Semiconductor Physics

🧗‍♀️Semiconductor Physics Unit 11 – Power Semiconductor Devices

Power semiconductor devices are the backbone of modern electronics, enabling efficient control and conversion of electrical energy. These devices, ranging from diodes to advanced transistors, operate by manipulating the flow of electrons and holes in semiconductor materials. This unit covers the fundamentals of semiconductor physics, types of power devices, and their operating principles. It delves into device structures, switching behavior, power loss management, and applications in power electronics. Advanced topics like wide bandgap semiconductors and future trends are also explored.

Fundamentals of Semiconductor Physics

  • Semiconductors are materials with electrical conductivity between insulators and conductors, and their conductivity can be controlled by doping or applying electric fields
  • Band structure of semiconductors consists of the valence band, conduction band, and the energy bandgap between them, which determines their electrical properties
  • Intrinsic semiconductors are pure materials with equal numbers of electrons and holes, while extrinsic semiconductors are doped with impurities to create excess carriers (electrons or holes)
    • N-type semiconductors are doped with donor impurities (phosphorus, arsenic) that provide extra electrons
    • P-type semiconductors are doped with acceptor impurities (boron, gallium) that create holes by accepting electrons
  • Carrier transport in semiconductors occurs through drift (under an applied electric field) and diffusion (due to concentration gradients)
  • PN junctions form the basis of many semiconductor devices, created by joining p-type and n-type regions, resulting in a built-in potential barrier and depletion region
  • Forward biasing a PN junction reduces the potential barrier and allows current to flow, while reverse biasing increases the barrier and blocks current flow

Types of Power Semiconductor Devices

  • Power diodes are two-terminal devices that allow current to flow in one direction (forward biased) and block current in the reverse direction, used for rectification and freewheeling
  • Thyristors (silicon-controlled rectifiers, SCRs) are three-terminal devices that can be switched on by a gate pulse and remain on until the current falls below a holding value, used for high-power switching and control
  • Triacs are bidirectional thyristors that can conduct current in both directions, used for AC power control (dimmer switches)
  • Power MOSFETs are voltage-controlled devices with high input impedance and fast switching speeds, suitable for high-frequency applications
    • Vertical double-diffused MOSFETs (VDMOSFETs) have a vertical structure with the drain on the bottom, enabling higher voltage and current ratings
  • Insulated Gate Bipolar Transistors (IGBTs) combine the high input impedance of MOSFETs with the low on-state voltage drop of bipolar junction transistors (BJTs), making them suitable for medium to high-power applications
  • Gate Turn-Off Thyristors (GTOs) are thyristors that can be turned off by applying a negative gate pulse, used in high-power applications (HVDC transmission)

Operating Principles and Characteristics

  • Power semiconductor devices operate in switching mode, transitioning between on-state (low voltage drop, high current) and off-state (high voltage blocking, low leakage current)
  • On-state characteristics include forward voltage drop (VF) and on-state resistance (RON), which determine conduction losses
    • Lower VF and RON are desirable for reducing power dissipation
  • Off-state characteristics include breakdown voltage (VBR) and leakage current, which determine the maximum voltage the device can block and the off-state power dissipation
  • Switching characteristics include turn-on and turn-off times, which affect the device's maximum operating frequency and switching losses
    • Faster switching times enable higher frequency operation and reduced passive component sizes
  • Safe Operating Area (SOA) defines the voltage and current limits within which the device can operate reliably without damage
  • Thermal characteristics, such as thermal resistance and maximum junction temperature, determine the device's power handling capability and cooling requirements

Device Structure and Fabrication

  • Power semiconductor devices have vertical structures to support high voltages and currents, with the current flowing perpendicular to the surface
  • Drift region is a lightly doped region that supports the high blocking voltage in the off-state, its thickness and doping concentration determine the breakdown voltage
  • Field stop layer is a highly doped region that terminates the electric field and improves the trade-off between breakdown voltage and on-state resistance
  • Carrier lifetime control techniques (gold or platinum doping, electron irradiation) are used to optimize switching speed and on-state voltage drop
  • Surface passivation and edge termination structures (field plates, guard rings) are used to prevent premature breakdown at the device edges
  • Packaging plays a crucial role in power devices, providing electrical insulation, thermal management, and mechanical protection
    • Power modules integrate multiple devices and passive components into a single package for improved performance and reliability

