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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.

5.1.1 Channel Hot-Electron Injection

Figure 5.1: At CHE stress conditions, $\ensuremath {V_\textrm {gs}}\approx \ensuremath {V_\textrm {ds}}$, ``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).
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 \ensuremath {\textrm {Si/SiO$_2$}} barrier at the drain end of the channel. The CHE injection can be measured as a gate current which has a maximum around $\ensuremath {V_\textrm {gs}}\approx \ensuremath {V_\textrm {ds}}$. 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.

5.1.2 Drain Avalanche Hot-Carrier Injection

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.
At stress conditions with high \ensuremath{V_\textrm{ds}} and lower \ensuremath{V_\textrm{gs}} 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.

5.1.3 Secondarily Generated Hot-Electron Injection

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.

5.1.4 Substrate Hot-Electron/Hole Injection

The substrate hot-electron or hot-hole (SHE/SHH) injection is a result of a high positive or negative bias at the substrate backside \ensuremath{V_\textrm{sub}} [49]. This leads to carriers in the substrate being driven to the \ensuremath {\textrm {Si/SiO$_2$}} 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 \ensuremath {\phi_\textrm{surf}} is pinned, the oxide field is solely controlled by the gate voltage. The potential drop in the substrate is determined by the substrate voltage \ensuremath{V_\textrm{sub}}. Therefore, several important conditions like the oxide field, the carrier energy, and the current intensity can be adjusted independently.

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Next: 5.2 Dielectric Wearout and Up: 5. Dielectric Degradation and Previous: 5. Dielectric Degradation and

R. Entner: Modeling and Simulation of Negative Bias Temperature Instability