Research Of Novel Nanoscale Junctionless MOSFET

As channel length of transistor scaled down,the formation of ultrasharp source and drain junctions which requires extremely high doping concentration gradients and very low ther-mal budget processing has become an increasingly difficult challenge for the semiconductor industry.Thus,a new concept of junctionless?JL?MOSFET has been recently simulated and fabricated.The JL MOSFET has a high and invariable doping concentration in all source,drain,and channel regions.Thus JL MOSFET requires no pn junction and it is simpler to fabricate.This thesis focused on JL MOSFET and JL Tunneling Field-Effect Transistors?TFET?,JL MOSFET is based on the carrier thermal injection and JLTFET utilizes band-to-band tun-neling as a source carrier injection mechanism.The influences of dual-material double-gate?DMDG?,asymmetric dual-material double-gate?ADMDG?,and high-k gate dielectric on JL MOSFET are analyzed.In addition,we investigated the impact of dual material gate?DMG?,hetero gate dielectric?HGD?,and source pocket on the performance of JLTFET.To verify the developed model,the model results are compared with the TCAD simulation results.In order to suppress drain induced barrier lowing?DIBL?and improve carrier transport effi-ciency,junctionless dual-material double gate?JLDMDG?MOSFET is firstly proposed.An-alytical model of channel electrostatic potential and subthreshold characteristics is derived by solving the 2D Poisson's equation using a variable separation technique.An analytical short-channel threshold voltage model for JLDMDG MOSFETs is successfully developed based on channel electrostatic potential expression.The model result shows that the poten-tial step in DMDG structure are helpful to the suppression of DIBL.There are two electrical field peaks along the channel direction for DMDG MOSFET which ensure a better average electric field across the channel,thus a higher drain current.By comparing the performance of JLDMDG MOSFET with different ratios of control gate and screen gate,it is found that threshold voltage roll-off and subthreshold swing can be reduced by increasing the control gate length.While a longer screen gate yields a better screening of the channel potential near the source from drain voltage,leading to a lower DIBL.The developed subthreshold current and threshold voltage models lay a theoretical foundation for further research of short channel effect and subthreshold behaviors.The perfectly symmetric top and bottom gates in a double gate device is difficult to realize,especially when device dimension is in sub-nano scale.It has been proved that asymmetric top and bottom gates do not harm the device performance,it has the advantage to enlarge the circuit design space.Therefore,it is of great concern to investigate the asymmetric dual-material double-gate?ADMDG?whose control and screen gates length ratios are different at front gate and back gate.The effect of HfO₂/SiO₂gate dielectric on the proposed device is also included.A 2D potential analytical model for the proposed device is developed by solving 2D Poisson's equation using a variable separation technique.Based on the channel potential,threshold voltage and subthreshold current are estimated.It is revealed that the ADMDG can significantly improve the electrical characteristics of the JL MOSFET without increasing the process steps and complexity.At the same time the device design is more flex-ible.Moreover,the channel potential and electric field distribution of ADMDG MOSFET is similar to that of tri-material double-gate MOSFET resulting in higher carrier transport efficiency and drain current.To achieve a subthreshold swing less than 60 mV/decade at room temperature,JLTFET us-ing electric field control of band-to-band tunneling?BTBT?as the current gating mechanism was proposed and has been intensively analyzed.However,JLTFET suffers from two ma-jor shortcomings,a low on-current in comparison to conventional MOSFET and ambipolar effect.To overcome these two problems,a novel JLTFET named dual-material double-gate hetero-gate-dielectric?DMDG-HGD?JLTFET is proposed for the first time.Device Perfor-mance for DMDG-HGD JLTFET is studied is performed and compared with corresponding HGD TFET and DMG TFET in terms of energy band profile,electric field,and drain current.The sharper band bending in DMDG-HGD TFET device leads to a reduction in tunneling barrier width and an increase in the lateral electric field,which helps in obtaining a high on-state current.Using low-?gate dielectric at the side of drain-channel junction can reduce the maximum electric field.Lower electric field implies a smaller tunneling probability,which indicates that ambipolar effect is suppressed by using a lower low-?gate dielectric constant.This thesis proposed several novel JLMOSFETs to suppress short channel effects and im-prove its subthreshold behaviors.By solving 2D Poisson's equation,analytical models are derived.Results of this thesis can provide some guidance for the further study of JLMOSFET.