Ageing of metal-semiconductor field effect transistors

The ageing effect in silicon metal-semiconductor field effect transistors is studied by taking into account the reaction of the gate contact with the native SiO2 layer on the silicon surface. Specifically, the effect of ageing on the threshold voltage and drain characteristics has been studied. It is shown that the change in the threshold voltage with time is quite significant until it becomes nearly constant after a considerably long time. The drain current-voltage characteristics of the device also show appreciable drift with time. It is also shown that, depending on the values of the interface parameters and doping, the device may make a transition from the normally on to a normally off state after prolong ageing. An expression for the critical time required to make such a transition in silicon devices is derived.