16 Dependence of carrier injection and gate current on gate bias in a conventional n-channel MOSFET. The thesis presents a study of the degradation of MOSFET parameters affecting circuit performance due to processes initiated by the injection of high-energy carriers into the gate oxide, the characterization techniques used to measure and attribute these parameter shifts. In this section, a brief overview of the phenomenon of hot carrier injection and the resulting device degradation will be given.
In addition to being trapped in the gate oxide, the injected carriers can also cause an increase in the density of interstitial traps present at the Si-SiO2 interface [8, 9, 10]. The presence of charge in the gate oxide and at the Si-SiO2 MOSFET interface results in modulation of the surface potential and carrier mobility on the semiconductor surface. As previously mentioned, the injection of carriers into the gate oxide is an important concern to ensure the long-term reliability of modern CMOS digital circuits.
CHARACTERIZATION TECHNIQUES
However, during channel injection of hot carrier, the threshold voltage at each point in the injection region may vary at a different rate. The two most important parameters that are numerically extracted from the measured data are the threshold voltage and the channel conductivity. The threshold voltage can be extracted from the measured IDS–VGS characteristics by extrapolating the curve in the high VGS region to IDS= 0.
The intersection of this extrapolated curve with the VGS axis gives the threshold voltage (Fig. 4). The threshold voltage obtained with this technique is called the linear extrapolated threshold voltage LVT,ext. The threshold voltage extracted with this definition is called the constant current threshold voltage VT,ci (Fig. 6).
Gate Bias (V)
This can be explained by the fact that the damage to the device is located near the drain area of the device. The charge pump technique is one of the most sensitive techniques for measuring the density of interface stages in MOS transistors. When removing the gate pulse, the charge from the inversion region flows back to the source/drain (in regions close to the junctions), while some of it recombines with the majority carriers of the substrate (in regions far from the junctions), forming a component of the charge pump current.
The main component of the charge pumping current arises from the recombination of fast surface states z. The decrease in ICP with increasing reverse bias can be explained by the widening of the source/drain depletion regions. After removing the gate pulse, the pumped charge density can be expressed as: Qss=αQinv+qNit.
If the geometric component of the charge pump current is small, the saturation of the charge pump current can be explained by the pinning of the surface potential once strong inversion is achieved. This is supported by the observation that the saturation occurs at the measured threshold voltage of the MOSFET. The charge pump current is proportional to the area of the band gap swept by the Fermi level when the gate pulse is applied and also to the surface drop density in this area.
This region becomes constant after strong inversion is reached which causes a saturation of the charge pump current. An alternative approach [23] is to measure the charge pump current as a function of the pulse base level keeping the pulse amplitude constant. Case A When the pulse base level is above the threshold voltage of the MOSFET, the.
Example B When the base pulse level is between 0 and VT, some surface states contribute to the charge pumping current and the ICP increases as the base pulse level approaches 0 V. The substrate doping concentration, ND, can be obtained by measuring the dependence of the threshold voltage on the substrate bias . Cox itself can be measured by three junction measurements of gate capacitance as a function of gate-substrate bias3.
HOT-CARRIER DEGRADATION IN N-CHANNEL MOSFETS
The hot-carrier-induced breakdown processes are initiated by the injection of high-energy carriers from the device channel into the gate oxide. This section provides a qualitative analysis of the dependence of electron and hole injection currents on the gate bias of a conventional n-channel MOSFET. Some of the main characteristics of the gate and injection current characteristics to be noted here are.
The peak of the hole injection current is significantly smaller than that of the electron-. The substrate current in n-channel devices mainly consists of the holes generated by impact ionization in the channel region. Thus, the variation of the relative equilibrium between the carrier supply and the lateral electric field leads to the well-known bell-shaped substrate current characteristics in n-channel devices.
Based on the lucky-electron concept, it can be shown that the ratio of the substrate and drain currents can be approximated by As shown in Section, the relative concentration of electrons and holes injected into the gate oxide is a strong function of the gate bias. Various device parameters were measured as a function of the strain time during each of these strain experiments.
The results shown here use the percent decrease in maximum linear transconductance (%∆Gm) as a monitor of device degradation. Similarly, the parameter A shows a strong dependence on the drain bias, while it is independent of the gate bias. However, to be used in a circuit reliability simulator, a device degradation model should predict the time dependence of the degradation rather than the lifetime.
To assess the validity and applicability of the existing degradation models, each of the above models was parameterized and applied to predict the device degradation during the bias sequences. Once again, the use of the conventional models to predict the device breakdown during the bias sequences results in very inaccurate prediction of the device behavior. In another attempt at empirical modeling, unit degradation under each of the three bias conditions was modeled using time-dependent equations based on lifetime models proposed by Mistry et al.[33] as given by.
Stress Time (seconds)
We tried a new approach to model the combined effect of the three mechanisms at each of the bias conditions using the following expressions. The three terms on the right-hand side of this equation include the contributions due to each of the three decay modes. As seen here, the coupled model is able to predict device degradation under varying bias conditions more accurately than any of the other models.
Since the model predicts the time dependence of the device degradation, it is also sufficient for use in circuit reliability simulators. All of the above empirical models have had a limited degree of success in predicting the hot carrier response under dynamic operation. It has been reported[51] that “strong transient effects directly related to intrinsic time constants of the MOSFET are absent up to slopes in the subnano-second regime”.
The dynamic response of the injected carriers is determined by their transport and trapping properties in the oxide and the interface. The presence of oxide defects near the hole can further reduce the effective mobility of the holes. This logarithmic nature of the recovery characteristics indicates that the detachment cannot be the result of field-assisted thermal emission or trap-band impact ionization with Fowler-Nordheim injected electrons, as the detachment is expected to be exponential in time under these mechanisms.
Extrapolation of the static stress results to dynamic conditions by accounting for duty cycle will only lead to a shift of the curves along the time axis without any change in the slope of the curves. Examination of any transient effects that may cause this behavior shows that there is no change in the nature of the dynamic voltage characteristics for frequencies ranging from 25Hz (tr = tf = 3ms) to 25MHz (tr = tf = 3ns). The presence of positive charge in the gate oxide leads to reduction of the channel electric field.
In some cases, the purpose of the experiment cannot be achieved unless these effects are eliminated.
CONCLUSIONS AND FUTURE WORK
ELECTRON SCATTERING IN IMAGE FORCE POTENTIAL WELL
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