AN1889 datasheet(5/33 Pages) STMICROELECTRONICS

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AN1889

ESBT: theory and evolution

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Being the negative base current equal to the collector current, the resulting turn-off time is

by far lower than any traditional power bipolar transistor and comparable with that of a high

voltage Power MOSFET. In fact, thanks to the floating emitter configuration, the high value of

the reverse base current results in a fast removal of the charges stored in the base,

achieving both reduced storage time and, most important, the structure results virtually free

of that tail current that characterizes all power bipolar based devices. It is worth to be noted

that the configuration gives an extra safety margin in reverse safe operating area, by

increasing the ruggedness versus the secondary breakdown, in fact since the emitter is

open the phenomenon of crowding current under the emitter finger (with possible creation of

hot spot) is practically absent. Cascade configuration can be implemented in a single four or

five pins package either as hybrid or as a monolithic single chip solution that combines a

vertical NPN Power transistor and a low voltage standard MOSFET. The choice of a suitable

value of thickness and resistivity for the collector drift layer coupled with the most

appropriate edge termination allows in principle to design devices with blocking voltage up

to 3.5 KV. The integration of a low voltage Power MOSFET inside the monolithic structure

had represented the main challenge in designing the ESBT. In particular the power

MOSFET has been integrated inside the emitter region of the Power bipolar stage realizing

a sort of a sandwich structure: thanks to the adopted solution the silicon area depends only

by the BJT size in spite the whole switch is formed by the series connection of the two

devices.

Figure 2.

ESBT switching operation

Figure 3.

ESBT cross section

VTH

VGATE

[µs]

[V]

VTH

VGATE

t [µs]

[V]

ESBT equivalent switching-on circuit

ESBT equivalent switching-off circuit

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