Switching Behavior and Control

  • Gate drivers are circuits that provide the necessary voltage and current to control the switching of power devices, ensuring fast and reliable turn-on and turn-off
  • Switching losses occur during transitions between on-state and off-state, due to the overlap of voltage and current waveforms
    • Hard switching involves abrupt transitions and higher switching losses
    • Soft switching techniques (zero-voltage or zero-current switching) reduce switching losses by shaping the waveforms
  • Electromagnetic interference (EMI) is generated during switching transitions due to high dv/dtdv/dt and di/dtdi/dt, requiring proper layout and filtering techniques to mitigate
  • Paralleling of devices is used to increase current handling capability, requiring careful consideration of current sharing and synchronization
  • Protection circuits, such as snubbers and clamps, are used to limit voltage and current stresses during switching transients

Power Loss and Thermal Management

  • Conduction losses are caused by the on-state voltage drop and resistance, proportional to the square of the current (Pcond=I2RONP_{cond} = I^2R_{ON})
  • Switching losses are caused by the overlap of voltage and current during transitions, proportional to the switching frequency (Psw=12VIontonfsw+12VIofftofffswP_{sw} = \frac{1}{2}VI_{on}t_{on}f_{sw} + \frac{1}{2}VI_{off}t_{off}f_{sw})
  • Thermal resistance (RthR_{th}) is a measure of the device's ability to dissipate heat, from junction to case (Rth,jcR_{th,jc}), case to heatsink (Rth,csR_{th,cs}), and heatsink to ambient (Rth,saR_{th,sa})
    • Lower thermal resistance enables better heat dissipation and higher power handling capability
  • Heatsinks are used to increase the surface area for heat dissipation and reduce the thermal resistance from case to ambient
  • Thermal interface materials (TIMs) are used to improve thermal contact between the device case and heatsink, filling air gaps and reducing thermal resistance
  • Liquid cooling systems, such as cold plates or immersion cooling, are used in high-power applications to provide more effective heat removal

Applications in Power Electronics

  • Power converters are circuits that process and control the flow of electrical power, using power semiconductor devices as switches
    • DC-DC converters (buck, boost, buck-boost) change voltage levels
    • AC-DC rectifiers convert alternating current to direct current
    • DC-AC inverters convert direct current to alternating current
    • AC-AC converters (matrix converters) directly convert between AC voltages and frequencies
  • Motor drives use power devices to control the speed and torque of electric motors, enabling energy-efficient operation and precise motion control
  • Renewable energy systems, such as solar inverters and wind power converters, use power devices to interface with the grid and maximize power extraction
  • Automotive applications, including electric vehicle (EV) traction inverters and on-board chargers, require high-power density and reliability
  • Uninterruptible power supplies (UPS) and energy storage systems use power devices for backup power and grid support
  • Wide bandgap (WBG) semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN), offer superior properties compared to silicon, including higher breakdown voltage, lower on-state resistance, and faster switching speeds
    • WBG devices enable higher efficiency, higher power density, and higher temperature operation
  • Integration of power devices, gate drivers, and protection circuits into monolithic power integrated circuits (PICs) reduces size, cost, and parasitic inductances
  • Advanced packaging techniques, such as 3D packaging and chip-on-chip (CoC), improve electrical and thermal performance by reducing interconnect lengths and increasing heat dissipation
  • High-voltage direct current (HVDC) transmission systems use power devices for efficient long-distance power transmission and grid interconnection
  • Fault-tolerant topologies and control strategies are being developed to improve the reliability and availability of power electronic systems
  • Intelligent power modules (IPMs) integrate sensing, protection, and communication functions to enable condition monitoring and predictive maintenance
  • Wide bandgap devices and advanced packaging technologies are expected to drive the future development of high-performance, compact, and reliable power electronic systems


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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.
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