Emitter Region Research Articles

An improved electron current model of AlGaAs/GaAs HBTs including self-heating effects

An improved electron current model of graded heterojunction AlGaAs/GaAs HBTs is presented to account for the effect of varied electron quasi-Fermi level from the emitter region to the base region, the base push-out effect and the self-heating effect. For the self-heating effect, two kinds of thermal boundary conditions in one dimension are included, which are heat insulation at the emitter contact and fixed temperature at the collector contact and the reverse case. In each boundary condition, the more accurate b-e junction temperature model is established, and incorporated into the electron. current formula to form the combined model. A comparison of this model with the numerical simulated results from the basic thermoelectric coupled equations is made, and excellent agreements are obtained.

Energy band structure parameters and their data, derived from the measurements of minority carrier current density in heavily doped emitters of silicon devices

In the n(p)-type heavily doped emitter region (HDER) of silicon devices, at room temperature, we have investigated the minority-carrier lifetime and the energy band structure parameters such as band-gap narrowing, apparent band-gap narrowing, unperturbed Fermi energy shift, optical gap, and reduced interacting density-of-majority conduction (valence) band states effective mass. As used in our previous paper, the present treatment is also based on the two assumptions for minority-carrier transport parameters. Gaussian impurity density profile, and accurate expression for minority-carrier mobility. Our empirical models for minority-carrier lifetime and band-gap narrowing are proposed and determined such that their curves versus the impurity concentration lie in between 3heir existing experimental data. Then, from a conjunction between electrical and optical phenomena, it is found that our theoretical values of such the energy band structure parameters are in good accordance with our own corresponding data, derived from the measurements of minority-carrier saturation current density in the n(p)-type HDER of silicon devices.

Lightly doped emitter HBT for low-power circuits

We report an approach to reduce the base-emitter capacitance in AlGaAs-GaAs heterojunction bipolar transistors (HBT's) by adding a lightly doped emitter (LDE) region together with appropriate planar (/spl delta/) doping region to a conventional base-emitter junction. This improves both the f/sub t/ and /spl beta/ for low collector current density (J/sub c/) operation while preserving the high peak f/sub t/ at high J/sub c/. When applied to a current mode logic 128/129 programmable prescaler, the LDE HBT results in a reduction in power dissipation and improved bandwidth without any circuit modifications.

Low-frequency noise of the quasi-neutral emitter region of bipolar transistors

A method is proposed for measuring the low-frequency resistance fluctuations of the quasi-neutral region of bipolar transistors and the noise properties of KT3102 transistors are investigated.

Electroluminescence spectroscopy of intervalley scattering and hot-hole transport in a GaAs/AlxGa1-xAs tunneling structure.

We have carried out an investigation by electroluminescence (EL) spectroscopy of hot-electron and -hole transport in a p-i-n GaAs/${\mathrm{Al}}_{0.35}$${\mathrm{Ga}}_{0.65}$As single-barrier tunneling structure at high electric fields (\ensuremath{\sim}150 kV ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). We observe EL arising from hot electrons in the p-type collector which reenter the \ensuremath{\Gamma} conduction-band valley from the L minimum. Analysis of the EL spectra provides an estimate of the relative \ensuremath{\Gamma}-L scattering rates by longitudinal and transverse phonon emission. In addition we observe EL due to recombination of hot holes injected into the spin-orbit split-off valence band of the n-type emitter region. Measurement of the pressure dependence of the EL spectra permits unambiguous identification of these spectral features. \textcopyright{} 1996 The American Physical Society.

Highly sensitive mangetotransistor with combined phenomena of Hall effect and emitter injection modulation operated in the saturation mode

We have designed and fabricated a highly sensitive magnetotransistor which employs the emitter region as a Hall plate for inducing a Hall voltage across the emitter. The induced Hall voltage modulates the emitter-base junction bias on both sides of the emitter so that a large collector-current difference results. The specially designed p + ring that surrounds the emitter enhances accumulation of the drifted electrons in the emitter and thus increases the Hall voltage. A relative sensitivity of 240 T −1 is measured by operating the device in the saturation mode

On the Estimation of Oxide Layer Breaking in a PET due to Annealing

A method for estimation of the percentage of interfacial oxide layer break-up in the emitter region of a polysilicon emitter transistor from a knowledge of the base saturation current is proposed. Values predicted by this method are compared with experimental results and good agreement is observed.

