Abrupt Heterojunction Bipolar Transistors Research Articles

The impact of interface charges at the heterojunction on the carriers transport in abrupt InP/InGaAs heterojunction bipolar transistor

The carriers transport at the base-emitter interface of abrupt heterojunction bipolar transistors (HBTs) is controlled by thermionic emission and tunneling, which depends on the form and height of the energy barriers. The interface charges at the heterojunction disturb the energy barriers, thus bringing about the change of the electrical characteristics of HBT. Based on thermionic-field-diffusion model which combines the drift-diffusion transport in the bulk of the transistor with the thermionic emission and tunneling at the interface, a conclusion can be drawn that the positive interface charges can improve the electrical characteristics of abrupt InP/InGaAs HBT, while the negative interface charges deteriorate the devices.

Comparative investigation of InP/InGaAs abrupt, setback, and heterostructure-emitter heterojunction bipolar transistors

In this paper, the characteristics of InP/InGaAs abrupt, setback, and heterostructure-emitter heterojunction bipolar transistors (HBTs) are comparatively investigated by twodimensional simulation analysis. In the setback (heterostructure-emitter) HBT, a thin 50 A undoped In0.53Ga0.47As (n-In0.53Ga0.47As) layer is inserted between n-InP emitter and p+-InGaAs base layers to lower the energy band at emitter side for decreasing the collector-emitter offset voltage. The simulated results exhibits that the abrupt HBT has the largest current gain, the largest collector-emitter offset voltage, and the smallest unity gain cutoff frequency. While, the setback and heterostructure-emitter HBTs exhibit the smallest current gain and offcet voltage, respectively. Consequentially, the demonstration and comparison of the three-type HBTs provide a promise for design in circuit applications.

Effect of heavy boron doping on the electrical characteristics of SiGe HBTs

A modified version of the Jain–Roulston (J–R) model is developed that takes into account the compensation effect of B to Ge in strained SiGe layers for the first time. Based on this new model, the distribution of the bandgap narrowing (BGN) between the conduction and valence bands is calculated. The influence of this distribution on the transport characteristics of abrupt SiGe heterojunction bipolar transistors (HBTs) has been further considered by using the tunnelling and thermionic emission mechanisms instead of the drift and diffusion mechanisms at the interfaces where discontinuities in energy levels appear. The results show that our modified J–R model better fits the experimental values, and the energy band structure has a strong influence on electrical characteristics.

Base Charge Dynamics of Abrupt Base–Emitter Junction HBTs and Its Representation in Transistor Models

Base charge dynamics in abrupt heterojunction bipolar transistors (HBTs) are determined by the thermionic/field emission boundary condition. As a result, their small and large signal models differ from those of bipolar junction transistors (BJTs), which obey the Shockley boundary condition. There is comparison of the base charge dynamics in the abrupt HBT to the BJT case and derive the small and large signal models of the abrupt HBT

Monte Carlo modelling of abrupt InP/InGaAs HBTs

AbstractIn this paper a Monte Carlo simulator which is focused on the modelling of abrupt heterojunction bipolar transistors (HBTs) is described. In addition, simulation results of an abrupt InP/InGaAs HBT are analysed in order to describe the behaviour of this kind of device, and are compared with experimental data.A distinctive feature of InP/InGaAs HBTs is their spike‐like discontinuity in the Ec level at the emitter–base heterojunction interface. The transport of electrons through this potential barrier can be described by the Schrödinger's equation. Therefore, in our simulator we have consistently included the numerical solution of this equation in the iterative Monte Carlo procedure.The simulation results of the transistor include the density of electrons along the device and their velocity, kinetic energy and occupation of the upper conduction sub‐bands. It is shown that the electrons in the base region and in the base–collector depletion region are far from thermal equilibrium, and therefore the drift–diffusion transport model is no longer applicable.Finally, the experimental and simulated Gummel plots JC(VBE) and JB(VBE) are compared in the bias range of common operation of these transistors, showing a good data agreement. Copyright © 2003 John Wiley & Sons, Ltd.

A self-consistent model for temperature and current distribution in abrupt heterojunction bipolar transistors

The thermal behavior of abrupt heterojunction bipolar transistors (HBTs) has been studied by coupling the thermionic-field-emission injection mechanism at the emitter-base heterojunction with the thermal-electric feedback phenomenon. The exact quantum mechanical injection mechanism rather than the semiclassical WKB approximation is used in the present calculation to self-consistently calculate the thermionic and tunneling components of current. Moreover, the total current and temperature are self-consistently evaluated by testing the convergence on both current and temperature. The calculation shows correctly that the degree of the partitioning between the thermionic and tunneling components are bias- as well as temperature-dependent. It is shown that even a single emitter finger can have a highly nonuniform temperature and current distribution across it, leading to the current collapse phenomenon. At high power levels, this may give rise to a current collapse phenomenon similar to that observed for the multifinger HBTs.

