Charge Control Analysis Research Articles

Acoustoelectric Amplifier Model Using Coupling of Modes and Charge Control Analysis.

This article develops a novel theoretical and modeling approach for a surface acoustic wave (SAW) acoustoelectric amplifier applicable to thin-film overlay resistive or semiconductor material that is within or outside the acoustic path. The theory is developed for a direct-coupled SAW and a thin-film resistive interaction layer. The analysis uses the combination of coupling of modes and charge control analysis to develop a complete model describing the small-signal operation and the large-signal saturation effect. The model is developed for the case of direct coupling of thin films in the propagation path or coupling electrodes either inside or outside the propagation path. The approach describes the phenomenological physics and amplifier's key parameters. Predictions are compared to previously published experimental results on lithium niobate using a monolayer graphene film that yielded continuous-wave operation and a net terminal gain.

Base transport factor and frequency response of transistor lasers

We report on a charge control analysis that relates the characteristic time constants of a three-port laser made of a quantum-well (QW) heterojunction bipolar transistor (HBT) to the electronic gain of the device. For this purpose, we take into account the linear variation of the base transport factor α with the HBT base current. Our approach enables us to obtain QW capture time, base recombination lifetime, and base transit time in terms of the experimental values of base current and of the transistor laser (TL) design parameters such as base width, QW width, and QW location. Whereas the base recombination lifetime is calculated to be a fraction of a nanosecond, the QW capture time is found to be of the order of a picosecond or less. The time constants obtained from our model are used to successfully reproduce the TL experimental optical frequency response.

A LED Driver With Switched Capacitor

This paper proposes a switched capacitor-based converter to drive high-power light-emitting diodes (LEDs). In contrast to conventional constant current dc drivers, the current pulse is provided by a switched capacitor. The proposed approach differs from the traditional switched-capacitor-based one, because it uses a small inductor to improve the switching behavior of the converter. Based on the charge control analysis, the effects of switching devices on the proposed converter are evidenced and evaluated. A 6 W, 24 V laboratory prototype has been implemented, while experimental results are presented and discussed to demonstrate the technical feasibility of the proposed converter applied as LED driver.

Analytical I–V model of SOI and SON MOSFETs: a comparative analysis

A generalised three-interface compact capacitive threshold voltage model for horizontal silicon-on-insulator/silicon-on-nothing (SOI/SON) MOSFET has been developed. The model includes different threshold voltage-modifying short-channel phenomena like fringing field, junction-induced 2D-effects, etc. Based on the threshold voltage model, an analytical current voltage model is formulated from the basic charge control analysis of MOSFET. In order to provide a better explanation to various observations and applicable to short-channel SOI and SON structures, the present current voltage model includes the effect of carrier velocity saturation and channel length modulation. Identical structures for both the devices, SOI and SON, are considered but for SON MOSFET, the buried oxide layer is replaced by air. The performance of the two devices are studied and compared in terms of threshold voltage roll-off, subthreshold slope, drain current and drain conductance. The SON MOSFET technology is found to offer devices with further scalability and enhanced performance in terms of threshold voltage roll-off, sub-threshold slope and greater current derivability, thereby providing scope for further miniaturisation of devices and much better performance improvement.

Estimated threshold base current and light power output of a transistor laser with InGaAs quantum well in GaAs base

We have solved the continuity equation for electrons in the base of an InGaP–GaAs–GaAs heterojunction bipolar transistor laser (TL) in which the position of an InGaAs quantum well (QW) in GaAs base is variable. The injected minority carrier is related to the two-dimensional carrier in QW via virtual states (VSs). The values for optical gain in the QW are obtained by considering subband energies and envelope functions in presence of strain, polarization dependent momentum matrix element and Lorentzian lineshape. Relating the gain with threshold current and the latter with base current via VS current, the threshold base current and power output from the TL are estimated. Good agreement between the calculated and the experimental threshold base currents is obtained and the match for light output power is satisfactory within experimental uncertainty. Our calculated charge distribution in the base shows similar behaviour as in the charge control analysis of the experimental data.

Microwave circuit model of the three-port transistor laser

Based on an earlier charge control analysis, we have constructed a microwave circuit model of a three-port quantum-well (QW) transistor laser (TL) by extending Kirchhoff’s law to include electron-photon interaction, to yield an electrical-optical form of Kirchhoff’s law. The TL circuit model includes both intrinsic device elements and extrinsic parasitic elements, and fits accurately measured microwave S-parameters upto 20 GHz and matches also measured eye-diagram data up to 13 Gb/s (equipment-limited). The TL model yields both electrical and optical device parameters as well as physical quantities such as QW charge density, nQW∼1016 cm−3, which is useful in the analysis of the device physics of TL operation. The low density indicates that the base QW charge level is not as important as the current driving the QW and supplying electron-hole recombination, and implies that the quasi-Fermi level is discontinuous in the TL base. The model is used to simulate a directly modulated TL up to 40 Gb/s, for example, a TL employed in an optical communication link.

