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Advanced Electrical Characterization of Charge Trapping in MOS Transistors

1.2 Reliability Issues in Field Effect Transistors

The stable and performant operation of devices and circuits can be affected by a wide range of reliability issues. Potential sources of malfunction of devices are mechanical stress, elevated temperature, harsh electrical stress or radiation. The operation of devices under such conditions can negatively impact the reliability of interconnects, can lead to an increase of the oxide leakage currents and in the worst case even to oxide breakdown, and can significantly alter the transfer characteristics of the device. The latter can manifest as a shift in threshold voltage, reduced sub-threshold slope and decreased on-current. The focus of this work is placed on the last mentioned issues, which are considered electrical issues and are caused by defects in the atomic structure of the device. These defects can capture and emit charge and thus lead to changes in the device performance. In this context the following four main device performance degradation mechanisms are typically investigated:

  • • Bias temperature instability (BTI): BTI degradation manifests itself when a bias is applied at the gate of a device. The resulting electrical field in the oxide causes defects located within the oxide to capture a charge either from the channel or the gate. These trapped charges reduce the threshold voltage and the mobility in the channel, thus affect the transfer characteristics of the device. The effect of charge trapping is accelerated by both gate bias and temperature and is at least partially recoverable when the gate bias returns to a low value. BTI can be separated into positive BTI (PBTI) and negative BTI (NBTI) depending on the sign of the gate bias applied and is relevant in both nMOS and pMOS devices. For silicon, the pMOS/NBTI combinations shows the strongest degradation due to the defect density and energetic alignment of the hole defect band [19, 20, 21].

  • • Random telegraph noise (RTN): The same type of defects responsible for BTI cause an effect called RTN, when their energy level is located close to the Fermi level. Even at static bias conditions the defects randomly capture and emit charge, causing noise in the channel current. In large devices this is seen as 1/f noise, while in small gate area devices RTN can be observed as discrete steps in the current. The noise produced by the devices causes various issues in integrated circuits, e.g increased failure probabilities in SRAM and jitter in ring oscialltors. The effect has attracted growing attention lately as the amplitude of the effect increases with device scaling [22, 23, 24].

  • • Stress induced leakage current (SILC) or trap assisted tunneling: Defects in the oxide not only affect the channel of the device, but can also aid tunneling of charge between the channel and the gate, either by allowing the carriers to hop between the gate and the channel or by inhibiting direct tunneling between these carrier reservoirs. This leads to increased gate leakage currents which in turn increase power consumption of the devices and can cause thermal failure of the device. For EEPROM and FLASH memory cells in particular, the increased leakage currents due to SILC decrease the retention time of the devices after repeated write and erase cycles [25, 26, 27, 28].

  • • Hot carrier (HC) degradation: When a drain-source voltage is applied to a MOSFET, carriers in the channel are accelerated towards the source or the drain region, depending on the carrier type. For high source-drain voltages, carriers with high kinetic energy—so-called hot carriers—cause damage close to the interface, where they can either get trapped or break Si-H bonds, and thereby create interface states. The carriers gain the highest energy close to the end of the channel, where most of the damage is typically observed. The interface states can then charge and thus affect the channel similar to BTI defects [29, 30].