Wangran Wu

Mechanical tensile strain induced gate and substrate currents change in n and p-channel metal-oxide-semiconductor field-effect transistors

To investigate and understand the reliability behavior of strained silicon devices, the changes of gate currents (Ig) and substrate currents (Isub) in n and p-channel metal-oxide-semiconductor field-transistors (MOSFETs) under different types of mechanically applied tensile stresses have been studied. It has been observed that, under the uniaxial tensile stress, both Ig and Isub of pMOSFETs increase with the increase of applied stress under the inversion and the accumulation conditions. However, an opposite stress dependence in nMOSFETs has been observed for Ig and Isub in both the inversion and the accumulation regimes. Similar changes have been found for Ig and Isub of nMOSFETs under biaxial tensile stress. The observations are explained by the strain induced band structure modulation and the repopulation of carriers.

Experimental Study on NBTI Degradation Behaviors in Si pMOSFETs Under Compressive and Tensile Strains

In this letter, we experimentally investigated the effects of four types of strains, uniaxial tensile strain, uniaxial compressive strain, biaxial tensile strain, and biaxial compressive strain, on the negative bias temperature instability (NBTI) degradation behaviors of Si pMOSFETs. The strains were applied using a wafer bending system to avoid processing effects on the NBTI characteristics as a result of strain engineering. We confirm experimentally, for the first time, that both uniaxial and biaxial compressive strains are advantageous in terms of the NBTI improvement in Si pMOSFETs. However, the NBTI reliability was degraded under both uniaxial and biaxial tensile strains. These results could not be explained by considering only the gate leakage current change due to the strain. The strain-induced modulation of the interaction between the carriers and Si-H bonds at the interface must also be considered.

Comparative study on strain induced electrical properties modulation of Si p-n junctions

Understanding the p-n junctions is of great importance because p-n junctions are the essential elements of semiconductor devices. In this study, we experimentally investigated the strain induced electrical properties modulation of Si p+-n and n+-p junctions. It is found that, under the uniaxial tensile stress, the current in the large-forward-bias region increases significantly, while a rather small current increase is observed in the diffusion-current-dominant region. Besides, the ideality factors in the diffusion-current-dominant region and the large-forward-bias region decrease when the amount of the applied stress increases. The observations are explained by the strain induced variations in energy band structure, their effect on minority carrier concentrations, and the piezoresistance effect.

Experimental Investigation on Alloy Scattering in sSi/

Alloy scattering in a sSi/Si_0.5Ge_0.5/strained Silicon on Insulator (SOI) (sSOI) quantum-well (QW) p-MOSFET is investigated by hole density modulation through applying back-gate biases. The hole mobility under negative back-gate biases is found degraded by intensified alloy scattering at low electrical field because more holes are distributed in the bulk Si_0.5Ge_0.5. At higher electrical field, the higher density of holes populated at the Si/ Si_0.5Ge_0.5 interface and less holes in the bulk Si_0.5Ge_0.5 result in less pronounced alloy scattering, leading to mobility enhancement under negative back-gate biases. This confirms experimentally that alloy scattering does not play a significant role in the hole mobility of sSi/ Si_0.5Ge_0.5/sSOI QW p-MOSFETs under normal operating mode.

{\rm Si}_{0.5}{\rm Ge}_{0.5}

Positive Bias Temperature Instability (PBTI) and Hot Carrier Injection (HCI) characterizations on InGaAs-On Insulator (OI) back gate nMOSFETs are presented and the degradation mechanism is discussed. Devices with two ultra-thin body thickness (15 nm and 8 nm) but same buried oxide (BOX) (15 nm) are investigated from a reliability perspective. Independent of the body thickness, the PBTI stress shows stronger impact on device performance than the HCI stress. Although the thinner body transistor exhibits a lower “off” current, it subjects to more severe degradations under both PBTI and HCI stresses. Pulsed HCI experiments confirmed that the self-heating effect (SHE) compounds the reliability challenge in ultra-thin body InGaAs-OI nMOSFETs. Additionally, it is also found that the different evolutions of the threshold voltage and the saturation current of the UTB InGaAs-OI nMOSFETs may be due to the slow border traps in the oxide.

/sSOI Quantum-Well p-MOSFET

In this paper, we have experimentally investigated the effects of all types of strains, including uniaxial tensile strain, uniaxial compressive strain, biaxial tensile strain and biaxial compressive strain, on the negative bias temperature instability (NBTI) of Si pMOSFETs. Strain is applied by using a wafer bending system to avoid processing effects on the NBTI characteristics that result from strain engineering. We confirm experimentally, for the first time, that both uniaxial and biaxial compressive strain in Si pMOSFETs is advantageous as demonstrated by suppressed NBTI. On the other hand, NBTI is enhanced under both uniaxial and biaxial tensile strains. Differences in measured in gate current (Ig) can be attributed to the varying NBTI degradation under different types of strains. The experimental results are partly explained by strain induced band structure modulation and hole repopulation among the heavy hole and light hole subbands.

PBTI and HCI degradations of ultrathin body InGaAs-On-Insulator nMOSFETs fabricated by wafer bonding

In this paper, we review the recent progresses about the effect of the uniaxial tensile strain on the electrical properties of the Si p-n junctions and MOS capacitors. We found that the uniaxial tensile stress could increase the junction current in the large-forward-bias region significantly. However, only a slight current increase has been observed in the diffusion-current-dominant region. In nMOSFETs the uniaxial tensile strain could enhance Isub significantly, while decreasing Ig slightly. Furthermore, in pMOSFETs, the uniaxial tensile strain could enhance both Ig and Isub. All these results have been explained by taking the strain induced subband structure modulation, current components variation and the piezoresistance effect into consideration.

Comprehensive study of NBTI under compressive and tensile strain

In this study, we experimentally examine the change of gate currents (I_g) and substrate currents (I_sub) in n and pMOSFETs under different types mechanically applied stress. It is found that, under the uniaxial tensile stress, both I_g and I_sub of pMOSFETs increase with the increase of the stress under the inversion condition. However, an opposite stress dependence in nMOSFETs could be observed for I_g and I_sub. Similar changes were found for I_g and I_sub of nMOSFETs under biaxial tensile stress. Furthermore, the results are explained by the strain altered band structure and the repopulation of carrier.

Strain-induced I-V characteristics modulation of p-n junctions and MOS capacitors in Si CMOS devices

p-n junctions are of great importance for both modern Si complementary metal oxide semiconductors (CMOS) devices and other semiconductor devices. In this study, we experimentally examined the strain induced modification of the current-voltage characteristics of Si p-n junctions. The strain was applied to the forward biased p+-n and n+-p junctions though a wafer bending method. It is observed that, under the uniaxial tensile stress, the ideality factor in the diffusion current region of a forward p-n junction decreases with the increase in the applied stress. Meanwhile, the junction current increases with the increase in the applied stress. It is also found that the applied uniaxial tensile stress causes a significant junction-current increase in the large forward biases region and a relative small current increase in the diffusion current region.