The Effects Of Phonon And Defect Assisted Zener Tunneling In Tunnel Field-effect Transistors

The tunneling field-effect transistor(TFET)has received tremendous attentions as a promising candidate for conventional metal-oxide-semiconductor field-effect transistor(MOSFET)due to its abilities to reduce the power consumption.The working principle of TFETs is based on band-to-band tunneling or Zener tunneling which is different from MOSFETs.Therefore,TFETs offer the possibility of overcoming the 60 mV/decade subthreshold slope limit of conventional transistors and making sharp-switching logic devices.However,it is very difficult to obtain a steeper SS as well as higher ON current due to the appearance of undesirable traps or phonon-assisted tunneling.The trap states in forbidden gap acting as stepping stones of tunneling from the valence band(VB)to the conduction band(CB),which both increase the ON-srtate current and OFF-state leakage,and deteriorate the SS.In this paper,under the theory of effective mass approximation,the influence of phonons and traps on Zener tunneling in direct and indirect semiconductors have been investigated respectively.The main research contents and results are summarized as follows:(1)Zener tunneling in direct band gap semiconductor.The BTBT currents of a monolayer MoS2 nano-junction with and without trap cases are calculated by using a transfer matrix method,which containing a corrected pre-factor and an exponential factor.The results suggest that the corrected formula can also applicable to the two-dimensional sinusoidal potential model,despite the model bases on the one-dimensional constant electric field.Different from the resonant tunneling from CB to CB,the energy levels of resonant tunneling are difficult to be determined in the trap-assisted tunneling.The numerical results consistent with the atomistic ab initio quantum transport simulations.The trap-assisted Zener tunneling increase the ON-state current,while the SS remains almost unchanged.It is found that the effects of traps on the transmission are mainly related to the position of resonant energy level.When the traps close to the edge of band or n/p-side,the influence become weak.(2)Phonon-assisted Zener tunneling.A formula for studying the phonon assisted Zener tunneling in an indirect band gap semiconductor is given by the Fermi gold rule.By comparing the different of phonon modes in silicon,we found that the transverse acoustic phonon mode makes the largest contribution to the tunneling current.The current density obtained from the transfer matrix model slightly higher than the Wentzel-Kramers-Brillouin approximation method,which ignores the effect of the reflection wave.And because of the overestimation of the coupling effect between the valence band and the conduction band,the result of this paper is about one order of magnitude lager than Kane's model.(3)Phonon assisted Zener tunneling in the case of defects.The effects of the trap states,spatial position and density distribution on Zener tunneling have been investigated in an indirect semiconductor.We found that the current density increases slightly with the increase of temperature.Additionally,there are two phonon-assisted tunneling peak,which corresponding to the absorption and emission phonon processes respeatively when the electron tunneling from the VB to the CB.The results show that the high dark currents under the low bias voltage and electric field are mainly caused by the deep-level traps or the traps closing to n-side.On the other hand,the traps closing to the p-side boost the ON-state current while maintaining the SS unchanged.We also find that the shallow-level traps almost no contribution to the tunneling current.The reduction of the trap densities can suppress the current near the low bias voltage region.It can be concluded that the performance of device can be improved by adjusting the trap position and trap level of channel material.The above results would play an important role in experiments for the design of TFETs.