A semiconductor device that switches high voltages and large currents to convert and control electric power. The main types are diodes, power MOSFETs, IGBTs and thyristors, made in Si, SiC or GaN.
Power semiconductor device appears in 53 practice questions across 15 topics, including Power semiconductors, Epitaxial growth, Compound semiconductors.
Power semiconductor devices, often just called power devices, are the switches and rectifiers that convert and control electric power: turning AC into DC, DC into AC in an inverter, one DC voltage into another in a DC-DC converter, and driving motors. Logic and memory ICs handle small signals at supply voltages around 1 V and gain density from scaling. Power devices instead block anywhere from a few tens of volts to several kilovolts and carry from a few amperes to thousands of amperes. Blocking a high voltage needs a thick, lightly doped drift region, so most power devices are vertical: current flows from the top of the chip to the back. They are sold as discrete semiconductors with one device per package, or as power modules that combine several chips on an insulating substrate to handle large currents.
Main device types
Diodes have no control terminal and are used for rectification and to carry freewheeling current in inverters; besides pn-junction diodes there are Schottky barrier diodes, which have almost no reverse recovery. Power MOSFETs conduct with majority carriers only, so they switch fast; in silicon they are used mainly up to a few hundred volts in power supplies. IGBTs are MOS-gated bipolar devices whose conductivity modulation keeps the on-state voltage low at high voltage ratings, which makes them the main silicon switch from about 600 V to 6.5 kV in electric-vehicle inverters, rail traction and industrial motor drives. Thyristors are four-layer pnpn devices that, once triggered by the gate, stay on until the current falls below the holding current; they are used for AC phase control and for very high-voltage, high-current conversion such as HVDC transmission.
Materials: Si, SiC and GaN
Silicon is still the most widely used material, thanks to large wafers and mature, low-cost manufacturing. Silicon carbide (SiC, bandgap about 3.3 eV) and gallium nitride (GaN, about 3.4 eV) are wide-bandgap semiconductors with a breakdown field about ten times that of silicon, so the same voltage can be blocked by a thinner, more heavily doped drift region, cutting both on-resistance and switching loss. SiC is used mainly as MOSFETs and Schottky barrier diodes rated from the 600 V class upward, in electric-vehicle inverters, fast chargers and solar inverters. Most GaN devices are lateral HEMTs on silicon substrates, used up to a few hundred volts where fast switching pays off, as in compact AC adapters and server power supplies.
Losses and main applications
Losses fall into two kinds: conduction loss from the voltage drop while the device is on, and switching loss from the overlap of voltage and current during each transition. Conduction loss is the current squared times the on-resistance for a MOSFET, and roughly the on-state voltage times the current for an IGBT or diode; switching loss is the energy lost per transition multiplied by the switching frequency. All of it ends up as heat, so packages with low thermal resistance and good cooling are needed to keep the junction temperature below its rating, typically 150 to 175 °C for silicon. Main applications include traction inverters and on-board chargers in electric and hybrid vehicles, switch-mode power supplies for PCs and servers, rail traction, inverter-driven home appliances such as air conditioners, industrial motor drives, solar and wind power conversion, and power transmission.
Try it: questions about Power semiconductor device
AIts breakdown electric field is about 10 times that of Si
BIts band gap is smaller than that of Si
CIts wafers are cheaper than Si and easier to make in large diameters
Show answer and explanation
Answer: A. Its breakdown electric field is about 10 times that of Si
SiC has a wide band gap of about 3.3 eV and a breakdown field about ten times silicon's (about 3 MV/cm). For the same voltage rating the drift layer can be about one tenth as thick and about 100 times more heavily doped, cutting its resistance theoretically by a factor of several hundred. Its thermal conductivity is about three times silicon's, so it handles heat well, and it is spreading in EV inverters. SiC wafers remain costlier, and as of the mid-2020s the move to 8-inch is still under way.
Switching loss arises because voltage and current coexist during each transition; it equals energy per switching event × switching frequency, so ten times the frequency means ten times the switching loss. Conduction loss (I²R while on for a MOSFET, roughly saturation voltage × current for an IGBT) does not depend on frequency. Higher frequency shrinks inductors and capacitors, so the trend is to raise frequency using SiC and GaN, whose switching losses are small.
FAQ
How do power semiconductors differ from logic ICs?
Logic and memory ICs process and store information at low voltage and integrate huge numbers of transistors. Power semiconductors handle the power itself, switching tens of volts to several kilovolts and large currents. To block high voltage they usually have a vertical structure with a thick drift region, and they are used as discrete devices or in modules.
What are the main types of power semiconductor devices?
The main ones are diodes for rectification and freewheeling (pn-junction and Schottky barrier diodes), power MOSFETs for fast switching, IGBTs for high voltage and high current, and thyristors for very high-voltage, high-current conversion. The choice depends on the voltage, current and switching frequency of the application.
Why are SiC and GaN power devices better than silicon ones?
SiC and GaN have wide bandgaps and a breakdown field about ten times that of silicon, so they block the same voltage with a much thinner drift region, which lowers on-resistance and switching loss. Power supplies and converters become more efficient and smaller, but substrates and devices still cost more than silicon.
Power semiconductor device in the explanations
Power device gates present large capacitances of several nF, so fast switching needs amperes of current to charge and discharge them quickly.
GaN power devices switch at hundreds of kHz to MHz, allowing much smaller transformers and capacitors in chargers; units of the same output can be about half the size.
Power semiconductors—diodes, MOSFETs, IGBTs, thyristors and more—efficiently control and convert large amounts of power by switching, as in motor drives and voltage conversion.