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Heterojunction Bipolar Transistor Using Si Alloyed With Ge For Greater Base Band Gap Reduction

IP.com Disclosure Number: IPCOM000102058D
Original Publication Date: 1990-Oct-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 3 page(s) / 100K

Publishing Venue

IBM

Related People

Iyer, SS: AUTHOR

Abstract

In order to achieve greater band gap reduction using the Si-Ge system, a Ge collector and emitter is used, using a lightly alloyed (< 25 at %) base layer to achieve a better heterojunction bipolar transistor.

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Heterojunction Bipolar Transistor Using Si Alloyed With Ge For Greater Base Band Gap Reduction

       In order to achieve greater band gap reduction using the
Si-Ge system, a Ge collector and emitter is used, using a lightly
alloyed (< 25 at %) base layer to achieve a better heterojunction
bipolar transistor.

      Heterojunction bipolar transistors offer several advantages
over conventional homojunction bipolar transistors for digital,
analog and optical applications. The band gap difference between
emitter and base allows for higher injection efficiency, and allows
for independent determination of doping parameters of the individual
active layers, yielding a fully optimized device.  In the Si-Ge
system, many of these advantages have been demonstrated using a
silicon emitter and collector and a Si-Ge base.  In this strained
layer system, the band gap of the base layer is reduced from that in
silicon.

      As can be seen from Fig. 1, band gap reduction of Si-Ge at
lower concentrations of Ge in Si depends on the amount of Ge in Si
and the strain in the layer.  This puts constraints on
      1.   the thickness of the strained layer
      2.   ultimate processing conditions of the structure
      3.   maximum obtainable band gap reduction.

      In this invention, the active layers of the transistor are
fabricated as follows (on an Si substrate):
      1.   a Ge or Si-Ge collector
      2.   a Ge-Si base layer
      3.   a Si-Ge emitter.

      As can be seen in Fig. 1, the band gap reduction in the Si
doped Ge is significantly greater than Ge doped Si.  For example, for
15% Ge in Si the reduction in band gap is 100 meV for a strained
system, and 45 meV for a relaxed (unstrained) system.  On the other
hand, to obtain 100 meV band gap change in the Ge-Si system, the Si
content need be 7.5%.  (These numbers are representative and depend
on temperature.)

      A schematic structure for our proposed device is shown in Fig.
2.  A matched substrate is generated with the required Ge-Si
concentration, denoted by Ge(1-x) Six, for example by CZ growth of
the alloy, or by substrate modification of si using met...