Meaning
Semiconductor aging appears as a shift in the threshold voltage of n-channel transistors when a positive voltage is applied to the gate at elevated temperatures. This positive bias temperature instability is primarily observed in advanced high-k metal gate technologies used in modern processors. The effect leads to a reduction in the drive current and can cause the timing of the circuit to drift.
Oxide Defect
Charge trapping in the dielectric layer happens when electrons are injected into existing or newly created states within the insulator. These trapped charges repel the electrons in the channel, effectively increasing the voltage needed to turn the device on.
nMOS Stress
High electric fields and thermal energy provide the conditions for the formation of oxygen vacancies in the gate oxide. These vacancies act as traps that capture and release electrons during the switching cycle of the transistor. As the density of these traps increases, the positive bias temperature instability becomes more pronounced and the performance of the logic gate degrades.
Precise characterization of this mechanism is necessary for the development of high-performance mobile chipsets.
Application Impact
Circuit failure occurs when the cumulative shift in threshold voltage exceeds the design margin of the system. This can lead to functional errors in memory cells or logic paths that are constantly held at a high voltage level. Manufacturers use specialized stress tests to characterize the rate of this shift and to set the safe operating voltage for the lifetime of the chip.
