Study On Mechanism And Structure Optimization Of The Novel TFET

The size of MOSFET becomes small gradually,and the problem of short channel effects and quantum effects have been rising.The limit of the transistors' Subthreshold Swing(SS)cannot exceed 60 mV/dec at 300 K at present,so it is difficult for the conventional micro-nano devices to satisfy the demand of low power consumption in IC.To continue the the Moore law,it is necessary to break through conventional the theory of MOSFET,and to develop the new kind of device to meet the low power consumption and imporve the SS.Tunnel Field-Effect Transistors(TFETs)have been raised in such a condition,because TFETs have the low power feature of the steeper slope and smaller SS than MOSFET devices.There is a different operating mechanism between TFET and MOSFET,and the current of the TFET was based on band-to-band tunneling of quantum effect but not the electrons and holes hot-injection so that can break the confine of the 60 mV/dec.Besides,TFET are kind of low power device that is super subthreshold slope and low subthreshold swing.However,experimental results demonstrated that Si-based TFETs are facing with the two challenges that the on-state current and SS could not be increased and improved,because of TAT tunneling.According to the BTBT,the ability and quantity of the electrons subject to semiconductor material and device structure.In order to obtain considerable current and improved SS,the electric-filed of the tunnel junction should be improved and carrier tunnel mass and Band-gap should be reduced respectively.This paper introduces three aspects,including the improvement of the structure of the gate,using of the new semiconductor materials in source region and proposing of the new form of tunneling.We analyzed and studied the electrical characteristics of the TFETs.There are four parts in this treatise.1)By analyzing the influence of point and line tunnel on Ion,and the increasing of the portion of the line tunnel can effectively improve the tunneling.The overlap of gate and source is the best way of increasing the line tunnel.The thesis presents Dual-Source SiGe U shape TFET.The drive currents could be improved by enlarging of the vertical source region and the line tunneling areas.Due to the isolation of the intrinsic channel,the ambipolar currents have been restrained.Comparing to the traditional dual sources TFETs,the device area can be reduced and the on-state current can be also improved.The results show that the Ion is 76 ?A/?m,and the average SS=30 mV/dec,they are 100 times.The ambipolar currents can be effectively reduced to 10-13 ?A/?m,because of the isolation of the channel2)The conventional TFETs improve tunneling effect by doping steep at pn junction.The difficult processing and diffusion doping results that the tunneling junction is not steep around the pn junction,and the SS is not ideal.The insulation layer was inserted into the pn junction is a new tunnel form device.This paper propose that TD-TFET was formed by high doping pn junction instead of the conventional TD-FET.The Ion is 11 times than normal TFETs and the Ioff is as small as 10-17 A/?m,due to the TD-TFET,because of the decrease distance tunnel.The ambipolar currents was suppressed absolutely.because of the steep doping pn junction,the SS reaches to 20 mV/dec.3)In order to improve the drive current of TFETs,it is necessary to increase the tunneling areas.EHB-TFET was formed by the stagger double-gate TFET and electrons and holes layers were formed by the asymmetric gate electrodes,so the tunneling happen through the channel.The EHB-TFET possesses the largest tunneling areas,but the ambipolar currents still affect the TFETs.The 3D EHB-TFET is in this study,the insulation layer was inserted into channel that is close to drain.The underlap length of the device was increasing and the ambipolar currents was reduced successfully.Si0.8Ge0.2 was used in source,the on-state current of 1 um gate height,200 nm gate length and 10 nm channel width of the new structure EHB-TFET is 4x 10-4 A.The off-state current was reduced to 5×10-15 A and the ambipolar currents is 10-14 A.The different tunneling mechanism between the EHB-TFET and the normal TFETs,Back-Gate and gate leakage were studied in this paper.4)On the basis of a lot of experimental data,the Ion,Ioff and SS are not ideal those values are far away from those of theory,especially ?-? semiconductor material TFETs.The SS values are not ideal.The influence of the TAT on the DG-TFET haves been analyzed in this paper because of the interface defect and lattice mismatch between the High-k dielectric and the channel.The TAT currents reach to 10-14 A/?m when the trap levels close to the mid-gap.The SS changes to 200 mV/dec from ideal 60 mV/dec by analyzing the impact of the different interface trap types,interface trap levels,TAT mechanism,trap positions and temperature on electric characteristics of the DG-TFET.The simulation results show that gate-channel traps have more influence than source-channel interface on TFET.The gate leakage current reduced to 10-11 A/?m from 10-9 A/?m by enlarging barrier height in gate dielectric.The simulations show that the performance degradation of the SS and Ion was affected by TAT and gate leakage currents,that have important reference to the performance degradation of TFETs.The deterioration properties of the TFET should be studied by improving processing,such as laser annealing,E-beam and surface inactivation,which improve characteristics of the interfaces and electrics.In conclusion,the new structure of the TFET was introduced in low power consumption,and the disadvantages of it was discovered and optimized.There are four new geometries to improve the property of the TFET,and there are new development prospects and theoretical foundation for the conventional TFETs.These theories have profound guiding significance for researching the new geometries TFET.