NBTI (Negative Bias Temperature Instability)
Definition: NBTI is a reliability issue that affects PMOS transistors under negative gate bias and high temperature stress. It causes an increase in the threshold voltage and a decrease in the drain current over time.
Key Characteristics:
- Affects only PMOS transistors
- Accelerated by negative gate bias and high temperature
- Causes an increase in threshold voltage and a decrease in drain current
- Can be partially recovered during device operation (recovery effect)
- Becomes more critical with technology scaling and increased operating temperatures
Physical Mechanism:
- Attributed to the generation of interface traps and oxide charges at the Si-SiO2 interface
- Negative gate bias and high temperature lead to the dissociation of Si-H bonds and the diffusion of hydrogen species
- Generated interface traps and oxide charges cause a shift in the threshold voltage and a reduction in carrier mobility
Impact on Device Performance:
- Increased threshold voltage leads to reduced drive current and slower switching speed
- Decreased drain current results in performance degradation and potential timing violations
- Can cause long-term reliability issues and limit the lifetime of PMOS transistors
Mitigation Techniques:
- Process optimization: Improving the quality of the Si-SiO2 interface and reducing the number of defects
- Device design: Using high-k dielectrics, metal gates, and nitrided oxides to reduce NBTI effects
- Circuit design: Applying adaptive voltage scaling, body biasing, and guard-banding to compensate for NBTI-induced performance degradation
- Recovery management: Exploiting the recovery effect by applying positive gate bias during idle periods to partially reverse NBTI degradation
NBTI is a significant reliability concern in modern VLSI designs, particularly as technology scales and operating conditions become more stringent. Understanding and mitigating NBTI is crucial for ensuring the long-term reliability and performance of PMOS transistors and circuits.