Research Of The Current Enhancement Technique For Nano-Scale Field-Effect Transistors

Current drive capability is important for various performance indicators such as the operating speed and gain of field-effect transistors.Therefore,it has been a research hotspot in the field of semiconductor devices.For nano-scale field-effect transistors,the current drive capability is susceptible to degradation due to a variety of parasitic effects.Therefore,improvement of the current drive capability for nano-scale field-effect transistors has become one of the research hotspots and difficulties for researchers.A large number of studies have shown that the optimization of the longitudinal electric field distribution can improve the carrier transport and current drive capability of the field-effect transistor.In this regard,researchers have also proposed a variety of device structures to achieve the modulation of the longitudinal electric field distribution.Due to the lack of a universal theory to guide the design of field-effect transistors,the conclusion on the optimal longitudinal electric field distribution remains unclear.In addition,carriers in the nano-scale field-effect transistors are extremely susceptible to the speed saturation effect and thus,leading to the current saturation.In order to further improve the current drive capability,new theories and new technologies must be proposed to break through the above-mentioned difficulties.In this thesis,the basic physical mechanism of nano-scale field-effect transistors is discussed.In addition,the optimal longitudinal electric field distribution theory and diffusion current enhancement theory are proposed,and a variety of device structures are proposed to further improve the current performance of field-effect transistors.The main contributions of this work are as follows:The relationship between longitudinal electric field and carrier transport efficiency is explored.By establishing the transport efficiency model,this work theoretically proved that the even longitudinal electric field distribution is the optimal case.Based on the diffusion-drift framework,a variety of device structures that can form an even longitudinal electric field distribution are theoretically deduced.In order to verify the effectiveness of the theory,this work designs a multi-material gate field effect transistor(MMG FET).Compared with the conventional field-effect transistor,the current of MMG FET is increased by 19%,and the transconductance transistor has an increase of 10.2%.Since the non-linear relationship between the carrier velocity and the longitudinal electric field widely exists in a variety of materials or devices,the realization of even longitudinal electric field distribution is expected to significantly improve the current driving capability of these devices.Therefore,this work will provide theoretical support for the structure design and optimization of devices including nano-field effect transistors.The linear doping junctionless field-effect transistor(LD JLFET)is proposed.Based on the theory of optimal longitudinal electric field distribution,a new type of junctionless field-effect transistor is designed in this thesis.First,this work establishes the potential field model of the junctionless field-effect transistor.The results show that the linear doping in the channel region can form an even longitudinal electric field distribution,thereby maximizing the carrier transport efficiency and current driving capability.Based on the CMOS process,the process flow required for linear LD JLFET is designed,and process simulations are also performed in this work.The results show that the proposed process flow can achieve well the linear doping distribution.Besides,using numerical calculation tools,the performance of the LD JLFET is studied and compared with the conventional uniform doping junctionless field-effect transistor(UD JLFET).The results show that the current drive capability of the LD JFLET is greater than that of the UD JLFET.The current increase is as high as 55.9%.For the subthreshold region,their subthreshold characteristics are similar,and the I_on/I_off has reached 10¹⁰,thereby showing excellent electrical performance.At the same time,the LD JLFET exhibits more significant transconductance performance with a maximum increase of 104.3%.The results also show that the even longitudinal electric field distribution technology will form a more significant current increase at high voltages and long channel lengths.The diffusion current enhancement theory and the titled-channel field-effect transistor(TC FET)is proposed.The TC FET has a titled channel direction.Therefore,diffusion current formed by the applied gate voltage will produce another diffusion current component along the transport direction.As a result,the current performance of the TC FET would be significantly improved.Based on the CMOS process,the process flow of the TC FET is designed and simulated.The process simulation results show that the designed process flow can obtain tilted field-effect transistors,and the tilt height can be controlled by process parameters.The influence of applied voltages and structural parameters on the current performance improvement of TC FET is also studied.The on-state current of the TC FET is improved up to 23.9%compared to the conventional FET.In addition,their subthreshold characteristics are almost the same.More importantly,the optimal value for the tilt height to maximize the current increase is also explored.The results show that the maximum on-state current,transconductance,final frequency,and maximum oscillation frequency would be maximum when the tilt height is 10 nm.A diffusion current enhancement fin field-effect transistor(DE Fin FET)with an asymmetric fin width is proposed.The variable fin width can also form a carrier gradient component along the transport,thereby forming a major diffusion current and a greater current enhancement.The current driving capability of the DE Fin FET is increased by 26%compared with the conventional Fin FET.In addition,the transconductance has also increased by 36%.The influence of the device structure parameters on the diffusion current enhancement effect of DE Fin FET is studied.The results show that the channel width of the source terminal has a minor significant effect on the current increase,while the degree of change in the channel width is related to the current improvement,amplifier quality factors and cut-off frequency.Therefore,an optimal value for the degree of change of the channel width exists,which is discussed in detail in this work.In addition,the subthreshold performance of DE Fin FET is also studied.The increasing channel width from the source region to the drain region will gradually weaken the gate control ability of the DE Fin FET.Therefore,its subthreshold performance includes subthreshold slope and characteristics such as threshold voltage drift will be degraded.In order to obtain higher gate control capability and subthreshold performance,the source channel width of the DE Fin FET should be reduced.