Hot-carrier reliability and device parameter extraction have become two major issues with continued size reduction and changes in structure for modem MOSFETs. Hot-carrier induced damages remain a reliability concern despite lowering of supply voltage and the introduction of hot-carrier resistant devices through drain engineering. As for device parameter extraction, established drain current based methods are starting to become limited for deep submicron devices.
A study on the small-signal characteristics of the drain-to-substrate junction of n- and p-channel MOSFETs configured as a gated diode to investigate hot-carrier induced degradation is presented. The small-signal admittance Ydb across the drain-to-substrate junction consists of the drain-to-substrate conductance Gdb and the drain-to-substrate capacitance Cdt. Based on both experimental and simulation results, it is established that similar to the dc gated-diode measurement, this technique uses the edge of the depletion region as a pointer to detect the presence and spatial distribution of hot-carrier induced interface states. Gdb is capable of detecting midgap states acting as recombination centers like the dc gated-diode current, while change in Cdb after stress reflects presence of ionized interface states and oxide trapped charges. The advantage of using Ydb is that complementary information on hot-carrier induced damages can be obtained with a single measurement. Also, it is shown to be very sensitive in detecting damages at the gate-to-drain overlap region.
The before and after stress experimental Cdb and Gdb curves are compared after non-uniform and uniform electrical stress. It is concluded that the hot-carrier induced damages are localized near the drain region after non-uniform stress. In n-channel MOSFET, these damages are observed to be mainly acceptor-like interface states, while, they are seen to be trapped electrons in p-channel MOSFET. On the other hands, after uniform electrical stress, the hot-carrier induced damages are distributed along the channel. The induced damages are trapped electrons in n-channel device and trapped holes in p-channel device.
The spatial distribution of hot-carrier induced interface states is extracted by modeling its distribution profile with two-dimensional numerical simulation to fit the after stress Cdb curve. The extracted profile is verified by incorporating it back in the simulated device to simulate the post-degradation I-V characteristics and compare with the experimental results.
Also, a new threshold voltage VT extraction method based on the gate-to-substrate capacitance Cgb is proposed. It does not require dc current to flow between the drain and source and thus eliminating the effects of source and drain series resistances. A symmetrical potential profile is also retained across the channel during the extraction measurement. With these advantages, this new method should be able to extract a more precise VT and both experimental and simulation results are compared with results obtained using the linear extrapolation method.