Physical origin and characteristics of gate capacitance in silicon metal-oxide-semiconductor field-effect transistors

Abstract

The physical origin of gate capacitance in both bulk and silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistors (MOSFETs) is studied. The gate capacitance is theoretically derived as the series capacitance comprising the oxide capacitance, number capacitance (CN), level capacitance (Clevel), and field capacitance (Cfield). CN is in proportion to the thermodynamic density of states under the hypothetical condition that the subband energy levels remain constant when the total electron density is differentiated by the Fermi level. Clevel is inversely proportional to the result of differentiating the subband energy level by the total electron density. In the case of bulk MOSFETs or SOI MOSFETs with thick buried oxide, Cfield, which originates from the electric field at the edge of the depletion layer, is negligible. In addition to the fact that our new theoretical model corresponds to the intuitive conventional model expressed in terms of the effective thickness of the inversion layer in bulk MOSFETs, it is demonstrated that the conventional capacitance model is also applicable to SOI MOSFETs at 300 K. We have calculated self-consistently the capacitance of a bulk MOSFET and of SOI MOSFETs with various top-silicon layer thicknesses at 300 K. At the fixed total electron density near 1012 cm−2, the gate capacitance becomes large with decreasing top-silicon layer thickness. This difference in the gate capacitance mainly comes from Clevel, which means that in the case of SOI MOSFETs with a thin top-silicon layer, a small increase in the subband energy level with increasing electron density results in large gate capacitance and high performance of the SOI MOSFETs at 300 K.

First-principles study on inversion layer properties of double-gate atomically thin silicon channels

“…This discrepancy corresponds to the contribution of the channel density of states ͑DOS͒ on the inversion layer capacitance. 10,16,17 In our calculation, the Fermi energies in the p-and the n-channels fall from the valence band top or rise from the conduction band bottom with the increase in V g since the finite DOS of the channels. This means that a part of V g is not used for establishing the electrostatic field between the channel and the gate.…”

“…With regard to metal-oxide-semiconductor-FETs with nanometer-scale Si film channels, the enhancement of the electron-phonon scattering and the additional term for the inversion layer capacitance are predicted, while these studies assumed the effective mass theory ͑EMT͒ with the continuum approximation. 11,[13][14][15][16][17] this contribution, we approach the inversion layer properties of double-gate atomically thin Si channels by using a first-principles method called the enforced Fermi energy difference ͑EFED͒ method, which was developed to handle charged structures under an external field. 10 We employ the Si͑111͒ thin films with one-or twobilayer Si as the prototype of intrinsic atomically thin Si channel double-gate FETs ͓Figs.…”

Quantum-mechanical effect in atomically thin MoS 2 FET

“…The next task is to explain why the saturation capacitance drastically increases at the monolayer limit. In analogy to Cinv in SOI MOSFETs, Cgc in MoS2 FETs at the accumulation region can be divided as 1/𝐶 𝑔𝑐 = 1/𝐶 𝑜𝑥 + 1/𝐶 𝐴 = 1/𝐶 𝑜𝑥 + 1/𝐶 𝐴 𝐷𝑂𝑆 + 1/ 𝐶 𝐴 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 [29,30]. 𝐶 𝐴 𝐷𝑂𝑆 comes from the finite DOS of MoS2 as 𝐶 𝐴 𝐷𝑂𝑆 = 𝑒 2 𝑔 2𝐷 [1 + 𝑒𝑥𝑝(𝐸 𝐺 / 2𝑘 𝐵 𝑇)/2𝑐𝑜𝑠ℎ(𝐸 𝐹 /𝑘 𝐵 𝑇)] , where g2D = gsgvmch/2πħ 2 is the band-edge DOS [8,20,31].…”

Model and analysis of gate leakage current in ultrathin nitrided oxide MOSFETs

“…Where n is the density of state, and at the room temperature we get From some related reference, we have the expression of , since the discussion is several pages long [8], so here we just use the result directly. Up to this point, we have got the expressions of those three capacitances respectively, where is a constant, and are both functions of ; that's to say, they are both the functions of surface potential…”