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Technique for Achieving Enhanced Electron Velocities in Highly-Doped Semiconductor Channels

IP.com Disclosure Number: IPCOM000108709D
Original Publication Date: 1992-Jun-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 3 page(s) / 153K

Publishing Venue

IBM

Related People

Masselink, WT: AUTHOR

Abstract

This report discloses a technique which allows electrons or holes to achieve higher velocities in a heavily doped semiconductor than they would normally attain. The technique requires the formation of several heterojunctions and is well suited for the GaAs/AlGaAs and the SiGe/Si systems.

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Technique for Achieving Enhanced Electron Velocities in Highly-Doped Semiconductor Channels

       This report discloses a technique which allows electrons
or holes to achieve higher velocities in a heavily doped
semiconductor than they would normally attain.  The technique
requires the formation of several heterojunctions and is well suited
for the GaAs/AlGaAs and the SiGe/Si systems.

      High current densities are vital for FETs used in digital
applications.  Current density is the product of sheet carrier
concentration and carrier velocity.  In order to obtain a high sheet
carrier concentration in doped-channel FETs, the semiconductor must
be heavily doped. Simply increasing the total thickness of the doped
region is not feasible because, with short gate lengths, a thick
doped region results in inferior performance.  With such heavy
doping, however, the carrier velocity is degraded due to ionized
impurity scattering.  This article describes a method using
semiconductor heterostructures which allows both high velocities and
high sheet carrier concentration.

      The invention consists of the formation of a quantum well or
set of quantum wells which have a thickness of the same order as the
Bohr radius of the electron or hole.  The barriers are thick enough
to confine the electrons in the wells.  With such an arrangement, the
ground-state wavefunction of the electron has its maximum in the
center of the well.  The dopants are in quasi-two-dimensional sheets
in the middle of each well.  Such a placement of dopant atoms is
referred to as "delta-doping" ("w-doping"). Although at very low
electric fields this structure results in a degraded electron
mobility because all of the dopant atoms are located precisely at the
maximum of the electronic wavefunction, as the electrons gain energy
in a higher electric field, many begin to occupy the next
higher-lying subband.  This subband has the opposite symmetry as the
ground state subband, which means that the electronic wavefunction
has a node in the center of the well.  Thus, electrons in this
excited-state subband have no overlap with the ionized impurities,
resulting in higher electron velocities.  At very high fields,
velocity is limited by mechanisms other than ionized impurity
scattering.  The key advantage to the arrangement described here is
that in the range of electric fields found in the greatest portion of
the operating transistor, the velocity is higher than would be
possible with a uniformly doped semiconductor.

      FETs based on w-doped bulk GaAs have been described before [1].
These FETs used structures without the confining barriers, and
showed, therefore, no velocity enhancement.  It is essential to
confine the electron (or hole) wavefunction within a quantum well and
that this potential energy of the barrier is much greater than the
conduction band bending due to the ionized impurities.
Heterostructures such as those formed from GaAs/AlGaAs or...