Physics of base charge dynamics in the three port transistor laser

The base charge dynamics of the quantum-well (QW) transistor laser (TL) is analyzed by constructing a model based on earlier incomplete charge control analysis. We extend Kirchhoff’s Law to include electrical and consistently “optical” elements. The model yields, via microwave measurements (and resolvable picosecond responses), physical quantities associated with TL base-charging, permitting the extraction of a base QW charge density, nQW=2×1016 cm−3, consistent with calculation by current continuity. The low density implies quasi-Fermi level discontinuity in the TL base, and indicates that the base QW charge level is not as important as the current driving the QW and supplying electron-hole recombination.

Charge control analysis of transistor laser operation

The authors report the calculation of the minority carrier distribution in the base region of the transistor laser (TL) employing the relevant continuity equations and experimental carrier lifetimes, spontaneous and stimulated, extracted from the transistor I-V characteristics. A charge control model of the TL is developed, consistent with the short recombination lifetime of the quantum-well base (which competes with the short emitter-to-collector transit time). The absence of carrier-photon resonance of a TL is demonstrated with the 3dB bandwidth (IB∕IB,th=1.5) estimated to be 30GHz for a 400μm long laser cavity length and 70GHz for a 150μm cavity.

Design analysis and circuit enhancements for high-speed bipolar flip-flops

The design of high-speed current mode logic latches is discussed, using analytical expressions for delay. An open circuit time constant method is utilized throughout this paper, though similar results were obtained from a charge control analysis. Emphasis is placed on the variables that are under the control of the circuit designer, as opposed to the device designer. Circuit delay is calculated with respect to device area, current density, amplitude, and a keep-alive current. In particular, the keep-alive current gives the circuit designer control over the average transconductance of switching transistors, independent of their bias currents. The cost of the keep-alive current is the loss of output amplitude. The effects of transmission lines and peaking inductors are discussed in a qualitative manner. Latch designs were tested with static divide-by-two frequency dividers. Results of several dividers (both SiGe and InP) are shown and compared with the theory.

An analytical parasitic resistance dependent Id– Vd model for planar doped InAlAs/InGaAs/InP HEMT using non-linear charge control analysis

An analytical parasitic resistance dependent model for the current voltage characteristics for InAlAs/InGaAs/InP HEMT is proposed. The model uses a new polynomial dependence of sheet carrier concentration on gate voltage to calculate I d– V d characteristics and has been extended to obtain transconductance, output conductance and cut-off frequency of the device. A maximum cut-off frequency of 83 and 175 GHz was obtained for channel length of 0.25 and 0.1 μm, respectively. Close agreement with published results confirms the validity of our approach.

Charge transport in low-dimensional nitride semiconductor heterostructures

Low-dimensional conducting channels in high electron mobility transistors (HEMTs) based on AlGaN/GaN heterostructures have been modeled and the results are described in this paper. The Schroedinger's equation and the Poisson's equation have been solved self-consistently in order to obtain a relationship between the sheet carrier density and the applied gate voltage. The relationship is treated using a non-linear exponential fit that enables a more accurate analysis of the transport saturation region compared to other models used hitherto. The current–voltage ( I– V) characteristics have then been determined by a charge control analysis that utilizes the exponential charge–potential relationship. This identifies the saturation point of channel conduction without parameters extracted from experiments. The intrinsic non-linearity of transport identified by the simulation explains why such devices do not have a singular channel “pinch-off” voltage.

Analysis of bipolar junction transistors with a Gaussian base-dopant impurity-concentration profile

A method for a quantitative charge-control analysis of bipolar base-junction transistors with a Gaussian dopant impurity-concentration profile is demonstrated. Analytical expressions for the base transit time are given for two different Gaussian impurity-concentration profiles with the peak concentration at the edge, and within the quasi-neutral base layer. It is also shown that approximating the Gaussian profile by a simple exponential profile results in only an insignificant error in the charge-control analysis.

Current-voltage characteristics and field distribution of pseudomorphic (AlGaAs/InGaAs) modulation-doped field-effect transistor for microwave circuit applications

A two-dimensional analytical model for the current–voltage characteristics of a pseudomorphic AlGaAs/InGaAs modulation-doped field-effect transistor is developed using charge-control analysis for its microwave circuit applications. The two-dimensional potential, field, electron concentration, and velocity profiles are expressed explicitly in the saturation region. The results so obtained are compared with the experimental data, and show excellent agreement, thereby proving the validity of the model. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 24: 407–412, 2000.

Capacitance-voltage characteristics and cutoff frequency of pseudomorphic (AlGaAs/InGaAs) modulation-doped field-effect transistor for microwave and high-speed circuit applications

A two-dimensional analytical model for the capacitance–voltage characteristics of a pseudomorphic AlGaAs/InGaAs modulation-doped field-effect transistor is developed using charge-control analysis for its microwave frequency applications. The model includes the effect of various fringing field capacitances, and a cutoff frequency of 94.8 GHz is obtained for an L=0.25 μm device. The effect of traps has been included, which decreases the cutoff frequency. The results so obtained are compared with experimental data, and show excellent agreement, thereby proving the validity of the model. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 23: 312–318, 1999.

