Effect Of Field-dependent Mobility Research Articles

Horizontal HALL devices: A lumped-circuit model for EDA simulators

A completely scalable lumped-circuit model for horizontal HALL devices is presented therein that can be efficiently implemented in SPICE-like EDA simulators. The model has been employed for the quantitative analysis of: (a) geometrical, (b) temperature, and (c) field-dependent mobility effects, as well as for (d) the dynamic response and (e) the noise behavior of several HALL sensors. A series of experimental data is presented along with a review to junction field effect transistor (JFET) operation theory.

Analytical model for the transconductance of microwave AlmGa1−mN/GaN HEMTs including nonlinear macroscopic polarization and parasitic MESFET conduction

AbstractAn improved nonlinear analytical model is presented for calculating the transconductance of AlGaN/GaN HEMTs. The model uses a polynomial dependence of the sheet carrier density on the position of the Fermi energy level, and is valid from subthreshold region to high conduction region. It also highlights the importance of including nonlinear macroscopic polarization in calculating the 2‐DEG sheet carrier concentration at the AlGaN/GaN heterointerface. Current–voltage characteristics developed from the present model include the effects of field dependent mobility, velocity saturation, channel length modulation, and parasitic source and drain resistances. The model is extended to include the parasitic conduction through the AlGaN layer, which reduces the transconductance (gm) at high gate voltages. The present model is based on closed form expression and does not involve any fitting parameter or excessive computation. The results obtained are in good agreement with published experimental data. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 382–389, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22126

Effects of field dependent mobility and contact barriers on liquid crystalline phthalocyanine organic transistors

Copper phthalocyanine (CuPc) transistors were fabricated using the Langmuir-Blodgett technique to produce bottom contact organic field effect transistors (OFETs) on silicon substrates. The resultant devices were measured and the performance was analyzed using a two-dimensional numerical simulation of the device structure. A hole barrier at the Au∕phthalocyanine source and drain contacts was seen from the experimental data. The numerical simulations were used to extract a barrier height of 0.415eV at the Au∕phthalocyanine contacts. Also, a Frenkel-Poole mobility model was used to account for the drain current in the transistors and a high field mobility of 0.018cm2∕Vsec was extracted from the experimental data. The resultant device parameters were compared to simple analytical results and the benefits of enhanced two-dimensional modeling of OFETs are shown.

Influence of Carrier Mobility and Interface Trap States on the Transfer and Output Characteristics of Organic Thin Film Transistors

We have used two dimensional drift diffusion simulations to calculate the electrical properties of bottom contact pentacene based organic thin film transistor, taking into account field-dependent mobility and interface or bulk trap states. In order to derive from basic principles the transport properties of the organic semiconductor we have developed a Monte Carlo simulator to calculate the field dependent mobility. We analyzed the presence of trap states at the interface between organic material and gate insulator and we show the influence of trap states combined with the effect of field-dependent mobility on the transfer characteristic. Numerical calculations are also reported for the output characteristics of the device showing a very good agreement with the available experimental results.

An accurate charge control model for spontaneous and piezoelectric polarization dependent two-dimensional electron gas sheet charge density of lattice-mismatched AlGaN/GaN HEMTs

The present paper proposes an improved charge control model of lattice-mismatched AlGaN/GaN HEMTs, valid over the entire operating region. The model for estimation of two-dimensional electron gas (2-DEG) sheet carrier concentration accounts for the strongly dominant spontaneous and piezoelectric polarization at the AlGaN/GaN heterointerface. The dependence of 2-DEG sheet carrier concentration on the aluminum composition and AlGaN layer thickness has been investigated in detail. Current–voltage characteristics developed from the 2-DEG model include the effect of field dependent mobility, velocity saturation and parasitic source/drain resistances. Close proximity with experimental data confirms the validity of the proposed model.

Impact of strain relaxation of AlmGa1−mN layer on 2-DEG sheet charge density and current voltage characteristics of lattice mismatched AlmGa1−mN/GaN HEMTs

An accurate charge control model to investigate the effect of aluminum composition, strain relaxation, thickness and doping of the AlmGa1−mN barrier layer on the piezoelectric and spontaneous polarization induced 2-DEG sheet charge density, threshold voltage and output characteristics of partially relaxed AlmGa1−mN/GaN HEMTs is proposed. The strain relaxation of the barrier imposes an upper limit on the maximum 2-DEG density achievable in high Al content structures and is critical in determining the performance of lattice mismatched AlmGa1−mN/GaN HEMTs. The model incorporates the effects of field dependent mobility, parasitic source/drain resistance and velocity saturation to evaluate the output characteristics of AlmGa1−mN/GaN HEMTs. Close proximity with published results confirms the validity of the proposed model.

The effect of field dependent mobility on the threshold voltage of a small geometry MOSFET

Threshold voltage calculations for small geometry MOSFET's typically assume a constant mobility. Calculations are performed with a two-dimensional computer model to determine the effect of field dependent mobility on the threshold voltage. The field dependent mobility is seen to decrease the threshold voltage as compared with the constant mobility case as the drain voltage is increased. For low to moderate drain voltages, this effect is not of significant magnitude to be of concern.