Compact modeling of the influence of emitter stored charge on the high frequency small signal AC response of bipolar transistors using quasi-static parameters

A model involving only quasi-static parameters is proposed which is capable of modeling the influence of high frequency nonquasi-static stored charge distributions in the neutral emitter region on the small signal high frequency ac response of bipolar transistors. Quasi-static ac model parameters are completely determined from static charge distributions for arbitrary impurity profiles, thereby enabling high frequency behavior to be accurately deduced for realistic device structures without the need for high frequency numerical simulation or measurement. The proposed model provides quantitative information regarding the transition from quasi-static to nonquasi-static behavior of emitter stored charges. An existing equivalent circuit representation that accurately models the influence of the stored base charge, and which is suitable for SPICE simulations, is modified to include the impact of quasi-static charge distributions in the emitter region.

High-temperature DC characteristics of Al/sub x/Ga/sub 0.52-x/In/sub 0.48/P/GaAs heterojunction bipolar transistors grown by metal organic vapor phase epitaxy

A series of Al/sub x/Ga/sub 0.52-x/In/sub 0.48/P/GaAs heterojunction bipolar transistors (HBT's) with x=0 to x=0.52 showed ideality factors close to unity for both base current and collector current and small variation in gain with temperature up to at least T=623 K across the whole range of x composition. Hole current injection from the base into the emitter in these devices was shown to be negligible. The current gain, /spl beta/, which is temperature insensitive was thought to be limited by bulk base recombination for x/spl les/0.3 and recombination at the graded emitter region for x>0.3. The optimum emitter composition (highest /spl beta/, and good /spl beta/ stability with collector current and temperature) was found to be x=0.18-0.30. Useful transistor action with very high gain and output resistance is possible up to at least T=623 K, limited only by the thermal performance of the unoptimized ohmic contacts employed in the devices.

Influence of pseudomorphic base-emitter spacer layers on current-induced degradation of beryllium-doped InGaAs/InAlAs heterojunction bipolar transistors

We present in this paper a basis for the design of high performance heterojunction bipolar transistors in which base dopant diffusion can be drastically reduced, or even eliminated. From previous theoretical and experimental studies we have established that the motion of point defects in a semiconductor can be impeded or enhanced by a thin (30-100 /spl Aring/) pseudomorphic layer. The path preferred by the defect will depend on the local strain exerted by it in the lattice and the strain tensor of the host layer. Based on this, we have studied the outdiffusion of Be dopant atoms from the base region of n-p-n In/sub 0.53/Ga/sub 0.47/As/In/sub 0.52/Al/sub 0.48/As heterojunction bipolar transistors (HBT's) during short-term high-current (t=18-24 h, J/sub c/=7/spl times/10/sup 4/ A/cm/sup 2/, T=80/spl deg/C) stress tests. The microwave transistors grown by molecular beam epitaxy have 150-200 /spl Aring/ of lattice-matched, compressively strained, or tensilely strained spacer layers incorporated between the base and emitter layers. Changes are observed in the dc and microwave characteristics of the transistors with lattice-matched and compressively strained spacers, while no changes are recorded in the devices with tensilely strained spacer layer after the current stress test. As expected, the tensiley strained spacer layer is very effective in controlling the outdiffusion of Be dopant atoms, which exert a local strain in the lattice, from the base to the emitter region.

Electron transfer between two coupled quantum wells in a resonant tunneling diode structure

Resonant tunneling effects between the quantum well of a double barrier heterostructure and the accumulation layer of the emitter region are investigated. Numerous singularities in the current-voltage characteristic of a GaAs Al 0.4Ga 0.6As diode, such as a negative differential conductance effect with a current contrast of 15:1, have been found and analyzed by means of self-consistent calculations of conduction band profile and tunneling probabilities. At relatively low bias, charge transfer can be interpreted as direct or scattering-assisted transitions with optical phonon emission, by considering the anti-crossing of the ground states or the energy conservation rules. At higher voltages, very fast tunneling processes at the time scale of relaxation mechanisms (through the second level of the accumulation layer) were demonstrated. Such a hot electron transition with direct evidence of negative differential conductance effect involves a non expected build-up of the resonance as a function of the symmetry of confining potentials in terms of transmission probabilities.

