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Subsections
5.1 Hot Carrier Degradation
In general ``hot carriers'' are particles that attain a very high kinetic
energy from being accelerated by a high electric field. These energetic
carriers can be injected into normally forbidden regions of the device, as the
gate dielectric, where they can get trapped or cause interface states to be
generated. These defects then lead to threshold voltage shifts and
transconductance degradation of MOS devices. To avoid, or at least minimize
hot carrier degradation, several device design modification can be made. These
are for example a larger channel length, double diffusion of source and drain,
and graded drain junctions to name a few.
For the injection of hot carriers into the dielectric there are four
distinguished injection mechanisms [49]: channel hot-electron (CHE)
injection, drain avalanche hot-carrier (DAHC) injection, secondary generated
hot-electron (SGHE) injection, and substrate hot-electron (SHE) injection.
Figure 5.1:
At CHE stress conditions,
, ``lucky electrons''
gain enough energy while drifting across the channel and are injected into
the dielectric causing a gate current, interface and oxide degradation. The
oxide electric field is the same as at the drain pulsed voltage measurement
technique (Section 6.3.3).
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Where the gate voltage is approximately equal to the drain voltage the channel
hot-electron (CHE) injection is at a maximum. Figure 5.1 depicts this
conditions where ``lucky electrons'' [50,51] which are
attracted by the high gate voltage gain enough energy from the electric field
across the channel to surmount the
barrier at the drain end of the
channel. The CHE injection can be measured as a gate current which has a
maximum around
. For lower gate voltages the field does not
attract electrons to the gate electrode anymore. For higher drain voltages the
electric field at the drain leads to avalanche multiplication due to impact
ionization (Section 2.3.4) and hot electrons and hot holes are injected
reducing the measured gate current.
Figure 5.2:
Hot carriers lead to impact ionization generating electron-hole
pairs. In the drain avalanche hot-carrier injection regime hot electrons
and hot holes are injected into the dielectric. Additionally some of the
carriers form a bulk current.
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At stress conditions with high
and lower
the drain avalanche
hot-carrier (DAHC) injection is significant [49] (Figure 5.2).
It is caused by the injection of holes and electrons generated by avalanche
multiplication as described in Section 2.3.4. The carriers gain their energy due
to a high electric field in the drain region. Measurement of DAHC is difficult
as both carrier types are injected simultaneously. Additionally some of the
generated carriers lead to a bulk current.
The origin of secondarily generated hot-electron (SGHE) injection is firstly a
photo induced generation process
(Section 2.3.1) [52,53]. Photons are generated in
the high field region near the drain and induce a generation process for
electron-hole pairs. The second effect is the avalanche multiplication
(Section 2.3.4) near the drain region leading to the injection of both, electrons
and holes into the dielectric. The injection process is supported by the
substrate bias which is additionally driving carriers to the interface.
The substrate hot-electron or hot-hole (SHE/SHH) injection is a result of a
high positive or negative bias at the substrate backside
[49]. This leads to carriers in the substrate
being driven to the
interface which gain further kinetic energy in the
surface depletion region. The substrate carriers are either generated by
optical generation or by electrical injection from a buried p-n junction.
These carriers can eventually overcome the energy barrier at the interface and
are injected into the oxide.
SHE/SHH is often used to investigate the insulator qualities and for
reliability tests [54,55]. The advantage is that the
energetic carriers at the interface are uniformly distributed along the channel
in contrast to the injection mechanisms described above, where the maximum of
the injection is near the drain end of the channel. Therefore the stress
conditions at the interface are well defined. As the surface potential
is pinned, the oxide field is solely controlled by the gate voltage.
The potential drop in the substrate is determined by the substrate voltage
. Therefore, several important conditions like the oxide field, the
carrier energy, and the current intensity can be adjusted independently.
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R. Entner: Modeling and Simulation of Negative Bias Temperature Instability