R Srinivasan

Dispersion of the stress optical coefficient of the alkali halides

The dispersion of the stress optic coefficient C=n3/2 (q11−q12) of the alkali halides, NaCl, KCl, KBr and KI have been measured from the visible to the ultraviolet region. In general the value of “C” decreases with wavelength for all crystals. While the dispersion is only a few per cent in the visible region of wavelengths, it is enormous in the ultraviolet. NaCl shows a dispersion of about 100% from 5800 to 2400 Å; KCl about 200% from 5000 to 2400 Å; KBr about 300% from 5000 to 2400 Å; and KI about 400% from 5000 to 2800 Å. Also the potassium halides exhibit a change in sign of their “C” values in the ultraviolet. In KCl the sign reversal occurs at about 2550 Å; in KBr at 2760 Å and in KI at 3380 Å. Below these wavelengths, the potassium halides belong to the same class inMuellers classification as sodium chloride. The theory ofRamaseshan andSivaramakrishnan based on the assumption that a stress causes a change in the frequencies and oscillator strengths of atoms is unable to explain the observed behaviour of the alkali halides. On the other hand, the mere variation of the ionic refractivities with wavelength is also unable to explain the observed dispersion onMuellers theory. One is forced to assume that the strain polarisability constantK inMuellers theory varies with wavelength. When “K” is calculated from the experimentally observed values of “C”, it is found to increase with decreasing wavelength for all alkali halides. The variation with wavelength of “K” for all the alkali halides can be fitted up well by a formula of the type given byRamaseshan andSivaramakrishnan. Hence it appears that the total dispersion ofC can be explained only when we take into account the variation with wavelength of 1. theLorentz andCoulomb contributions fromMuellers theory and 2. the strain polarisability constant fromRamaseshan andSivaramakrishnans theory.

Effect of scaling on the non-quasi-static behaviour of the MOSFET for RF ICs

The effect of scaling (1 /spl mu/m to 0.09 /spl mu/m) on the non-quasi-static (NQS) behaviour of the MOSFET has been studied using process and device simulation. It is shown that under fixed gate (V/sub gs/) and drain (V/sub ds/) bias voltages, the NQS transition frequency (f/sub NQS/) scales as 1/L/sub eff/ rather than 1/L/sup 2//sub eff/ due to the velocity saturation effect. However, under the practical scaling guidelines, considering the scaling of supply voltage as well, f/sub NQS/ shows a turn around effect at the sub 100 nm regime. The relation between unity gain frequency (f/sub t/) and f/sub NQS/ is also evaluated and it is shown that f/sub t/ and f/sub NQS/ have similar trends with scaling.

Effect of gate-drain/source overlap on the noise in 90nm N-channel metal oxide semiconductor field effect transistors

In this paper, we have studied the effect of gate-drain/source overlap (LOV) on the drain channel noise and induced gate current noise (SIg) in 90nm N-channel metal oxide semiconductor field effect transistors using process and device simulations. As the change in overlap affects the gate tunneling leakage current, its effect on shot noise component of SIg has been taken into consideration. It has been shown that “control over LOV” allows us to get better noise performance from the device, i.e., it allows us to reduce noise figure, for a given leakage current constraint. LOV in the range of 0–10nm is recommended for the 90nm gate length transistors, in order to get the best performance in radio frequency applications.

Scaling characteristics of fNQS and ft in NMOSFETs with uniform and non-uniform channel doping

Extensive process and device simulations are performed to investigate the non-quasi-static transition frequency (fNQS ) and unity gain frequency (ft ) behaviour of the NMOSFETs at different technology nodes, varying from 0.5u2009µm to 90u2009nm. The scaled transistors are constrained to have identical leakage current (IOFF ) at scaled voltages to facilitate a fair comparison. fNQS exhibits a turn-around in the 100u2009nm regime irrespective of the channel engineering. We attribute this effect to the reduced gate over-drive (VGS-Vt ) and lower mobility; which inturn degrades the transconductance (gm ). ft also shows a similar trend. The turn around effect of fNQS and ft disappears, when IOFF constraint is relaxed or the gate over-drive is increased.

Optimisation of Gate-Drain/Source Overlap in 90 nm NMOSFETs for Low Noise Amplifier Performance

The effect of gate-drain/source overlap (L ov ) on LNA performance has been studied, in 90 nm NMOSFETs using process, device and mixed mode simulations. In order to have a fair comparison, the off-state leakage current (I OFF ) of MOSFETs is kept constant by adjusting the pocket halo dose as a function of varying L ov . A basic LNA circuit with two transistors in cascode arrangement is constructed and the input impedance, gain and noise-figure have been used as performance metrics. It has been shown that control over L ov allows us to get better power and noise performance from the LNA i.e., it allows us to get minimum noise figure (NF) and maximum gain from the LNA. To get the better noise performance and gain, L ov in the range of 0-10 nm is recommended.

Impact of channel engineering on unity gain frequency and noise-figure in 90nm NMOS transistor for RF applications

In this paper, we have studied and compared the RF performance metrics, unity gain frequency (f/sub t/) and noise figure (NF), of the devices with channel engineering consisting of halo and super steep retrograde channel (SSRC) implants, and the devices with uniform channel doping concentration, using process, device, and mixed mode simulations. The simulation results show that at 90nm gate lengths, for a given off-state leakage constraint (I/sub OFF/), devices with uniform channel doping concentration deliver higher f/sub t/ and lower NF than the devices which used halo and SSRC, due to better sub-threshold slope and transconductance. However, at 0.25 /spl mu/m technology the same is not true. Therefore, in the 90 nm devices uniform channel doping profile is recommended to get better RF performance.