Photoreflectance temperature dependence of graded InAlAs/InGaAs heterojunction bipolar transistor layers

Photoreflectance(PR) spectra of the graded InAlAs/InGaAs heterojunction bipolar transistor layers were investigated at various temperatures between 8 K and 300 K. The energy features of the PR spectra were fitted and identified as band-to-band transitions in the graded layers which were grown by pulsed molecular beam epitaxy (pulsed-MBE) and InGaAs as well as InAlAs layers. The temperature variation of energy gaps can be described by the Varshni and Bose-Einstein equations. A linear variation relationship of band gaps with Al composition (z) was observed and approximated to be E0(z)=0.809+0.769z eV at T=0 K. However, the parameters aB and ΘB derived from the Bose-Einstein expression do not change meaningfully in the whole range of Al composition. From the observed Franz-Keldysh oscillations (FKOs) we have evaluated the built-in dc electric fields in the i-InGaAs collector, i-InGaAs spacer and n-InAlAs emitter regions. The electric fields are in good agreement with the continuity condition of electric displacements in the interfaces between emitter and base.

Development of HBT structure to minimize parasitic elements

Several approaches to decrease the parasitic base-emitter and base-collector capacitances, and parasitic extrinsic base resistance, of III–V HBTs are reviewed. Base-emitter depletion capacitance reduction is achieved through modifications of the epitaxial layer structure to reduce doping in the emitter. A doping dipole is introduced in order to compensate for effects of conduction band offset that can adversely affect current gain and transconductance in AlGaAs GaAs HBTs. Base-collector capacitance reduction is achieved with compensating implants of hydrogen and helium into the collector and subcollector regions (through the base layer), which decrease the extrinsic portion of the capacitance to a fringe component. Finally, to reduce extrinsic base resistance and to allow base-emitter capacitance reduction, a new structure is presented that allows HBT lateral scaling without excessive emitter edge recombination. This structure is based on multiple epitaxial growths, using GaInP, GaAs and InGaAs selective-area regrowth in the emitter region of the HBT. Concepts and initial experimental results for these approaches are discussed.

A simple accurate solution to minority electron injection in the p-type heavily doped emitter region of silicon devices

In the p-type quasi-neutral and heavily doped emitter region of silicon devices at room temperature, with the aid of our accurate empirical models for the apparent band-gap narrowing and minority-electron lifetime, a simple accurate theory of minority-electron (emitter) saturation current density JEO, and effective minority-electron transit time, which are expressed as functions of the surface acceptor density Ns, the electron surface recombination velocity S, and the emitter width W, is derived and discussed. It is found that, under transparent emitter region condition, our theoretical values of JEO(Ns) for S = ∞ and 0.19 μm ≦ W ≦ 0.25 μm are in good agreement with the corresponding ones measured by Cuevas et al., with a precision of the order of 22%. Dans la région d'émitteur très dopé et quasi-neutre de type p des dispositifs à silicium, à la température ambiante, à l'aide de nos expressions empiriques et précises de réduction de la bande interdite et de durée de vie des électrons minoritaires, une théorie précise mais simple de densité de courant-saturation de l'émitteur JEO, et de temps de transit des électrons, exprimés en fonction de la concentration maximale des accepteurs Ns, la vitesse de recombinaison électronique à la surface S, et la largeur de l'émitteur W, est obtenue et discutée. Dans la condition de transparence de la région d'émitteur, pour S = ∞ and 0,19 μm ≦ W ≦ 0,25 μm, nos valeurs théoriques de JEO(Ns) sont en bon accord avec celles mesurées par Cuevas et al., avec une précision de l'ordre de 22%.

Influence of low doped emitter and collector regions on high-frequency performance of SiGe-base HBTs

The high-frequency behavior of a Si/Si 1−xGe x/Si heterojunction bipolar transistor is investigated by means of a two-dimensional device simulation program. A simplified method based on the quasi-static approximation and the well-known formula f max =√ f t 8πR B C c is compared with the more accurate a.c. small-signal analysis in order to estimate the limits of the first approach. For a given Gaussian boron distribution in the base an optimized Ge profile is derived exploiting its influence on the spatial distribution of the transit time. Our main attention is directed to the dependence of f T and f max on the vertical profile in the low doped regions of emitter and collector. The study of the tradeoff between the capacitance of the BC junction and the transit frequency shows that a small capacitance C C at the expense of f T is favorable for obtaining high f max values.

