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A P+n+ip+n+ Base-collector Structure for Minimization of Impact Ionization

IP.com Disclosure Number: IPCOM000120483D
Original Publication Date: 1991-Apr-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 1 page(s) / 41K

Publishing Venue

IBM

Related People

Crabbe, EF: AUTHOR [+2]

Abstract

Disclosed is a class of base-collector designs which improves the breakdown voltage BVceo of very-high-frequency bipolar transistors. A npn bipolar transistor with a p+n+ip+n+ base-collector, such that the n+ip+ region is depleted at zero bias, allows much greater design flexibility of the shape of the base-collector potential to minimize impact ionization.

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A P+n+ip+n+ Base-collector Structure for Minimization of Impact Ionization

      Disclosed is a class of base-collector designs which improves
the breakdown voltage BVceo of very-high-frequency bipolar
transistors.  A npn bipolar transistor with a p+n+ip+n+
base-collector, such that the n+ip+ region is depleted at zero bias,
allows much greater design flexibility of the shape of the
base-collector potential to minimize impact ionization.

      Impact ionization occurs when some electrons gain energies of
the order of or above the silicon bandgap.  The purpose of the n+ and
p+ layers at both ends of the space-charge region is to create two
potential cliffs (see Fig. 1).  The first cliff is designed to be on
the order of 1.5 eV.  The i layer should be much larger than the
electron mean free path and allows the "warm" electrons to partially
thermalize before they reach the second potential cliff. The
potential cliff on the collector side should itself be no higher than
1.5 eV (this second potential cliff is optional).  With such a
structure, very few electrons are allowed to reach the threshold
energy in any region of the base-collector space-charge region and
impact ionization is greatly minimized.  An additional advantage of
the i-layer is the resulting decrease in base-collector capacitance.

      A symmetrical design, that is, a n+p+in+p+ structure, can be
used for pnp bipolar transistors.

      Disclosed anonymously.