29-08-2014, 12:57 PM
Quasi-Ballistic Transport in MOSFET Seminar Report
[attachment=67323]
ABSTRACT
The formidable progress in microelectronic has pushed the short channel length of mosfet into decanano scale .This progress imposes new challenges on the device simulation as the essential physics is different from that of the old convential old techniques because the effect of the quasi –ballistic transport on device.Therefore it become essential to understand essential physics of quasi ballistic transport and its implication on the nanoscale device.under this we disscuss two topics for understanding the Quasi-Ballistic Transport in Nano-MOSFET.
Introduction
SILICON MOSFET channel lengths are decreasing to below 50 nm where traditional compact models lose validity. Now a days the carrier transport tends toward the quasi-ballistic regime and the models based on the drift-diffusion theory and on the notion of statistical mobility fail to describe transistor operating close to the ballistic limit.This models assumes that the mean-free path is negligible as compared with the channel length i.e., channel length and mean-free path are in
the same order of magnitude. The distribution function is no more maxwellian like and “ballistic peaks” appear in the velocity distribution . Therefore the use of the notion of
mobility in compact models is questionable. In this theory the key factor is the backscattering
coefficient Rc which is linked to the degree of transport ballisticity.
BACKSCATTERING THEORY
In very short MOSFETs the entire channel or a more significant part of the channel participates in the backscattering. A carrier can
even be sent back to the source by a scattering event in the drain. n a short channel where the transport is nonstationary we cannot consider the mean-free path to be independent of
the carrier energy. Moreover, in order to determine the mean-free path =vtherm*ἰ . The
backscattering effect is expected to be strongly influenced by effective mass. In order to obtain good quantitative results
in the case of short channel, model of mobility depending on the bias VDS and VGS to determine the mean-free path , and use two fitting parameters βand α , to calibrate the critical distance Ɩ according to
Characteristics of Ballistic Tansport in Short-Channel MOSFETs
Introduction
In a long channel MOSFET whose transport is basedon a drift-diffusion model, charge carriers (electrons and holes) experience many scattering events when they are moving in the channel. It is well studied that these scattering events deteriorate transport characteristics resultingin a decrease of the drain current.Recently, due to the remarkable development of semiconductor device fabrication technology, the MOSFET channel length has become shorter and comparable to a carrier scattering length ; as a result, the carriers can encounter less scattering
QUASI-BALLISTIC MOSFET MODEL
The drain current of the quasi-ballistic MOSFET can be expressed by
where r is a back-scattering coeffcient given
where l , called kT-layer length, is defined as a characteristiclength corresponding to a potential drop by kT/q near the source, as indicated in Fig. 1
RESULTS AND DISCUSSION
In the quasi-ballistic MOSFET, the back-scattering phenomenon makes the drain current decrease, so that it is essential to reduce the back-scattering events to enhance
the characteristics of the ballistic transport. Backscattering occurring within a characteristic length called the kT-layer near the source plays a key role in the quasiballistic transport, as illustrated in Figure 1.
Dependence of quasi-ballistic characteristics on drain voltage
As drain bias is increased, the magnitude
of the potential barrier becomes smaller, due tothe DIBL (Drain-Induced Barrier Lowering) effect, and the kT-layer also decreases simultaneously. increasing drain voltage, the back-scattering coefficient (r ) becomes smaller, due to the reduction of the kT-layer length , and its value is saturated at a lower drain voltage with decreasing channel length. So, these results support the idea that as the channel length becomes shorter, both the back-scattering coeffcient and the drain current saturate at a lower drain voltage.
Dependence of quasi-ballistic characteristics on temperature
Temperature can also be used as one of the external parameters to alter the properties of the quasi-ballistic MOSFET. Since carrier transmission in the channel is strongly dependent on carrier mobility, lowering the temperatures can enhance carrier mobility, leading to a larger transmission coefficient.
As the temperature decreases, more inversion charges can move to the channel, because of the lowered e_ective potential barrier which carriers experience, leading to the enhancement of the transmission coefficient. In addition, increased carrier mobility makes their mean free path longer, resulting in the reduction of the back-scattering coeffcient in the channel.
Dependence of quasi-ballistic characteristics on doping concentration in the source
It is assumed in this study that carriers can be transmitted over the barrier
through thermionic emission. So,making a degenerate condition in the source region can increase the carrier density injected into the channel. Since a high doping concentration in the source increases
the number of carriers having energy higher than the channel barrier, the saturation current is expected to increase at a higher source doping concentration. As expected, with increasing doping concentrations in the source, the saturation current is enhanced.
CONCLUSION
We have studied the characteristics of the quasiballistic transport and drain current in ultra small MOSFETs,taking into account the back-scattering and transmission coefficients.
The characteristics of the quasi-ballistic MOSFET vary dramatically according to temperature, doping concentration
in the source, oxide thickness and drain & gate
voltage. Dependences of these parameters on the quasiballistic transport are more pronounced as the channel length becomes shorter.
