Channel Charge Density Research Articles

Dyakonov–Shur plasma excitations in the channel of a real high-electron mobility transistor

The arguments leading to the prediction of instability of plasma excitations in a drifting 2D electron gas given by Dyakonov and Shur are made more rigorous by treating a real high-electron mobility transistor (HEMT) channel biased in its ohmic region (i.e., below pinchoff), including nonuniformity of the channel charge density. Expressions are derived for the changes in resonant frequency and gain of the Dyakonov–Shur modes arising from finite channel mobility and density nonuniformity to first order in the momentum relaxation time. It is found that channel nonuniformity (constriction) weakens the instability, in keeping with the recent results of Cheremisin and Samsonidze [Semiconductors 33, 578 (1999)], possibly explaining why it has not been observed in real HEMTs.

Channel electron mobility dependence on lateral electric field in field-effect transistors

A method is described to measure the channel charge density-mobility product in field-effect transistors as a function of lateral channel field. From knowledge of the channel charge density, the channel carrier mobility-lateral field relation is determined. This method was applied to n-channel Si metal–oxide–semiconductor field–effect transistors. The channel charge density was determined at low lateral field as a function of gate-to-channel voltage from capacitance measurements and this value was corrected to account for electron saturation velocity effects at higher fields. The electron mobility-field results were obtained for lateral fields up to 2×105 V/cm, a factor of 5–10 larger than previously obtained, and were fit to the conventional empirical relation for mobility, μ=μlf[1+(μlfξL/vsat)β]−1/β, where μlf is mobility at small lateral fields, vsat is electron saturation velocity, ξL is the magnitude of the lateral channel field, and β is an empirical fitting parameter; a best fit to this relation was obtained for unity β and vsat of 4×106 cm/s.

Monte Carlo simulations of electron transport in coupled Si quantum wells

Coupled wide and narrow tensile strained Si X2-valley quantum wells could provide the basis for an SiGe transistor operating in a velocity modulation mode. Ideally, charge would be rapidly switched between high- (wide) and low- (narrow) mobility channels under the action of an applied gate bias, with little or no modulation of the total channel charge density. With this application in mind, the Monte Carlo technique has been used to simulate in-plane electron transport for a back-doped Si/Si0.85Ge0.15 double-well structure. The equilibrium band profile from the Schottky or oxide top gate to the SiGe virtual substrate is such that electrons are confined to the narrow well when the gate is unbiased. The calculated in-plane mobility is sensitive to the transverse field applied across the quantum well structure - the maximum:minimum mobility ratio is 13:1 at 77 K. At 77 K, a lower mobility in the narrow channel (4000 cm2 V s-1) is mainly due to surface roughness and Coulomb scattering by supply layer impurities. The simulations also predict that mobility modulation would be far less effective at 300 K; scattering by acoustic and inelastic g phonons (LO, TA, LA modes), and consequent distribution across the available subbands (9) of the entire well structure reduces confinement to either the wide or narrow well, and hence the predicted maximum:minimum mobility ratio is less than 2:1.

The role of background concentration in AlInAs/GaInAs/InP based double heterojunction HEMTs

The influence of background concentration on threshold voltage and other small signal parameters is studied theoretically. It is shown that varying the background concentration (unintentional) in the GaInAs channel significantly affects the threshold voltage, the channel charge density, the drain current and other small signal parameters. Varying the background concentration from ∼10 14 to ∼10 16 cm −3, a shift in the threshold voltage of 54 and 87 mV was obtained for 250 and 350 Å channel thickness. The role of background concentration is diminished for increasing channel thickness. The threshold voltage shift is approximately insensitive to a higher indium content in the GaInAs channel.

The role of acceptor density in [formula omitted] based quantum well HEMTs

Numerical results based on a self-consistent solution of Poisson's and Schrődinger's equations for GaAs AlGaAs quantum well HEMTs are presented. It is shown that varying the acceptor density in a quantum well significantly affects the threshold voltage, the channel charge density, and hence the drain current. Particularly, near threshold and the subthreshold region, for a given gate bias a change of acceptor density from ∼ 10 13 to ∼ 10 16 cm −3 decreases the drain current by two orders of magnitude. For typical acceptor densities existing in the MBE-grown GaAs channel, a shift in the threshold voltage of 120 mV was obtained. The role of acceptor density is enhanced at increasing well width.

Modelling the MOS transistor

Issues associated with the static and transient modelling of the MOS transistor in subthreshold are investigated. The assumptions used in the literature to derive the DC model are examined. These assumptions are justified for long channel transistors while their invalidity for short channel transistors are demonstrated. The phenomena responsible for the departure from the long channel behaviour are discussed. DC models for long and short channel MOS valid in all regions of operation are presented. The key features in modelling the transient behaviour are analysed with special attention to the issue of obtaining an analytical expression for the channel charge density.

Unified nonquasi-static modeling of the long-channel four-terminal MOSFET for large- and small-signal analyses

An iterative technique based on device transport and continuity equations is used to formulate a unified nonquasi-static model for the long-channel four-terminal MOSFET for both transient and small-signal analyses in all regions of operation (weak, moderate, and strong inversion). The model is physically derived without resorting to the concept of channel charge partitioning or the use of a priori assumptions about the functional form of the channel charge density. It is shown that the Ward charge-based model is only a 0th-order solution of this formulation. A first-order solution is presented that holds for arbitrary time-varying input voltages and can be reduced exactly to a small-signal nonquasi-static admittance model. Relaxation due to the channel resistance is included to account for the device nonquasi-static transient behavior. The first-order model consists of simple ordinary differential equations, which can be easily discretized for solution. Results from the proposed model are examined and compared with numerical simulation results and experimental data. Good agreement has been obtained. >

The foundation of a charge-sheet model for the thin-film MOSFET

Based on charge-sheet ideas, a physical model for the thin-film n-channel inversion-mode single-crystal MOSFET has been developed. This model, which is valid for moderate and strong inversion, describes analytically the front-channel charge density and accounts for the charge coupling between the front and the back gates. The utility of this model, which has a maximum relative error of less than 5%, follows from the elimination of the need to numerically evaluate the integral for the (front) channel charge density. This is the first step toward the development of a complete charge sheet model for the thin-film transistor.

A sub- and near-threshold current model for silicon MESFETs

An analytical model for silicon MESFETs that predicts static current-voltage characteristics continuously from subthreshold through above-threshold operation has been developed. The model is based on an expression for the channel charge density derived from an analytical solution of Poisson's equation in one dimension including mobile electrons and holes. The closed-form current model is verified with measured data in all regimes of operation. >

Monte Carlo study of two-dimensional electron gas transport in Si-MOS devices

By using Monte Carlo simulations the effective mobility and high field drift velocity of a quasi-two-dimensional electron gas in Si-MOS devices have been calculated. For this calculation the three lowest subbands and an ensemble of scattering quantities (fixed oxide charge, surface roughness, and several modes of phonons) have been taken into account. The relative population of the E 0- and E 0′-subbands is calculated for channel charge densities between 10 12 and 10 13 cm −2. The dependence of the carrier drift velocity on the field in the inversion layer is studied for several values of fixed oxide charge and surface roughness in a wide field range. Finally the effective mobility is considered as a function of temperature, channel charge density, surface field, fixed oxide charge, and surface roughness. The computed results are compared with the most recent experimental data. A good agreement is found.