A DIFFERENT JOB

Hold off the voltage, carry the amps

The task Block hundreds or thousands of volts when off, and carry many amps with little loss when on

Vertical structure Current flows from top to bottom through a thick, lightly doped drift region

The compromise Blocking more voltage needs a thicker, more resistive drift region

Figures of merit On-resistance times area, and the switching energy per cycle

Power devices are judged by how little energy they waste, not by how small they are.

THE MATERIAL LIMIT

Why wide bandgap wins

R_on,sp = 4V_B² ÷ (ε μ E_c³)

The drift region has to hold the off-state voltage with a field below the critical field E_c of the material. Silicon carbide and gallium nitride have critical fields about ten times that of silicon, so their drift regions are ten times thinner and far more heavily doped. Because E_c is cubed, the ideal on-resistance falls by two to three orders of magnitude.

Wide-bandgap materials turn a thick, resistive layer into a thin, conductive one.

THE DEVICE ZOO

Four power devices

Silicon IGBT A MOS gate on a bipolar output. Electrons and holes flood the drift region and lower its resistance, but the stored charge makes it slow to turn off

Superjunction MOSFET Alternating n and p columns let a silicon MOSFET block hundreds of volts at low resistance

SiC MOSFET 650 V to 3.3 kV and above: electric-vehicle inverters and solar converters

GaN HEMT Lateral devices up to about 650 V that switch very fast, shrinking chargers and data-center supplies

Each material and structure has a window of voltage and frequency where it is best.

SWITCHING LOSS

Where the energy goes

P_loss = P_cond + E_sw · f

The on-state loss does not depend on frequency, but every switching event wastes a little energy as voltage and current overlap. At high frequency this switching loss dominates, so a faster device lets designers switch faster, and a higher frequency means smaller inductors and capacitors, which is where chargers got their small size.

Smaller switching energy means higher frequency, and higher frequency means smaller parts.

CHECK THE MATCH

Pick the power device

IGBT for big and slow, silicon MOSFET for low voltage, SiC for high voltage, GaN for fast and small.

UNIT 22 STUDY COMPLETE

⚡

Ready for the Fab Challenge?

You've seen how the transistors that run electric cars, chargers and the grid are built to hold off voltage, and why new materials took over.