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Control of Contact-Induced Threshold Voltage in Mosfets

IP.com Disclosure Number: IPCOM000099541D
Original Publication Date: 1990-Feb-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 5 page(s) / 192K

Publishing Venue

IBM

Related People

Vinal, AW: AUTHOR

Abstract

This article describes junction potentials of ohmic metal-semiconductor contacts and their effects on threshold voltage of MOSFET. Figs. 1A through 1D illustrate various contact potentials that occur at the substrate-metal, diffusion-metal, and polysilicon gate-metal junctions. Figs. 1A and 1B illustrate N-channel devices, and Figs. 1C and 1D illustrate P-channel devices.

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Control of Contact-Induced Threshold Voltage in Mosfets

       This article describes junction potentials of ohmic
metal-semiconductor contacts and their effects on threshold voltage
of MOSFET.  Figs. 1A through 1D illustrate various contact potentials
that occur at the substrate-metal, diffusion-metal, and polysilicon
gate-metal junctions. Figs. 1A and 1B illustrate N-channel devices,
and Figs. 1C and 1D illustrate P-channel devices.

      It will become apparent from the following analysis that metal-
gage and substrate contact potentials can add unwanted threshold
voltage to MOSFETs.  In the past, the effects of contact potentials
on threshold voltage may have been mistaken for flat-band voltage.

      Doping polarity and concentration of the polysilicon gates, for
both P and N-channel devices, can be chosen to compensate substrate
contact potential, thus eliminating this undesired source of
threshold voltage.

      Referring to the N-channel technology (Figs. 1A and 1B), metal
contact 1 to the substrate 2 is made by depositing aluminum 1 on a
heavily doped P++ region 3 provided at the surface of the substrate
2.  A potential is developed across the P++-metal junction.  The
potential Vjx is given below. Vjx = KT/q Ln(N µ Na/Ni2)

      NA is the acceptor concentration of the P++ pocket and N is the
effective density of conduction electrons in the contacting metal.
The depletion depth within the P++ pocket region resulting from metal
contact must be shallow by design to allow electron tunneling to
occur in order to achieve ohmic-contact properties at the
metal-semiconductor junction.  Depletion depth within the P++ pocket
region is approximated below.

                            (Image Omitted)

      This depletion depth needs to be less than about 1.5 x 10-6 cm
in order to support the tunneling mechanism. Assuming Nd µ = 1021
cm-3 and Na = 1019 cm-3, we find that Vjx = 1.17 V, Xd =
1.17X10-6 cm, and the electric field at the junction is qNaXd/es =
1.87X106 V/cm.  A significant result is realized when the
aluminum-substrate contact is grounded.  This ground connection
places the substrate potential below true ground potential, i.e., Vjs
.

      To assess true MOSFET threshold voltage we are concerned with
the quiescent substrate-to-gate voltage. Referring to Fig. 1A, the
N-polysilicon gate-aluminum contact potential is neglibible and
assumed to be zero volts.  Therefore, grounding the gate yields a net
positive gate- to-substrate voltage Vjs = 1.17 V.  Therefore, the
total MOSFET threshold voltage is reduced by the value of the
substrate contact potential Vjs .  Given a grounded source N-channel
DI-MOSFET provided with a N- poly gate, threshold voltage Vt would be
Vt = 2  Df -Vjs and, quite likely, threshold voltage could have a net
negative value.

      Now refer to Fig. 1B where it is shown that the poly gate is
doped P++.  If the doping density of the polysi...