Determination of defects Concentration from C-V and G-V Curves in a MOSFET Structure
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1195-B14-05
Determination of defects Concentration from C-V and G-V Curves in a MOSFET Structure Nadine Abboud1 and Roland Habchi1 1 LPA, Department of Physics, faculty of sciences 2, 90656 Lebanon
ABSTRACT The gate oxides of Si based MOSFET devices are subjected to a high field in order to induce defects in the oxide bulk and at the Si/SiO2 interface. The defects are characterized by a series of gate to source capacitance and conductance measurements. Shifts in the flat band voltage and the threshold voltage are observed and are related to the position of charged defects. The difference of the equivalent charge between the two types of defects is also determined. Conductance measurements are performed to determine the difference of interface states concentration as a function of the high field exposure time. INTRODUCTION Defects in MOS structures are common in all devices especially after a certain operating time in relatively harsh environments such as in high current or high voltage or even high temperature conditions. We know that the MOS structure is essentially a capacitor [1] in which one plate is a semiconductor, therefore the C-V characteristic is of major importance. When the defects are charged, they tilt the band diagram and therefore alter the C-V and G-V curves in such a way that information could be retrieved. The nature of charges and their concentration could be determined with a proper calculation. The creation of charged defects after high field stress is known to be a major concern in microelectronics where hot carriers pile up a number of oxide charges and interface states [2-5]. EXPERIMENT The devices that have been used are n-channel Si based VDMOSFET structures, manufactured by standard commercial processes and presented in a low cost TO220 plastic package, the gate material is SiO2. A high field is applied through a positive voltage on the gate contact (45V), in this particular case [6]. The field is applied for different periods going from 10 to 90 minutes using a controlled voltage source. After each period, C-V and G-V (at 1 MHz frequency) characteristics are measured by applying a gate to source voltage going from -8V to 8V with a step of 0.2V, which will allow the establishment of the various regimes going from strong accumulation, Flat Band, depletion, weak inversion to finally reach strong inversion. DISCUSSION Parameters like the Flat Band, the threshold voltage and the nature of the defects affecting the MOS structure can all be deduced from the study of C-V curves [7]. Therefore the quality of the oxide and the interface, which is a determining factor of the MOSFET performance, still govern the shape of C-V curves [8]. In the following, the C-V curves of the gate-source capacitance will be studied according to the presence of defects created by the high field.
We can determine the Flat Band voltage and the threshold voltage from the curves of capacitance in such a way that the Flat band voltage is related to the descent of the capacitance (well shape), and the threshold voltage
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