanche photodiodes and transient voltage suppressors rely on the controlled breakdown characteristics of p-n junctions

Applications of p-n junctions

• P-n junctions find extensive applications in various electronic devices and systems, leveraging their rectifying, light-emitting, and light-sensing properties
• Some of the key applications of p-n junctions include rectification and power conversion, light-emitting diodes (LEDs), solar cells, and photodetectors
• Understanding the principles and characteristics of p-n junctions is essential for designing and optimizing these applications

Rectification and power conversion

• P-n junctions are used as rectifiers to convert alternating current (AC) to direct current (DC)
• The unidirectional current flow property of p-n junctions allows them to conduct current only in the forward bias direction while blocking current in the reverse bias direction
• Rectifier diodes are used in power supply circuits to convert AC mains voltage to DC voltage for electronic devices
• Half-wave rectifiers use a single diode to rectify the positive or negative half-cycle of the AC waveform, while full-wave rectifiers use multiple diodes to rectify both half-cycles

Light-emitting diodes (LEDs)

• LEDs are p-n junctions that emit light when forward biased
• When electrons and holes recombine in the depletion region, they release energy in the form of photons, resulting in light emission
• The wavelength (color) of the emitted light depends on the bandgap of the semiconductor material used in the LED
• LEDs are widely used for lighting, displays, indicators, and optical communication due to their high efficiency, long lifetime, and fast switching capabilities

Solar cells and photovoltaics

• Solar cells are p-n junctions that convert light energy into electrical energy through the photovoltaic effect
• When photons with sufficient energy are absorbed by the semiconductor, they generate electron-hole pairs
• The built-in electric field of the p-n junction separates the photogenerated carriers, creating a photocurrent and a photovoltage
• Solar cells are connected in series and parallel to form solar panels and arrays for large-scale power generation
• The efficiency of solar cells depends on factors such as the semiconductor material, junction design, and light trapping techniques

Photodetectors and optical sensors

• P-n junctions can be used as photodetectors to convert light signals into electrical signals
• When reverse biased, the p-n junction acts as a photodiode, generating a photocurrent proportional to the incident light intensity
• Photodiodes are used in various applications, including optical communication receivers, camera sensors, and light detection systems
• The sensitivity, response time, and spectral range of photodiodes depend on the semiconductor material and the junction design
• Other types of optical sensors, such as phototransistors and avalanche photodiodes, also rely on the light-sensing properties of p-n junctions