Channel length modulation

About: Channel length modulation is a research topic. Over the lifetime, 1790 publications have been published within this topic receiving 34179 citations.

Effects of diffusion current on characteristics of metal-oxide (insulator)-semiconductor transistors☆

Abstract: A qualitative discussion of the device operation is first given using three-dimensional energy band diagrams to show the significance of the diffusion current. The theoretical static I–V characteristics are the computed including both the diffusion and the drift currents, based on the one-dimensional and gradual channel model. Drain current saturation phenomena are evident in these exact solutions which are in good agreement with the calculations based on the bulk charge approximation and with the experimental data for the entire non-saturating and saturated ranges. The relative importance of the two current components along the length of the channel is illustrated. The effects of the diffusion current on the three more important low-frequency dynamic characteristics (the short-circuit gate capacitance, the transconductance, and the drain conductance) are discussed. The surface potential, the quasi-Fermi potential, the surface electric field and the surface carrier concentration along the channel are examined. The complete one-dimensional gradual channel model is inadequate to account for the large drain conductance observed in the saturation range, and it is shown that the electric field longitudinal to the channel current flow must be taken into account near the drain junction where it is larger than the transverse field due to the voltage applied to the gate electrode.

A charge-sheet model of the MOSFET

Abstract: Intuition, device evolution, and even efficient computation require simple MOSFET (metal-oxide-semiconductor field-effect transistor) models. Among these simple models are charge-sheet models which compress the inversion layer into a conducting plane of zero thickness. It is the purpose of this paper to test one such charge sheet model to see whether this approximation is too severe. This particular model includes diffusion which is expected to be important in the subthreshold and saturation regions. As a test the charge sheet model is applied to long-channel devices. Long-channel MOSFET behavior has been thoroughly studied, and is very well explained by the Pao-Sah double-integral formula for the current. Hence, a clear-cut test is a comparison of the charge sheet model with the Pao-Sah model. We find the charge sheet model has two advantages over the Pao-Sah model. (1) It leads to a very simple algebraic formula for the current of long-channel devices. The same formula applies in all regimes from subthreshold to saturation. Neither splicing nor parameter changes are needed. No discontinuities occur in either the current or the small-signal parameters, or in the derivatives of the small-signal parameters. (2) It is simpler to extend the charge sheet model to two or three dimensions than the Pao-Sah model. This simplification is a result of dropping the details of the inversion layer charge distribution. An important aspect of the gradual channel approximation is brought out by the analysis. Suppose the boundary condition relating the quasi-fermi level at the drain, φfL, to that at the source, φfo, namely φ ƒL =φ ƒ0 +V D where VD is the drain voltage, is applied in all bias regimes. Then it is shown that this means the potential at the drain end of the channel, φsL is not related to the potential at the source end of the channel, φso, by φ sL =φ s0 +V D Instead, φsL is computed, not imposed as a boundary condition. It is suggested that this failure of the potential to satisfy the boundary condition at the drain is justifiable. That is, φsL should be reinterpreted as the potential at the point in the channel where the gradual channel approximation fails. Hence, (2) may be relaxed. However, the “channel length” in the gradual-channel approximation now becomes a fitting parameter and is not the metallurgical source-to-drain separation. In addition several aspects of the long-channel MOSFET are brought out: (1) Pinch-off is achieved only asymptotically as the drain voltage tends to infinity. This is in marked contrast to the often-stated, textbook view that pinch-off is achieved for some finite drain voltage, the saturation voltage. (2) The channel or drain conductance approaches zero only asymptotically. (3) The transconductance saturates only asymptotically. Figures comparing the simple charge-sheet model formulas with the usual textbook formulas are included for direct-current vs drain voltage, channel conductance vs drain voltage, and transconductance vs drain voltage. The charge-sheet model agrees with the original Pao-Sah double-integral formula for the current at all gate and drain voltages, and possesses the correct subthreshold behavior. The textbook formulas do not.

Threshold voltage model for deep-submicrometer MOSFETs

Abstract: The threshold voltage, V/sub th/, of lightly doped drain (LDD) and non-LDD MOSFETs with effective channel lengths down to the deep-submicrometer range has been investigated. Experimental data show that in the very-short-channel-length range, the previously reported exponential dependence on channel length and the linear dependence on drain voltage no longer hold true. A simple quasi-two-dimensional model is used, taking into account the effects of gate oxide thickness, source/drain junction depth, and channel doping, to describe the accelerated V/sub th/ on channel length due to their lower drain-substrate junction built-in potentials. LDD devices also show less V/sub th/ dependence on drain voltage because the LDD region reduces the effective drain voltage. Based on consideration of the short-channel effects, the minimum acceptable length is determined. >

Controlling short-channel effects in deep-submicron SOI MOSFETs for improved reliability: a review

Abstract: This paper examines the performance degradation of a MOS device fabricated on silicon-on-insulator (SOI) due to the undesirable short-channel effects (SCE) as the channel length is scaled to meet the increasing demand for high-speed high-performing ULSI applications. The review assesses recent proposals to circumvent the SCE in SOI MOSFETs and a short evaluation of strengths and weaknesses specific to each attempt is presented. A new device structure called the dual-material gate (DMG) SOI MOSFET is discussed and its efficacy in suppressing SCEs such as drain-induced barrier lowering (DIBL), channel length modulation and hot-carrier effects, all of which affect the reliability of ultra-small geometry MOSFETs, is assessed.

Lucky-electron model of channel hot-electron injection in MOSFET'S

Abstract: The lucky-electron concept is successfully applied to the modeling of channel hot-electron injection in n-channel MOSFET's, although the result can be interpreted in terms of electron temperature as well. This results in a relatively simple expression that can quantitatively predict channel hot-electron injection current in MOSFET's. The model is compared with measurements on a series of n-channel MOSFET's and good agreement is achieved. In the process, new values for many physical parameters such as hot-electron scattering mean-free-path, impact-ionization energy are determined. Of perhaps even greater practical significance is the quantitative correlation between the gate current and the substrate current that this model suggests.