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Integrated Pressure Sensor Using Silicon And Germanium Bipolar Transistors

IP.com Disclosure Number: IPCOM000120416D
Original Publication Date: 1991-Apr-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 2 page(s) / 107K

Publishing Venue

IBM

Related People

Cressler, JD: AUTHOR

Abstract

This article describes a method for fabricating an integrated pressure sensor using silicon and germanium bipolar transistors.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Integrated Pressure Sensor Using Silicon And Germanium Bipolar Transistors

      This article describes a method for fabricating an
integrated pressure sensor using silicon and germanium bipolar
transistors.

      There has been considerable recent interest in fabricating
analog sensors from microelectronic components (1).  Such
"micro-sensors" are attractive because they can be easily
mass-produced using well-established wafer processing techniques, and
yield highly reliable yet inexpensive components.  One such type of
micro-sensor is a pressure sensor which typically consists of an
etched diaphragm suspended above a silicon (or other) substrate. This
article describes a novel method for making a pressure sensor which
is far more compatible with conventional integrated circuit
processing.  The basic mechanism underlying this present pressure
sensor is electrical in origin rather than mechanical in origin
(i.e., a suspended diaphragm), thus making it unique.

      The physical basis of the present disclosure is to be found in
different pressure dependences of the bandgaps of two conventional
semiconductors, silicon (Si) and germanium (Ge).  The bandgap of
silicon has a negative dependence on increasing pressure, while
germanium has a positive dependence of increasing pressure.  The
precise values for Si are: dEg/dP = -2.4x10-6 eV/(kg/cm2) and for Ge:
dEg/dP = 5.0x10-6 eV/(kg/cm2) (2).  The recent introduction of Si and
SiGe epitaxial base bipolar technologies allows one to exploit this
difference in pressure dependence for the first time.

      To exploit these pressure dependences for a sensor note that
the collector current of a bipolar transistor depends exponentially
on the bandgap of the base region according to (2),

                            (Image Omitted)

 (1) which can
be written as,

                            (Image Omitted)

 (2)
where h is a weakly pressure-dependent term, and Eg(P) is the
pressure dependent bandgap of the semiconductor.  Thus, the collector
current of the SiGe or Ge base transistor is an exponentially
decreasing function of pressure while that of the Si transistor is an
increasing function of pressure. Note that the collector current of
the Ge (or SiGe) transistor is higher than the pure Si device at zero
pressure because its base has a smaller bandgap (1.12 eV vs .66 eV
for Si vs Ge at zero pressure).  For a typical sensitivity, a
pressure of 1000 kg/cm2 gives a bandgap change of 5 meV for a Ge
device.  This translates into an 18% change in collector current over
zero pressure, and is far larger than the minimum delta required for
a detection circuit.

      A...