Effects of field-dependent mobility on transfer efficiency in m.o.s. b.b.d. analogue delay lines

The effects of field-dependent mobility on transfer inefficiency due to intrinsic transfer limitations, dynamic-dram conductance and threshold-voltage modulation are calculated for an m.o.s. bucket-brigade delay line. The results show significant differences over previous theories at higher clock frequencies, where line performance is critical if quantisation errors are to be minimised in analogue applications.

Effect of field-dependent mobility on m.o.s. transistor characteristics

The authors calculate the characteristics of m.o.s.t.s, taking full account of the surface electric-field dependence of mobility in the channel, which is very significant for thin-oxide or high-gate-voltage devices. An approximation is used for the average effect of background ionised impurity charge. Fields along the channel are assumed to have a negligible influence on mobility.

Input capacity of a junction field-effect transistor

A new expression is obtained for the input capacity of a junction field-effect transistor (JFET) in the prepinch-off region, by taking into consideration the effect of field-dependent mobility.

Double injection in semiconductors heavily doped with deep two-level traps

Double injection is discussed for semiconductors heavily doped with deep recombination centers which act both as acceptors and donors. The current-voltage characteristics are derived using a generalization of the method due to Lampert for long structures in which diffusion can be neglected. Thermal emission from the traps and trapped space-charge effects are included. The resulting j- V curve shows in order of increasing current the following general features: an Ohmic regime, a low injection square law related to thermal emission, a negative resistance regime, a high injection square law, and finally a space-charge-limited cube law. A complete computer solution for the case of gold-doped silicon is given. Simple power laws for the various regimes, derived using the quasineutrality approximation, are found to agree well with the numerical solution. An expression is obtained for the voltage at the threshold of the negative resistance, and the effects of field dependent mobility are discussed quantitatively. Experimental results are presented for long (∼100 μm) gold-doped (∼10 16 cm −3) silicon p- i- n structures. The devices are fabricated from high purity n-type silicon, and large area alloy junctions are used. Pulsed I– V measurements at room temperature show an Ohmic regime, a square-law regime, and a negative resistance regime. These data are found to agree quantitatively with the theoretical calculations up to and including the onset of the negative resistance. In particular a strong temperature dependence is observed in the low injection square-law regime, which is different from that found in the Ohmic regime. This difference is explained in terms of the level structure of the gold impurity. The current for voltages above the onset of the negative resistance is approximately two orders of magnitude below the theoretical predictions. This indicates that filamentary conduction is probably occurring.

Current-voltage characteristics of the junction-gate field-effect transistor with field-dependent mobility

Current-voltage characteristics of the junction-gate field-effect transistor are analyzed based on a depletion layer model of the channel which is divided into subregions of constant mobility and of field-dependent mobility to reproduce the actual situation under various voltage conditions in order to clarify the effect of field-dependent mobility on the characteristics of the device. New formulas for design parameters are given and experimentally verified on specially designed and fabricated samples as well as on commercial samples. Agreement between theory and measurement is good.

Field-dependent mobility effects in the excess noise of junction-gate field-effect transistors

According to Sah's theory (1964) of excess noise in junction-gate field effect transistors the equivalent saturated diode current Ieq of the drain noise should vary as Vd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5/2</sup> below saturation (where Vd is the drain to source voltage) and rapidly turn over into its saturated value at saturation. Experimentally, one finds in many units that Ieqa ries as Vd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5/2</sup> at low currents, roughly as Vd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3/2</sup> at intermediate currents and that IeQ gradually turns over into its saturated value at saturation. We shall see that this behavior can be explained by field-dependent mobility effects.

Carrier mobility and current saturation in the MOS transistor

Consideration is given to the phenomenon of saturation of drain current in the insulated-gate field-effect transistor. The effect of limited carrier drift velocity in the transistor channel is evaluated, and a quantitative theory developed to predict in such a region of current flow. It is shown that the paradox of complete pinch-off at the drain need not necessarily be resolved by postulating a limiting carrier velocity to explain drain-current saturation. It is found that saturation of drain current may be meaningfully ascribed to saturation of carrier velocity only for special cases, such as n -type germanium transistors, and, in addition, this current saturation is only to first order. Detailed consideration is given to of drain-current saturation, and the physical mechanisms controlling saturation drain resistance and voltage gain are discussed and compared. It is proposed that for high-resistivity substrates, electrostatic drain to channel feedback is dominant, whereas for low-resistivity substrates modulation of the length of the space-charge region at the drain is dominant. Experimental evidence is presented for silicon metal-oxide-semiconductor (MOS) transistors which indicates that the effect of the channel mobile charge on the field distribution in the drain space-charge region is negligible as compared to several sources of fixed charge. Based on this it is concluded that carrier mobility within this region does not play a significant role in determining device characteristics. Brief consideration is also given to the constant-carrier-mobility approximation for in the source region of the channel. It is found that due to fairly strong surface region scattering in silicon units, the effects of field dependent mobility are reduced substantially below that observed in single-crystal bulk material. The constant-mobility approximation is also experimentally confirmed by the excellent square-law behavior demonstrated by these transistors.