🧗♀️Semiconductor Physics Unit 5 – P-N Junctions in Semiconductor Devices
P-N junctions are the foundation of semiconductor devices. These structures, formed by joining P-type and N-type materials, create a depletion region and built-in potential, enabling controlled current flow under different biasing conditions.
Understanding P-N junctions is crucial for grasping the operation of diodes, solar cells, and transistors. Their current-voltage characteristics, described by the Shockley equation, explain the behavior of these devices in various applications, from rectification to amplification.
Semiconductors are materials with electrical conductivity between insulators and conductors
Their conductivity can be controlled by doping with impurities (phosphorus, boron)
Intrinsic semiconductors are pure materials without any added impurities
Extrinsic semiconductors contain added impurities that change their electrical properties
The most common semiconductors are silicon (Si) and germanium (Ge)
Semiconductors have a band gap between the valence band and conduction band
Electrons can be excited from the valence band to the conduction band by applying energy (heat, light)
This creates electron-hole pairs, which are responsible for electrical conduction
P-Type and N-Type Materials
P-type semiconductors are doped with acceptor impurities (boron, gallium)
Acceptor impurities create holes in the valence band, which are the majority carriers
N-type semiconductors are doped with donor impurities (phosphorus, arsenic)
Donor impurities provide extra electrons in the conduction band, which are the majority carriers
The doping concentration determines the conductivity of the semiconductor
Minority carriers are electrons in P-type and holes in N-type semiconductors
The Fermi level in P-type semiconductors is closer to the valence band, while in N-type it is closer to the conduction band
The majority carrier concentration is much higher than the minority carrier concentration
The mobility of electrons is higher than that of holes due to their smaller effective mass
Formation of P-N Junctions
A P-N junction is formed by bringing P-type and N-type semiconductors into contact
Diffusion of majority carriers occurs across the junction due to the concentration gradient
Electrons diffuse from the N-type to the P-type region
Holes diffuse from the P-type to the N-type region
Diffusion creates a depletion region near the junction, which is depleted of free carriers
The diffusion of carriers leaves behind fixed ionized impurities (acceptors and donors)
The fixed charges create an electric field that opposes further diffusion
Drift current is generated by the electric field, which balances the diffusion current at equilibrium
The P-N junction reaches thermal equilibrium when the Fermi levels align on both sides
Energy Band Diagrams
Energy band diagrams represent the energy levels of the conduction and valence bands in a semiconductor
In a P-N junction, the energy bands bend near the junction due to the built-in potential
The built-in potential is caused by the electric field created by the fixed charges in the depletion region
The Fermi level is constant throughout the P-N junction at thermal equilibrium
The conduction and valence bands are shifted by the built-in potential
The potential barrier prevents the flow of majority carriers across the junction at equilibrium
The width of the depletion region depends on the doping concentrations and the applied voltage
Under forward bias, the potential barrier is reduced, allowing current to flow
Depletion Region and Built-in Potential
The depletion region is a space charge region formed near the P-N junction
It is depleted of free carriers due to the diffusion of majority carriers
The width of the depletion region depends on the doping concentrations and the applied voltage
The built-in potential (Vbi) is the potential difference across the depletion region at equilibrium
Vbi is caused by the electric field created by the fixed charges in the depletion region
The magnitude of Vbi depends on the doping concentrations and the semiconductor material
The depletion region acts as a potential barrier, preventing the flow of majority carriers at equilibrium
The capacitance of the depletion region is important in various applications (varactor diodes)
Biasing P-N Junctions
Forward bias is applied when the P-type region is connected to the positive terminal and the N-type to the negative terminal
Forward bias reduces the potential barrier and allows current to flow
Reverse bias is applied when the P-type region is connected to the negative terminal and the N-type to the positive terminal
Reverse bias increases the potential barrier and the depletion region width
Under forward bias, the diffusion current dominates, and the junction conducts
Under reverse bias, the drift current dominates, and the junction acts as an insulator
The applied voltage affects the width of the depletion region and the magnitude of the current
The current-voltage relationship of a P-N junction is described by the Shockley diode equation
Breakdown occurs under high reverse bias due to impact ionization or tunneling (Zener breakdown)
Current-Voltage Characteristics
The current-voltage (I-V) characteristics of a P-N junction are described by the Shockley diode equation:
I=Is(eqV/kT−1), where Is 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
Under forward bias, the current increases exponentially with the applied voltage
The forward bias current is mainly due to the diffusion of majority carriers
Under reverse bias, the current is small and saturates at −Is
The reverse saturation current depends on the doping concentrations and the minority carrier lifetimes
The ideal diode equation assumes negligible recombination and generation in the depletion region
Real diodes deviate from the ideal behavior due to series resistance, leakage current, and high injection effects
The I-V characteristics are temperature-dependent, with the current increasing with temperature
Applications and Devices
P-N junctions are the building blocks of various semiconductor devices
Diodes are the simplest P-N junction devices, used for rectification, switching, and protection
Solar cells are P-N junctions that convert light energy into electrical energy (photovoltaic effect)
Light-emitting diodes (LEDs) emit light when forward-biased, used for displays and lighting
Bipolar junction transistors (BJTs) consist of two P-N junctions, used for amplification and switching
Photodiodes are P-N junctions that generate current when exposed to light, used for detection and sensing
Zener diodes are designed to operate in the reverse breakdown region, used for voltage regulation and reference
Varactor diodes have a voltage-dependent capacitance, used for tuning and variable capacitance applications
Schottky diodes are formed by a metal-semiconductor junction, having lower forward voltage drop and faster switching compared to P-N diodes