[attachment=67323]
ABSTRACT
The formidable progress in microelectronic has pushed the short channel length of mosfet into decanano scale .This progress imposes new challenges on the device simulation as the essential physics is different from that of the old convential old techniques because the effect of the quasi –ballistic transport on device.Therefore it become essential to understand essential physics of quasi ballistic transport and its implication on the nanoscale device.under this we disscuss two topics for understanding the Quasi-Ballistic Transport in Nano-MOSFET.
Introduction
SILICON MOSFET channel lengths are decreasing to below 50 nm where traditional compact models lose validity. Now a days the carrier transport tends toward the quasi-ballistic regime and the models based on the drift-diffusion theory and on the notion of statistical mobility fail to describe transistor operating close to the ballistic limit.This models assumes that the mean-free path is negligible as compared with the channel length i.e., channel length and mean-free path are in
the same order of magnitude. The distribution function is no more maxwellian like and “ballistic peaks” appear in the velocity distribution . Therefore the use of the notion of
mobility in compact models is questionable. In this theory the key factor is the backscattering
coefficient Rc which is linked to the degree of transport ballisticity.
BACKSCATTERING THEORY
In very short MOSFETs the entire channel or a more significant part of the channel participates in the backscattering. A carrier can
even be sent back to the source by a scattering event in the drain. n a short channel where the transport is nonstationary we cannot consider the mean-free path to be independent of
the carrier energy. Moreover, in order to determine the mean-free path =vtherm*ἰ . The
backscattering effect is expected to be strongly influenced by effective mass. In order to obtain good quantitative results
in the case of short channel, model of mobility depending on the bias VDS and VGS to determine the mean-free path , and use two fitting parameters βand α , to calibrate the critical distance Ɩ according to
Characteristics of Ballistic Tansport in Short-Channel MOSFETs
Introduction
In a long channel MOSFET whose transport is basedon a drift-diffusion model, charge carriers (electrons and holes) experience many scattering events when they are moving in the channel. It is well studied that these scattering events deteriorate transport characteristics resultingin a decrease of the drain current.Recently, due to the remarkable development of semiconductor device fabrication technology, the MOSFET channel length has become shorter and comparable to a carrier scattering length ; as a result, the carriers can encounter less scattering
QUASI-BALLISTIC MOSFET MODEL
The drain current of the quasi-ballistic MOSFET can be expressed by
where r is a back-scattering coeffcient given
where l , called kT-layer length, is defined as a characteristiclength corresponding to a potential drop by kT/q near the source, as indicated in Fig. 1
RESULTS AND DISCUSSION
In the quasi-ballistic MOSFET, the back-scattering phenomenon makes the drain current decrease, so that it is essential to reduce the back-scattering events to enhance
the characteristics of the ballistic transport. Backscattering occurring within a characteristic length called the kT-layer near the source plays a key role in the quasiballistic transport, as illustrated in Figure 1.
Dependence of quasi-ballistic characteristics on drain voltage
As drain bias is increased, the magnitude
of the potential barrier becomes smaller, due tothe DIBL (Drain-Induced Barrier Lowering) effect, and the kT-layer also decreases simultaneously. increasing drain voltage, the back-scattering coefficient (r ) becomes smaller, due to the reduction of the kT-layer length , and its value is saturated at a lower drain voltage with decreasing channel length. So, these results support the idea that as the channel length becomes shorter, both the back-scattering coeffcient and the drain current saturate at a lower drain voltage.
Dependence of quasi-ballistic characteristics on temperature
Temperature can also be used as one of the external parameters to alter the properties of the quasi-ballistic MOSFET. Since carrier transmission in the channel is strongly dependent on carrier mobility, lowering the temperatures can enhance carrier mobility, leading to a larger transmission coefficient.
As the temperature decreases, more inversion charges can move to the channel, because of the lowered e_ective potential barrier which carriers experience, leading to the enhancement of the transmission coefficient. In addition, increased carrier mobility makes their mean free path longer, resulting in the reduction of the back-scattering coeffcient in the channel.
Dependence of quasi-ballistic characteristics on doping concentration in the source
It is assumed in this study that carriers can be transmitted over the barrier
through thermionic emission. So,making a degenerate condition in the source region can increase the carrier density injected into the channel. Since a high doping concentration in the source increases
the number of carriers having energy higher than the channel barrier, the saturation current is expected to increase at a higher source doping concentration. As expected, with increasing doping concentrations in the source, the saturation current is enhanced.
CONCLUSION
We have studied the characteristics of the quasiballistic transport and drain current in ultra small MOSFETs,taking into account the back-scattering and transmission coefficients.
The characteristics of the quasi-ballistic MOSFET vary dramatically according to temperature, doping concentration
in the source, oxide thickness and drain & gate
voltage. Dependences of these parameters on the quasiballistic transport are more pronounced as the channel length becomes shorter.