6. Negative Bias Temperature Instability

BIAS temperature instability (BTI) is a degradation phenomenon affecting mainly MOS field effect transistors. The highest impact is observed in p-channel MOSFETs which are stressed with negative gate voltages at elevated temperatures. The stress conditions for this negative bias temperature instability (NBTI) typically lie below 6MV/cm for the gate oxide electric field and temperatures ranging between 100-300^C. Higher electric fields can cause additional degradation due to hot carriers (Section 5.1) and should be avoided for the evaluation of NBTI. A very interesting aspect of device degradation caused by NBTI is its capability to anneal to a certain extend when the stress conditions are diminished.

The most important transistor parameters which degrade because of NBTI are:

• decreasing transconductance ,
• decreasing linear drain current and saturation current ,
• decreasing channel mobility ,
• decreasing subthreshold slope $S$,
• increasing off current , and
• increasing absolute value of the threshold voltage .

The consequence can be a reduced circuit switching speed as charging times for interconnect or load capacitances are increased or even circuit failures.

The effect of NBTI has already been reported 40 years ago [83], but gained much attention in recent years [84,85,86,87] due to modern semiconductor technologies. The following aspects have been found to lead to increasing susceptibility to NBTI:

• higher oxide electric fields due to oxide scaling,
• higher temperatures due to higher power dissipation,
• replacement of buried p-channel MOSFETs with surface devices,
• the introduction of CMOS elevating the importance of p-channel MOSFETS, and
• nitrided oxides with a higher permittivity.

In this chapter the observed characteristics of NBTI are outlined and the recent understanding of the involved physical degradation processes described. An important topic discussed here is the measurement and characterization of the level of degradation followed by state-of-the-art modeling approaches presented by different groups as found in literature. A new model is presented which was implemented in the numerical device simulator Minimos-NT [44] and gives excellent agreement with measurement data as shown in Chapter 7.