Quantitative analysis of Si/Ge x Si 1– x /Siheterojunction bipolar transistors with linearly graded Ge profile

A new experimental methodology for the analysis of Si/GexSi1–x/Si HBT yields quantitative values for the important transistor parameters, such as the energy gap reduction and the built-in electric field due to Ge, and the minority-carrier base diffusion length, lifetime and transit time. The method also enables a charge-control analysis of the base current to be made. A demonstration is presented for a GeSi heterojunction bipolar transistor with a linearly graded Ge profile in the base. The methodology can be extended for arbitrary material composition and doping profile in the base.

Analysis of self-excited electronic ballasts using BJTs/MOSFETs as switching devices

Self-excited electronic ballasts are analysed and compared using the same circuit configuration but two different switching devices, BJTs and MOSFETs. These two different charge-driven switching devices normally result in different circuit behaviour and design. According to the operating points of saturable driving transformers and switching devices, a complete circuit operation is divided into six stages. Based on the charge control analysis, the effects of switching devices on the self-excited ballasts are discriminated and evaluated. The analysis reveals that BJTs are more suitable than MOSFETs in this transformer-driven self-excited ballast circuit, and it also provides a clear insight into the utilisation of switching devices.

MODFET versus MESFET: the capacitance argument

In this paper a detailed charge control analysis is presented for a MESFET and a MODFET to show how the various capacitances associated with the region under the gate contact influence device performance. Where necessary, an exact description of electron confinement is included in the solution scheme by solving the effective mass Schrodinger equation for a single band and within a self-consistent framework. In the case of the MESFET, the analysis shows that the filling of donor impurity states, which are coincident in real space with the conducting channel, gives rise to a large parasitic capacitance which severely degrades both the intrinsic transconductance and cutoff frequency performance. In the case of the MODFET, the separation of the donor impurity level in real space from the conducting channel, and in energy from the Fermi-level, results in a low parasitic capacitance over the whole operating region of the device. Thus it is shown that the MODFET appears to be more suited to millimeter-wave applications, regardless of any electron transport effects.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A quantum gate current model

Gate current plays a critical role in the maximum gate voltage swing, the maximum transconductance, and as a direct consequence, the maximum noise margin of digital devices. A quantum gate current model based on a charge-control analysis and the WKB approximation is presented for heterostructure field effect transistors (HFETs). Both the tunneling and thermionic emission gate currents are included and treated in a unified way in this model. Along with the general model itself, a specific calculation on a semiconductor insulator semiconductor (SISFET) is presented. The model shows that for advanced HFETs with thin hetero-barrier layers, the tunneling component dominates over the thermionic component and essentially constitutes the gate current. >

Characterization of Ultrahigh-Speed Pseudomorphic InGaAs/AlGaAs Inverted High Electron Mobility Transistors

A detailed characterization of ultrahigh-speed pseudomorphic InGaAs/AlGaAs inverted high electron mobility transistors (pseudomorphic I-HEMT's) is reported. Charge control analysis indicates that the pseudomorphic I-HEMT accomplishes improved transconductance (gm) and reduced short channel effects due to higher concentration and excellent confinement of two-dimensional electron gas (2DEG) in the InGaAs channel. The 0.2 µm gate pseudomorphic I-HEMT's reported here exhibit superior DC and RF performances, i.e., maximum transconductance (gmmax) of 565 mS/mm, cutoff frequency (fT) of 110 GHz and propagation delay times (τpd's) of 6.6 ps/gate at R.T. and 4.9 ps/gate at 120 K. Delay time analysis reveals the reduction of both the electron transit time (τtransit) and the channel charging time (τchannel) in the pseudomorphic I-HEMT, which are significant factors in the ultrahigh-speed performance. Higher electron saturation velocity (υs:2.1×107 cm/s) and improved gm are considered to contribute to the reduction in τtransit and τchannel.

Charge-control analysis of collector transit time for (AlGa)As/GaAs HBTs under a high injection condition

Charge-control analysis of collector transit time ( tau /sub C/) for (AlGa)As/GaAs n-p/sup +/-n heterojunction bipolar transistors has been carried out under the restriction of low-frequency approximation to explain the remarkable tau /sub C/ reduction at the onset of the Kirk effect. The fundamental idea of the model is that the increment of the current density must depend not only on the increment of the carrier density but also on the increment of the carrier drift velocity in the collector space-charge region. The obtained model well explains the previous Monte Carlo results, thus indicating the validity of the above idea as well as the possibility that the actual tau /sub C/ is by far smaller than that estimated from the conventional model. It also leads to a concept for the reduction of transit times, i.e. that an increasing rate of the electron overshoot velocity is as essential for the reduction of tau /sub C/ as the magnitude of the overshoot velocity itself. >