Research On Traps Effect And Structure Optimization Of The L-shaped TFET

The scaling of conventional semiconductor devices significantly improves the circuit in-tegration.But meanwhile,the subthreshold swing(SS)limit of circuit unit-metal-oxide-semiconductor field-effect transistor(MOSFET)becomes more serious.This is because the primary injection mechanism in MOSFET is the thermionic injection of electrons over an energy barrier,so the SS of MOSFET has a minimum value of 60 m V/decade at room tem-perature.So according to scaling law,the reduction of threshold voltage in MOSFET will result in higher off-current and power dissipation.Among the various techniques to solve the problems,tunneling field-effect transistor(TFET)has attracted much attention.In TFET,the current-switching process involves the band-to-band tunneling so that it could offer sub-60m V/decade subthreshold behavior.In addition,TFET also could provide a much lower off-current than MOSFET.Also,the fabrication process of TFET is compatible with the mature complementary metal oxide semiconductor(CMOS)process technology,so researchers do not have to spend a lot of time to study new processes.Therefore,TFET is regarded as one of the most promising replacements of the MOSFET.However,the conventional TFET also has some issues,such as lower on-current,serious ambipolar effect and traps effect.Much literature has been published to boost on-current and suppress the ambipolar effect in the recent years,but the traps effect in TFET is rarely addressed.This paper takes the L-shaped TFET(L-TFET)as an example,to investigate the traps effect,and then proposes new device structures.The main research work can be summarized as the following four parts:1)An analysis on the effect of trap assisted tunneling(TAT)in L-TFET is proposed,includ-ing the location and concentration of traps,temperature and doping function.The traps in the band gap can be regarded as the recombination centers that provide springboards for the carrier tunneling,resulting in the formation of TAT current and the degradation of transfer characteristics.And the TAT mechanism is dependent to the location and concentration of traps,temperature and doping function.The simulation results show that the traps on the tunneling path could significantly degrade the SS,while the traps on the oxide interface could degrade the off-current of TFET.And TFET is less affected by the TAT once the traps density is lower than 10¹⁰cm^-3.Besides,lower temperature and higher doping gradient are beneficial to suppress the TAT.2)The impact of interface trap charge(ITC)on L-TFET's characteristics is investigated in terms of the electric field,dc,analog/RF,linearity and transient performances,and then an effective solution is proposed.After absorbing or releasing electrons,traps can act like neutral,positive or negative charges.As a result,the nearby electric field will change.The simulation results show that for L-TFET,the traps on the oxide interface near source region dramatically degrade the device stability.It is proved that hetero-gate-dielectric structure could effectively suppress the effect of ITC.It can be explained that the presence of high-?dielectric improves the electric field,which is equivalent to reducing the electric field variation caused by ITC,so the device stability is ensured.3)A novel L-shaped dopingless TFET(LDL-TFET)with tunneling gate structure is pro-posed.Besides,the process flows of the proposed device are introduced as well.Instead of physical doping,LDL-TFET employs the electrical doping to induce the desired doping region,so reduces the density of traps in the silicon.And in order to overcome the effect of ITC,the hetero-gate-dielectric structure is also introduced into the LDL-TFET.Besides,by using the tunneling gate structure,the dc and analog/RF performances of LDL-TFET are further boosted.4)A novel raised source and recessed drain dopingless TFET(RSRD-DL-TFET)is pro-posed.Similarly,electrical doping and homo-junction structures are used in RSRD-DL-TFET to reduce the density of traps.By using the raised source structure,RSRD-DL-TFET employs the line tunneling to boost on-current.Besides,the introduction of the recessed drain structure in RSRD-DL-TFET leads to a wider tunneling distance at drain/channel in-terface,so the ambipolar effect is suppressed and the power dissipation is reduced.