Investigation of the performance limits of III-V double-gate n-MOSFETs

Abstract

The performance limits of ultra-thin body double-gated (DG) III-V channel MOSFETs are presented in this paper. An analytical ballistic model including all the valleys (Γ-, X-and L-), was used to simulate the source to drain current. The bandto-band tunneling (BTBT) limited off currents, including both the direct and the indirect components, were simulated using TAURUS TM . Our results show that at significantly high gate fields, the current in the III-V materials is largely carried in the heavier L-valleys than the lighter Γ-valleys, due to the low density of states (DOS) in the Γ, similar to current conduction in Ge. Moreover, these high mobility materials like InAs, InSb and Ge suffer from excessive BTBT which seriously limits device performance. Large bandgap III-V materials like GaAs exhibit best performance due to an ideal combination of low conductivity effective electron mass and a large bandgap.Introduction Due to their small Γ-valley electron mass, Ge and III-V materials like GaAs, InAs and InSb are being investigated as high mobility channel materials for high performance NMOS [1,3,4,5]. Under ballistic conditions, the main advantage of a semiconductor with a small transport mass is its high injection velocity. However, these materials also have a very low density of states in the Γ-valley, which tends to greatly reduce the inversion charge and hence reduce drive current. Further, the very high mobility III-V materials like InAs and InSb, have a much smaller direct band gap which gives rise to high band to band tunneling (BTBT) leakage. Materials such as InAs and InSb have a high dielectric constant and hence are more prone to short-channel effects (SCE). In this paper we have thoroughly investigated and benchmarked DoubleGate (DG) n-MOSFETs with different channel materials (GaAs, InAs, InSb) under ballistic transport taking into account band structure, quantum effects, BTBT and SCE.Device Structure and Simulation Methodology The device structure simulated is shown in Fig. 1. The effective masses used in this work are listed in Table I. To calculate the electron wavefunction and sub-band energy levels we solve the 1D-Poisson-Schrödinger as described in [6]. Due to the small effective mass in the Γ-valley, the quantization and the lower DOS causes the inversion charge to populate the higher (X-and L-) valleys, which cannot be neglected. We include all the valleys, Γ-, X-and the L-, for the III-V materials due their relatively small valley shifts. Parabolic E-k relationship is assumed in all valleys. The drive current for the device is calculated using a ballistic transport model [8]. Due to the higher dielectric constant, the SCE in these high mobility materials must be taken into account [7]. The relative performance of these devices is highly dependent on their sub-threshold characteristics [10]. We use TAURUS TM to estimate the sub-threshold characteristics and SCE. The BTBT leakage is calculated taking into account QM effects using the model and parameters from [3,9]. In this paper, Ge (110) orientat...

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