🧗♀️Semiconductor Physics Unit 3 – Equilibrium Carrier Statistics in Semiconductors
Equilibrium carrier statistics in semiconductors form the foundation for understanding how these materials behave electrically. This unit covers key concepts like the Fermi-Dirac distribution, density of states, and carrier concentration equations, which are crucial for analyzing semiconductor properties.
Temperature effects, doping impacts, and the distinction between equilibrium and non-equilibrium states are explored. These principles are essential for designing and optimizing various semiconductor devices, from solar cells and LEDs to transistors and lasers.
Semiconductors materials with electrical conductivity between insulators and conductors (silicon, germanium)
Carriers charge carriers responsible for electrical conduction in semiconductors
Electrons negative charge carriers in the conduction band
Holes positive charge carriers in the valence band
Intrinsic semiconductors pure semiconductors without any added impurities
Extrinsic semiconductors semiconductors with added impurities (dopants) to modify electrical properties
Fermi level energy level with a 50% probability of being occupied by an electron at thermal equilibrium
Band gap energy difference between the top of the valence band and the bottom of the conduction band
Density of states (DOS) number of available energy states per unit volume and energy interval in a semiconductor
Fermi-Dirac Distribution
Describes the probability of an electron occupying an energy state at thermal equilibrium
Represented by the equation: f(E)=1+e(E−EF)/kT1
E energy of the state
EF Fermi level
k Boltzmann constant
T absolute temperature
At T=0K, the distribution is a step function, with all states below EF occupied and all states above EF empty
As temperature increases, the distribution becomes smoother, with some states above EF being occupied and some below EF being empty
The Fermi level determines the carrier concentrations in semiconductors
The position of the Fermi level relative to the band edges affects the electrical properties of the semiconductor
Density of States
Quantifies the number of available energy states per unit volume and energy interval in a semiconductor
Depends on the band structure and the effective mass of carriers
For a parabolic band, the density of states is proportional to the square root of energy: DOS(E)∝E
The conduction band density of states is given by: NC=2(h22πme∗kT)3/2
me∗ effective mass of electrons
h Planck's constant
The valence band density of states is given by: NV=2(h22πmh∗kT)3/2
mh∗ effective mass of holes
The density of states plays a crucial role in determining the carrier concentrations in semiconductors
Carrier Concentration Equations
Carrier concentrations in semiconductors are determined by the Fermi-Dirac distribution and the density of states
For intrinsic semiconductors, the electron and hole concentrations are equal: ni=pi
ni intrinsic electron concentration
pi intrinsic hole concentration
The intrinsic carrier concentration is given by: ni=NCNVe−Eg/2kT
Eg band gap energy
For extrinsic semiconductors, the majority carrier concentration is determined by the doping level
n-type semiconductors: n≈ND, where ND is the donor concentration
p-type semiconductors: p≈NA, where NA is the acceptor concentration
The minority carrier concentration is calculated using the mass action law: np=ni2
Temperature Effects on Carrier Concentration
Temperature significantly influences the carrier concentrations in semiconductors
As temperature increases, more electrons are excited from the valence band to the conduction band
The intrinsic carrier concentration increases exponentially with temperature: ni∝e−Eg/2kT
In extrinsic semiconductors, the majority carrier concentration is relatively insensitive to temperature changes
Determined by the doping level, which is fixed
The minority carrier concentration increases with temperature, following the intrinsic carrier concentration
At high temperatures, the semiconductor may become intrinsic, with the intrinsic carrier concentration dominating over the doping-induced carriers
Temperature effects on carrier concentration have important implications for semiconductor device performance and reliability
Doping and Its Impact
Doping intentional introduction of impurities into a semiconductor to modify its electrical properties
n-type doping introduces donor impurities (phosphorus, arsenic) that provide extra electrons to the conduction band
p-type doping introduces acceptor impurities (boron, gallium) that create holes in the valence band
Doping shifts the Fermi level towards the corresponding band edge
n-type doping: Fermi level moves closer to the conduction band
p-type doping: Fermi level moves closer to the valence band
The doping concentration determines the majority carrier concentration in extrinsic semiconductors
Heavily doped semiconductors have a higher conductivity and lower resistivity compared to lightly doped or intrinsic semiconductors
Doping enables the creation of p-n junctions, which are the building blocks of many semiconductor devices (diodes, transistors, solar cells)
Equilibrium vs. Non-Equilibrium States
Equilibrium state: no external forces acting on the semiconductor, carrier concentrations determined by the Fermi-Dirac distribution
Characterized by a constant Fermi level throughout the semiconductor
Carrier generation and recombination rates are balanced
Non-equilibrium state: external forces (electric field, light, temperature gradients) disturb the equilibrium carrier concentrations
Characterized by a non-constant Fermi level or quasi-Fermi levels for electrons and holes
Carrier generation and recombination rates are not balanced
In non-equilibrium states, the carrier concentrations deviate from their equilibrium values
Excess carriers are generated or injected into the semiconductor
Carrier lifetime and diffusion play a role in the transport and recombination of excess carriers
Understanding the difference between equilibrium and non-equilibrium states is crucial for analyzing semiconductor devices under various operating conditions
Applications and Real-World Examples
Solar cells: p-n junctions that convert light into electrical energy
Carrier generation by photon absorption creates a non-equilibrium state
Separation of photogenerated carriers by the built-in electric field produces a photocurrent
Light-emitting diodes (LEDs): p-n junctions that emit light when forward-biased
Injection of minority carriers leads to radiative recombination and photon emission
The wavelength of the emitted light depends on the band gap of the semiconductor material
Transistors: three-terminal devices used for amplification and switching
Bipolar junction transistors (BJTs) rely on minority carrier injection and transport
Field-effect transistors (FETs) control the conductivity of a channel by applying an electric field
Semiconductor lasers: p-n junctions that produce coherent light through stimulated emission
Population inversion achieved by injecting high current densities
Used in fiber-optic communication, barcode scanners, and laser pointers
Photodetectors: devices that convert light into electrical signals
p-n junctions or PIN structures that generate a photocurrent when illuminated
Used in digital cameras, optical receivers, and light sensors