Structural Design And Characterization Of Novel Tunnel Field-effect Transistors

With the scaling down of the Metal-Oxide-Semiconductor Field Effect Transistor?MOSFET?,the performance of integrated circuits is improving.With the size of the transistor into Nano scale,subject to the physical limitations of the thermionic emission process during the carrier transport,the subthreshold swing?SS?of the conventional MOSFETs cannot lower than 60mV/decade at 300K.This causes the power and performance of integrated circuits becomes a serious contradiction.It is difficult to reduce the threshold voltage of the device while the performance and power consumption of the device are not degraded.To solve this problem,a great deal of research has been devoted to new materials,structures and even new physical mechanisms.Due to the physical mechanism of band to band tunneling,tunnel field-effect transistor?TFET?can achieves the SS smaller than 60mV/dec and the good device performance.Thus makes it become a promising candidate for future ultralow power applications.The driven current of conventional Silicon-based TFET is determined by the gate-modulated tunneling diode.However,the large bandgap and carrier mass in silicon means the small drive-current for Si-TFET.Furthermore,due to the relatively large tunneling barrier and width,the homojunction TFET still faces challenges in improving the driven current.On the other hand,the asymmetric source/drain structure as an inherent disadvantage can bring trouble and inconvenience to the circuit design.This paper focuses on these shortcomings of TFET and proposes two novel TFET structures to overcome these shortcomings.The two novel devices are verified by simulation and compared with some existing novel TFET structures under the same parameters.The frequency characteristics of these two devices are also investigated and compared with other novel devices.The main contents of this paper are as follows:1.To solve the asymmetric source/drain structure caused unidirectional current path of the TFET.This paper presents and designs a symmetric U-shape gate tunnel field-effect transistor?SUTFET?for the first time.This device is verified by simulation and compared with three existing novel TFET structures under the same parameters.SUTFET has a recessed gate structure,a symmetry channel structure,and a unique source drain heterojunction design.Different from traditional TFET devices,SUTFET can turn on both ways like a conventional MOSFET and have fully symmetrical bidirectional current path.Thus it can be compatible with MOSFET circuit design rules.In this paper,the structure,working mechanism,device performance and influence of parameters on device characteristics are carried on the thorough analysis and the discussion.The results show that the device has a good performance in comparison with other novel TFET at the same time with symmetrical current path.The operating current at 0.5V supply voltage reaches12.3?A/?m,the switching ratio is over 10⁶,and the minimum subthreshold swing is15.2mV/dec.The SUTFET also has a certain reliability and process robustness,and is very suitable for future ultra-low power integrated circuit applications.2.To clarify the potential of SUTFET in analog applications and frequency applications,the frequency characteristics of SUTFET are analyzed and studied for the first time by simulation in this paper and compare with another device with symmetry structure?STFET?under the same parameter condition.The operating current(I_on),gate capacitance(C_gg),transconductance?g_m?,output conductance(g_ds),gain bandwidth product?GBW?and cut-off frequency?f _T?of these two devices are compared and analyzed.The influence of device structure on frequency characteristics is analyzed.In which,the maximum cut-off frequency of 27.7GHz and the maximum gain bandwidth of 3.2GHz are obtained by SUTFET.The maximum cut-off frequency of 17.8GHz and the maximum gain bandwidth of 2.2GHz are obtained by STFET.The results show that,compared to STFET,the SUTFET has better analog and frequency characteristics.3.To further improve the performance of tunneling transistors with recessed gate structures.A T-shape gate tunneling transistor TGTFET with recessed gate structure has been proposed and designed for the first time.This device is verified by simulation and compared with three existing novel TFET structures under the same parameters.This device has a T-shape gate and dual source structure.Thus,larger effective tunneling junction area and tunneling probability can be obtained under the same device area and material parameters.In this paper,the structure,working mechanism,device performance and influence of parameters on device characteristics are carried on the thorough analysis and the discussion.The operating current at the 1V supply voltage reaches 81?A/?m,the switch ratio is 10¹¹,and the minimum subthreshold swing is 29.8mV/dec.The results show that the device has a good performance in comparison with other novel TFET,and is very suitable for future ultra-low power integrated circuit applications.4.To clarify the potential of TGTFET in analog applications and frequency applications,the frequency characteristics of TGTFET are analyzed and studied for the first time by simulation in this paper and compare with other two devices with recessed gate under the same parameter condition.The operating I_on,C_gg,g_m,g_ds,GBW and f_T of these two devices are compared and analyzed.The influence of device structure on frequency characteristics is analyzed.In which,the maximum cut-off frequency of 11.9GHz and the maximum gain bandwidth of 2.3GHz are obtained by TGTFET.The maximum cut-off frequency of4.1GHz and 8.7GHz and the maximum gain bandwidth of 0.5GHz and 2.1GHz were obtained by UTFET and LTFET,respectively.The results show that,the TGTFET has very good analog and frequency characteristics.