Impact of non-equilibrium transport and series resistances in 0.1um bulk and SOI MOSFETs

Impact of non-equilibrium transport on current performances has been investigated by means of device simulation for 0.1μm bulk and SOI MOSFET’s. Although performance degradation due to series resistances is more pronounced than in bulk counterparts, SOI devices take more advantage of velocity overshoot. IIntroduction Optimization of SOI MOSFETs in the 0.1μm regime has been systematically investigated by means of device simulation, including both drift-diffusion and Monte Carlo techniques. Advantages of thin film SOI transistors in terms of short-channeleffect immunity are already known [1]. Moreover, current capability of 0.1μm devices deviates from the behavior predicted by drift-diffusion models, due to non-equilibrium transport.The enhancement of current capability, crucial for high speed applications, has been investigated for fully depleted SOI and bulk structures. The transconductance of both types of devices is also compared with experimental data. IISimulations and results Thin film SOI devices and bulk counterparts have been simulated and the influence of various technological parameters have been investigated (effective channel length L, SOI film thickness tSi, gate oxide thickness tox, buried oxide thickness tBox, bulk or SOI film doping Csub). Results of drift-diffusion simulations are used as an initial solution for Monte Carlo computations, which include non-equilibrium transport. The Monte Carlo simulator uses classical values for scattering coefficients [2]. Scattering at the Si/SiO2 interfaces is treated as a mixture of reflections (82%) and diffusions (18%), in order to obtain the same currents with drift-diffusion and Monte Carlo for a 1μm long bulk transistor (Fig.1). As shown in Fig.2, electron velocity may reach 107cm/s (saturation velocity) even at the source junction, suggesting possible enhancement of current performance due to velocity overshoot in the shorter devices. A comparison of Id-Vd characteristics extracted from drift-diffusion (DD) and Monte Carlo (MC) simulations gives the increase of current directly related to non-stationary transport. At Vg=Vd=2V, this is estimated as +28% for the bulk structure (Fig.3) and +34% for the thin-film SOI device (Fig.4). Non stationary transport also results in transconductance (gm) enhancement as reported in Fig.5. However, the values of gm are well below the maximum of transconductance predicted by the velocity saturation (about 860μS/μm for tOx=4nm). This is confirmed by a comparison with measured values given in Table 1 for devices similar to those investigated in this study. Several factors may explain this limitation: mobility degradation, access resistances, SOI film self heating [4]. According to our study, although non-equilibrium transport significantly enhances current performances, source-drain resistances is still a main limiting factor. IIIConclusion The enhancement of current capability due to non-equilibrium phenomena has been investigated for bulk and SOI MOSFETs in the 0.1μm regime. SOI devices suffer from a more pronounced degradation of current performances due to series resistances. However, this is partially compensated as thin film transistors take more advantage of velocity overshoot than their bulk counterparts, thus leading to comparable transconductances. This work is supported by DRET under contract 95-34-055-00-470-75-01 References [1] J.-P. Colinge, Silicon-on-insulator technology: Materials to VLSI, Kluwer Aca- demic Publishers, 1991 [2] C. Canali, C. Jacoboni, F. Nava, G.Ottaviani, A. Alberigi-Quaranta, Phys. Rev. B, vol.12, p2265, 1975 [3] D. P. Kern, Sub-0.1μm silicon MOSFETs, Proc. ESSDERC, p. 633, 1989 [4] K. Ohuchi, R. Ohba, H. Niiyama, K. Nakajima, T. Mizuno, A high-performance 0.05 μm MOS FET : possibility of a velocity overshoot, Jpn. J. Appl. Phys.,Vol.35, Part 1, No. 2B, p. 960, Feb. 1996 0. 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Vd (V) 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 Id ( A ) Vg=0.4V Vg=0.8V Vg=1.2V Vg=1.6V Vg=2.0V Monte Carlo drift-diffusion *1 0 -5 -300. -100. 100. 300. 500. Distance from source junction(nm) -5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 6 L=107nm 157 207 257 307nm Vg=2V Vd=2V *1 0 Fig.1: Id-Vd characteristic. L=1.0 μm, Csub=8.10 17 cm-3, tox=4 nm Fig.2: Mean electron velocity along the interface, calculated on a 10nm depth. Csub=2.10 17 cm-3, tSi=50 nm, tox=8 nm, tBOX=200 nm 0. 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Vd (V) 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Id ( A ) Vg=0.4V Vg=0.8V Vg=1.2V Vg=1.6V Vg=2.0V MC DD * 10 -4 Fig.3: Id-Vd characteristic of bulk MOSFET. L=0.1 μm, Csub=8.10 cm-3, tox=4 nm 0. 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Vd (V) 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 -4 *1 0 Id ( A ) Vg=0.4V Vg=0.8V Vg=1.2V Vg=1.6V Vg=2.0V MC DD Fig.4: Id-Vd characteristic of SOI MOSFET. L=0.1 μm, Csub=8.10 cm-3, tSi=25 nm, tox=4 nm, tBOX=200 nm 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 0. 100. 200. 300. 400. 500. 600. 700. 800. 900. vsat.Cox SOI DD Bulk DD SOI MC Bulk MC Fig.5: Mean transconductance of the 0.1mm SOI transistor and its bulk counterpart. Table 1 (*) : gm max Origin Conditions Gm / (vsat.Cox) measure [3] BULK Vd=Vg=0.8V 66%* Monte Carlo figure 4 BULK Vd=Vg=0.8V 46% measure [4] SOI Vd=1.5V 47% Monte Carlo figure 5 SOI Vd=1.5V 58%