Improving Thermal Reliability of FETs and MMICs

The constant need for higher speed leads to the requirement of field effect transistors (FETs) with shorter gate lengths, smaller gate widths, and narrower gate-finger pitches. The relationship between various FET parameters and the device lifetime is not readily determined due to the complexity of the problem, along with the elaborate, time-consuming process, and expense of the measurements to establish reliability. This paper presents analytical expressions relating FET reliability [the change in the mean-time-to-failure (MTTF)] to its gate length, width, pitch, substrate thickness, thermal conductivity, and dissipated power. Experimental observations support the model's conclusions. The expressions and results are useful to device/circuit designers to assess the projected MTTF value in their device/MMIC designs.