Photoreflectance characterization of graded InAlAs/InGaAs heterojunction bipolar transistor layers

The photoreflectance (PR) spectroscopic technique has been used to investigate the microstructures of the graded InAlAs/InGaAs heterojunction bipolar transistor at room temperature. The energy features of the PR spectrum were fitted and identified as band-to-band transitions in the graded layers which were grown using pulsed molecular beam epitaxy and InGaAs as well as InAlAs layers. A linear variation relationship of band gaps with Al composition z was observed and approximated by Eg=0.737+0.759z eV. From the observed Franz-Keldysh oscillations, we have evaluated the built-in dc electric fields in the i-InGaAs collector and n-InAlAs emitter regions. These electric fields are in good agreement with the continuity condition of electric displacements in the interfaces.

Γ-X Mixing studied by electroluminescence spectroscopy under hydrostatic pressure in a GaAs/AlAs p-i-n superlattice resonant tunnelling device

Electroluminescence measurements under hydrostatic pressure have been performed to study the Γ-X mixing in GaAs AlAs p-i-n double barrier resonant tunnelling structures incorporating superlattice energy filters in the emitter and collector regions. For a sample with a 5 nm quantum well, an anti-crossing with the lowest confined X-states in the AlAs barriers is observed under hydrostatic pressure. The observed anti-crossing agrees well with the expected position and pressure coefficient of the confined X-states. A sample with a narrower 3.1 nm quantum well exhibits a recombination which is higher in energy than the lowest confined X-states in the AlAs barrier and decreases with an anomalously large pressure coefficient.

Direct measurement of minority carrier diffusion length in planar devices

The minority carrier diffusion length in a semiconductor device is a very important parameter for its design, understanding or modeling; usually this parameter is taken from graphs or tables made for uniform doping. Unfortunately in a real device the doping is rarely uniform and usually has a specific doping distribution (error function, Gaussian, etc…); the diffusion length for these distributions is difficult to evaluate, especially for heavy doping. In this paper we show how this parameter can be measured directly and precisely on localized regions of a device with any doping distribution. The method used is based on the Electron Beam Induced (EBIC) technique; we expose the experimental set up, the principle and discuss the conditions of the validity of the measurements. We have tried to measure the minority carrier diffusion length in the base region of two bipolar transistors with a Gaussian doping distribution and in an emitter region of an npn transistor with a heavy doping, error function-type distribution. The value of the minority carrier diffusion length found is compared with those available in the literature and the validity of the measurements is discussed.

Investigation of GaAs homojunction bipolar transistor with delta doping emitter structure

A GaAs homojunction bipolar transistor with a delta doping emitter structure is proposed and demonstrated. The proposed device makes use of a delta doping structure inducing a triangular barrier for minority carrier confinement, resulting in a high emitter injection efficiency. Based on the minority carrier transport in the bulk emitter region with drift-diffusion mechanisms, and in the triangular barrier region with tunnelling and thermionic-emission mechanisms, an analytical derivation of current-voltage characteristics, including the effect of bandgap shrinkage, was obtained. The calculated results show that the triangular barrier is the key parameter in determining the electrical properties. In addition, due to the absence of heterojunction, the proposed device exhibits more nearly constant current gain with collector current than for the AlGaAs/GaAs heterojunction bipolar transistor (HBT). The proposed device, grown by molecular beam epitaxy, shows a differential current gain of 13 and an offset voltage of 60 mV at a base-to-emitter doping ratio of 10. The offset voltage is attributed mostly to the geometric limits. With a simple chemical treatment of Na/sub 2/S/spl middot/9H/sub 2/O, the differential current gain is enhanced to be 16 due to the reduced surface recombination. Theory and experiment indicate the potential application of the proposed device.

Charge-injection theory of bipolar junction transistors

A physical theory of the bipolar junction transistor which provides closed-form solutions for current/voltage relations for generalized bias conditions is introduced. Included are the new concepts of emitter and collector collection efficiency. Both emitter and collector regions are treated symmetrically to allow for accurate treatment of operation in both the saturation region and the forward-active region, as well as the reverse-active region. There are six components each of emitter and collector current, resulting from inclusion of emitter injection efficiency, surface recombination at the pn junction peripheries, and bulk base recombination as well as finite minority-carrier collection efficiency. Direct comparison between theory and experiment over the entire transistor operating range from deep saturation where IC=0 continuously into the forward-active region where VCE is much greater than the thermal voltage VT shows excellent agreement.