Power BJT

UNLIKE a field effect transistor (FET), a Bipolar Junction Transistor uses both electrons and electron holes as charge carriers.

They are current-controlled devices made out of a sandwich of either NPN or PNP doped silicon substrate.

Uses

Cheap: BJTs are the oldest kind of #transistor
largely supplanted by MOSFETs or IGBTs for power applications.
supplanted by CMOS for computers.
lowest impedance (output and input).
- used for amplifiers for #RF
- and other specialized #power applications

construction

To the right is the cross section view of a planar NPN BJT. Current flowing into the base terminal $(I_{B})$ causes a diffusion of charge carriers in the *p-region* which then allows current to flow from collector to emitter $(I_{C}\approx I_{E})$.

comparison table

Name	Voltage rating	Current rating	Input drive	Input impedance	Output impedance	Switching speed	Cost (relative)
IGBT	Very High (>1 kV)	High (>500A)	Voltage Driven, VGE ~4-8V	High	Low	Medium	high
Power BJT	High (<1 kV)	High (<200A)	Current Driven, hfe ~20-200	Low	Low	slow (μs)	Low
Power MOSFET	High (<1 kV)	Low (<500A)	Voltage Driven, VGS ~3-10V	High	Medium	Fast (ns)	Medium