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Lateral Thin Film Bipolar Transistors Isolated from the Substrate by Heterojunctions

IP.com Disclosure Number: IPCOM000110497D
Original Publication Date: 1992-Dec-01
Included in the Prior Art Database: 2005-Mar-25
Document File: 5 page(s) / 239K

Publishing Venue

IBM

Related People

Dennard, RH: AUTHOR [+4]

Abstract

A high performance lateral bipolar device structure is disclosed which can be fabricated in a thin layer of a narrow-gap material such as Ge grown on a substrate or buffer layer of a wider-gap material such as GaAs. Reasonable isolation from the substrate can be achieved using the heterojunction properties, proper biasing and low operating temperatures.

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Lateral Thin Film Bipolar Transistors Isolated from the Substrate by Heterojunctions

       A high performance lateral bipolar device structure is
disclosed which can be fabricated in a thin layer of a narrow-gap
material such as Ge grown on a substrate or buffer layer of a
wider-gap material such as GaAs.  Reasonable isolation from the
substrate can be achieved using the heterojunction properties, proper
biasing and low operating temperatures.

      Germanium is a very suitable material for high-performance
bipolar devices because of the high carrier mobility.  In particular,
the hole mobility is much better than that found in other common
materials at moderately high doping level majority and minority
carriers within the base region [1], which can be very important for
complementary (npn and pnp) devices for both majority and minority
carriers within the base region.

      The relatively low bandgap (N 62 percent of Si at 77K) is also
an important factor since it can lead to reduced voltage levels, and
therefore lower power consumption, in high-performance bipolar
integrated circuits.  At a given speed, the power is proportional to
V2, and therefore can be expected to be only 38 percent as great as
for Si, assuming that all the voltage levels are scaled proportional
to the bandgap.  Low-temperature operation (for example, 77K) would
be used to relieve any deleterious leakage effects due to the lower
bandgap, while providing much more highly-conductive interconnections
which would be needed with the lower-impedance devices.

      It is proposed that a relatively simple and effective way to
produce germanium bipolar devices is to build lateral transistors in
a thin epitaxial layer on a GaAs substrate, as shown in Fig. 1.  The
substrate can effectively be an insulator by proper biasing of the
heterojunction between the lower-bandgap epitaxial material and the
higher-bandgap substrate, as will be shown.

      A principal advantage of the lateral structure is that it
provides low-resistance contacts to all the terminals on the top
surface without elaborate reach-through, subcollector, and isolation
structures.  Isolation can be provided between devices in this case
by, for example, etching the thin Ge film into islands, although some
reasonable passivation layer is still required for the edges of the
base region.  Another advantage of the lateral device is that the
active surface of the emitter (corresponding to the emitter width in
a vertical device) is controlled by the thickness of the Ge layer
rather than a lithographically-defined pattern which is subject to
more variations.  On the other hand, the critical base dimension in
the symmetrical structure of Fig. 1 must be defined by lithography,
and is similar to the channel length of a field-effect transistor
(FET).  An unsymmetrical transistor with relaxed lithography
requirements is shown later.

      The fabrication of the lateral bipolar would be sim...