Semiconductor Physics

🧗‍♀️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.

Basics of Semiconductors

  • 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 (VbiV_{bi}) is the potential difference across the depletion region at equilibrium
  • VbiV_{bi} is caused by the electric field created by the fixed charges in the depletion region
  • The magnitude of VbiV_{bi} 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/kT1)I = I_s(e^{qV/kT} - 1), where IsI_s is the reverse saturation current, qq is the electron charge, VV is the applied voltage, kk is Boltzmann's constant, and TT 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-I_s
  • 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


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© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.