Predrag Habas

Charge-pumping characterization of SiO/sub 2//Si interface in virgin and irradiated power VDMOSFETs

The applicability of charge-pumping technique to characterize the oxide/silicon interface in standard power Vertical Double-diffused (VD)MOS transistors is studied. Qualitative analysis of the charge-pumping threshold and flat-band voltage distributions in the VDMOS structure, supported with rigorous transient numerical modeling of the charge-pumping effect, shows that the measurements can be carried out in the subthreshold region. This conclusion is confirmed by various experimental results. The characteristics, i.e. charge-pumping current versus gate top level, is studied in detail. The changes in the characteristics after /spl gamma/-ray irradiation are analyzed. A charge-pumping-based method for separate extraction of interface state density and density of charge trapped in the oxide after irradiation of VDMOSFETs is proposed. The validity and limitations of the method are studied by experiments and modeling.

Investigation of the physical modeling of the gate-depletion effect

The physical modeling of the gate for the analysis of the gate-depletion effect is investigated. The accuracy of the assumed rigid-parabolic-band model is examined by comparing the numerical simulation with the experimental C-V data. Disagreement between calculation and measurement is observed and physical phenomena which may be responsible for it are proposed. An engineering approach to remove the disagreement is given. The presented investigations may be of general interest for modeling of heavily doped space-charge regions. >

On the effect of non-degenerate doping of polysilicon gate in thin oxide MOS-devices—Analytical modeling

Abstract A one-dimensional model of the polysilicon-gate-oxide-bulk structure is presented in order to analyze the implanted gate MOS-devices. The influence of the ionized impurity concentration in the polysilicon-gate near the oxide and the charge at the polysilicon-oxide interface on the flat-band voltage, threshold voltage, inversion layer charge and the quasi-static C – V characteristic is quantitatively studied. The calculations show a considerable degradation of the inversion layer charge due to the voltage drop in the gate, especially in thin oxide devices. The calculated quasi-static C – V curves agree with the recently published data of implanted gate devices.

Time-resolved analysis of self-heating in power VDMOSFETs using backside laserprobing

Abstract A laserprobe technique for characterization of transient self-heating in power devices is proposed and its applicability is demonstrated on VDMOSFETs. The change in the silicon lattice temperature in the device active region under pulsed operation is sensed by an infrared laser beam focused on a single VDMOS cell from the substrate side. The temperature-induced change in the silicon refractive index causes an optical phase change which is detected interferometrically. The transient phase measurements are carried out under various VDMOSFET bias conditions and for heating pulse widths in the μs range. The laser beam modulation is numerically modeled using a solution of the transient heat transport equation and a transmission line approach to model the beam propagation. Calculated optical phase signals show a very good agreement with the experiments. The phase change calculated by using a simple geometric optic approximation was compared with the phase variation obtained by the rigorous transmission line approach. The results show that the geometric optic approach can accurately be used in the analysis of transient self-heating of VDMOSFETs.

A study of backside laser-probe signals in MOSFETs

Abstract The performance of a laser-probing system due to analyzing signals in MOSFETs is studied. A heterodyne laser interferometer is used for the detection of changes in the free carrier concentrations induced by terminal biases in n-channel and p-channel MOSFETs. Measurements of the phase shift and intensity changes are carried out in accumulation, inversion and depletion and compared to the results of a rigorous numerical modeling of the wave propagation. In the depletion region, the effect of the standing wave resulting from the reflection from the gate is observed in experiment. As the sensitivity of the phase shift measurements is in the order of 10 −5 rad, doping profiles could be derived from optical experiment. We found that a strong impact of the reflection properties of the polysilicon gate on the intensity signal poses a restriction to the applicability of the intensity modulation measurements for the inversion and accumulation conditions.

Charge-pumping characteristics of virgin and stressed lightly-doped drain MOSFETs

Abstract Qualitative differences between the charge-pumping characteristics of lightly-doped drain (LDD) MOSFETs against the characteristics of conventional MOSFETs with abrupt junctions are analyzed and explained. The contribution of the particular interface regions in LDD devices to the charge-pumping characteristics is clarified by studying the spatial distributions of the charge-pumping threshold and flat-band voltages. A two-dimensional transient numerical model of the charge-pumping effect is also applied in the analysis. In order to understand the deep tail at the rising edge of the characteristics of LDD devices, an analytical model of the gate/LDD electrical-field fringing is derived. The accuracy of the analytical solution to the field-fringing problem is investigated regarding the self-induced lateral field in the semiconductor near the gate edge. The degradation of n -channel LDD MOSFETs under electrical stress is studied. The spatial distribution of stress-generated traps is extracted at different stress moments. Assuming these distributions as input the charge-pumping characteristics are calculated by employing the transient numerical model and compared with experimental data.

The application of charge-pumping technique to characterize the Si/SiO 2 interface in power VDMOSFETs

Abstract The possibility to perform charge-pumping measurements on power VDMOS transistors is investigated. By analyzing the spatial distributions of the charge-pumping threshold and flat-band voltages it is concluded that the charge-pumping measurements can be carried out in the subthreshold region of the device. This conclusion is confirmed by two-dimensional transient numerical modeling of the charge-pumping effect and supported by experiments. The method for a separate extraction of interface state and fixed charge densities generated by irradiation in the oxide of VDMOSFETs is proposed.

A model study of the hot-carrier problem in LDD and overlapped LDD MOSFETs

Abstract The role of the gate overlap and the LDD-implantation concentration in the hot-carrier aging of n -channel LDD and overlapped LDD MOSFETs is extensively studied by means of numerical modeling. The device drive current, the carrier heating and the impact ionization in devices at the stress bias and the sensitivity of the drain current to the amount and the location of the damage are considered. The results provide an understanding of the experimentally obtained reliability behavior of 0.35μ m -devices and can be applied to formulate the design rules for these devices.

Geometric current component in charge-pumping measurements

A detailed experimental and modeling study of the geometric current component in charge pumping measurements is carried out for bulk MOSFETs. Nand p-channel devices with various gate lengths and widths are studied. Electron and hole components in the charge-pumping current are separated by applying five terminal measurements. A transient two-dimensional numerical model is used for a rigorous analysis of the effect. A one-dimensional semi-analytical model of the free-carrier removal from the channel is developed and applied with proper boundary conditions at the channel ends. The usually applied results from the CCD-theory are found to be inadequate to model the geometric component in MOSFETs. The proposed semi-analytical model enables understanding of all effects observed in experiments and numerical modeling, and provides results which quantitatively agree with numerically calculated curves. The impact of the inversion gate bias, the pulse fail (rise) time and the junction reverse bias on the geometric component is explained. Both experiments and modeling show that the turn-off slew rate is the only parameter of the gate pulse practically relevant for the magnitude of the geometric component.

Transient Two-Dimensional Numerical Analysis of the Charge-Pumping Experiment

A transient model of the charge-pumping experiment on bulk MOSFETs and SOI devices has been developed, It is based on the self-consistent numerical solution of the time-dependent basic semiconductor equations including the trap dynamics equations, assuming arbitrary voltage pulses at the terminals. In this paper some details of the approach are presented. The model is applied to study the non-ideal (geometric) components in the charge-pumping current for the two-dimensional cases. Additional applications of the presented charge-pumping simulation are pointed out.