Abstract
In this thesis, we present a new reverse transconductance method to investigate hot-carrier degradation on high-voltage (HV) lateral-diffused metal-oxide-semiconductor field-effect transistors (LDMOSFETs). This new method can extract asymmetric drain and source series resistance separately with only one single device. By using this extraction method in 16V LDMOSFETs before and after hot-carrier stress, drain series resistance is extracted and found increasing while source series resistance is remained the same. In addition, the threshold voltage and subthreshold slope has no degradation after hot carrier stress. Therefore, it is suggested that the current degradation in 16V LDMOSFETs after hot carrier stress is not due to the damage under channel but to the drift region under spacer oxide. This is confirmed by the simulation results of a two-dimensional (2-D) simulator. Besides, the differences in hot carrier degradation between 16V LDMOSFETs and low voltage lightly doped drain (LV LDD) MOSFETs are also discussed in detail. In addition, hot-carrier stresses in 40V LDMOSFETs are performed at ISub,max conditions with different VDS and VGS. The power index of the hot-carrier degradation rates are found not always 0.5 when stressing at different ISub,max conditions, and they do not consist with the hot-carrier degradation behavior in LV MOSFETs. In order to explain why the degradation rates at the ISub,max conditions are different with different biases, two dimensional simulators are used to find the degradation mechanism in the 40V LDMOSFETs. It is found that at different ISub,max conditions the highest impact ionization region is located at different positions in the drift region of the device. Due to the different gate-control abilities among these regions, the current densities affected by the hot-carrier induced interface trapped charges are different, thus the hot-carrier degradation rates are different. Also, due to the different depths of the impact ionization region, the amount of the initial hot-carrier induced interface trapped charge are different, thus the power-law pre-coefficient A are different. Nowadays, the HV LDMOSFETs are widely used in many applications such as driver ICs, power switches, etc. Under the real operation conditions, the HV LDMOSFETs are switched on and off at a certain frequency. Thus, for reliability issue, AC hot-carrier stress is more close to the real operation condition than DC hot-carrier stress. However, most of the previous hot-carrier stress studies on the HV LDMOSFETs focused on the DC hot-carrier stress and only few study mentioned about the AC hot-carrier stress. In this thesis, AC hot-carrier stress was performed on two different HV LDMOSFETs’ structures. Their degradation behaviors and relationships with the stress frequencies are discussed. The 1/f noise measurement is believed to be highly sensitive to hot-carrier injection. However, the noise degradation in 16V LDMOSFETs after hot-carrier stress has never been discussed before. In this thesis, 1/f noise is measured before and after hot-carrier stress under linear and saturation regions. The 1/f noise power spectral density in linear region is increasing with the stress time while the 1/f noise power spectral density in saturation region has no change. Therefore, the hot-carrier induced traps are located at the pinch-off region in the drain end. Further, the increment of 1/f noise power spectral density is up to 180% after 1000s stress, which is much larger than DC static parameter’s 6.5%. Thus the 1/f noise measurement is indeed a sensitive tool to hot-carrier degradation.