These positive charges partially cancel the negative gate voltage without contributing to conduction through the channel as electron holes in the semiconductor are supposed to.
NBTI has become significant for portable electronics because it interacts badly with two common power-saving techniques: reduced operating voltages and clock gating.
Some low-power designs switch to a low-frequency clock rather than stopping completely in order to mitigate NBTI effects.
The details of the mechanisms of NBTI have been debated, but two effects are believed to contribute: trapping of positively charged holes, and generation of interface states.
Even with the introduction of new materials like hafnium oxide in the gate stack, NBTI remains and is often exacerbated by additional charge trapping in the high-κ layer.