Ultrahigh Performance Staggered Lineup (“Type-II”) InP/GaAsSb/InP NpN Double Heterojunction Bipolar Transistors

We study the performance of staggered lineup NpN InP/GaAsSb/InP abrupt double heterojunction bipolar transistors (DHBTs) intended for ultrahigh speed applications. With a peak fT of 305 GHz (and fMAX=300 GHz), InP/GaAsSb/InP DHBTs are currently the fastest bipolar transistors ever implemented, and as such may challenge sub-100 nm gate InP HEMTs for > 40 Gb/s applications: previously published criteria suggest current device performance should be suitable for 80–100 Gb/s OEICs. InP/GaAsSb/InP DHBTs feature high breakdown voltages and low offset and knee voltages, and extremely high current drive levels enabled by the lack of collector current blocking at the staggered base/collector junction. InP/GaAsSb/InP DHBTs also feature important manufacturability advantages because the structure is entirely made up of uniform composition binary and ternary alloy layers.

300 GHz InP/GaAsSb/InP double HBTs with high current capability and BV/sub CEO/>6 V

We report MOCVD-grown NpN InP/GaAsSb/InP abrupt double heterojunction bipolar transistors (DHBTs) with simultaneous values of f/sub T/ and f/sub MAX/ as high as 300 GHz for J/sub C/=410 kA/cm/sup 2/ at V/sub CE/=1.8 V. The devices maintain outstanding dynamic performances over a wide range of biases including the saturation mode. In this material system the p+ GaAsSb base conduction band edge lies 0.10-0.15 eV above the InP collector conduction band, thus favoring the use of nongraded base-collector designs without the current blocking effect found in conventional InP/GaInAs-based DHBTs. The 2000 /spl Aring/ InP collector provides good breakdown voltages of BV/sub CEO/=6 V and a small collector signal delay of /spl sim/0.23 ps. Thinner 1500 /spl Aring/ collectors allow operation at still higher currents with f/sub T/>200 GHz at J/sub C/=650 kA/cm/sup 2/.

Parallel finite element method to solve the 3D Poisson equation and its application to abrupt heterojunction bipolar transistors

In this work we present a parallel solver for the Poisson equation for 3D abrupt heterojunction bipolar transistors (HBT). Three-dimensional simulation is essential for studying devices of small geometry as in the case we have studied. We have used an unstructured tetrahedral mesh and we have applied the finite method element (FEM), making a specific formulation for the nodes located on the interface of the regions with different characteristics. For HBT devices, it is necessary to take into account that on both sides of the interface between the different regions exist materials with different properties. Our formulation implies situating pairs of nodes in the same physical positions of the interface, associating each nodes to a region of the HBT. This way, the effects due to thermionic emission and the tunnel effect may be simulated when the Poisson and the electron and hole equations are solved in an abrupt HBT. We have applied domain decomposition methods to solve the associate linear systems. This code has been implemented for distributed memory multicomputers, making use of a message passing standard library, MPI. Copyright © 2000 John Wiley & Sons, Ltd.

Parallel finite element method to solve the 3D Poisson equation and its application to abrupt heterojunction bipolar transistors

Parallel finite element method to solve the 3D Poisson equation and its application to abrupt heterojunction bipolar transistors

The importance of bandgap narrowing distribution between the conduction and valence bands in abrupt HBTs

Abrupt heterojunction bipolar transistors (HBTs) show interfaces where discontinuities in the energy levels appear. Currents through these interfaces are controlled by tunneling and thermionic emission. The values of these currents depend on the form and height of the energy barriers, which are disturbed by the heavy doping effects on semiconductor energy band structure. In this work, the real bandgap narrowing is distributed between the conduction and valence bands according to Jain-Roulston model, and its effect on the base and collector currents of Si/SiGe and InP/InGaAs HBTs is analyzed. This analysis is carried out through a numerical model which combines the drift-diffusion transport in the bulk of transistor with the thermionic emission and tunneling at the base-emitter interface, and an empirically determined surface recombination current.

Trends in the emitter-base bias dependence of the average base transit time through abrupt heterojunction bipolar transistors

The average base transit time is computed using a current impulse response technique for three typical abrupt Npn heterojunction bipolar transistors as a function of the emitter-base bias, VBE. This technique is based on a hybrid model of carrier transport incorporating a quantum-mechanical analysis of carrier injection at the emitter-base junction and a Monte Carlo analysis of base transport. For typical AlGaAs/GaAs and InP/InGaAs structures, the base transit time first increases with VBE, reaches a maximum, and then decreases towards a value close to the one predicted using a semi-Maxwellian injection of carriers into the base at an energy equal to the emitter-base conduction band spike. For a typical InAlAs/InGaAs structure, the average base transit time is found to decrease with an increase in VBE. For all structures, we show that there is a correlation between the bias dependence of the average base transit time and the bias dependence of the average number of collisions per carrier (calculated for carriers transmitted across the base).

Thermionic diffusion model for abrupt HBTs including self-heating inside the multilayer nonplanar device structure

A thermo-electrical model which analytically describes the current flow inside abrupt heterojunction bipolar transistors (HBTs) using thermionic diffusion theory was developed. This model calculates numerically the three-dimensional temperature distribution inside the multilayer, nonplanar device structure with the help of a semi-analytical approach. This semi-analytical approach consists of a finite element program computing the heat flow in the mesa structure and an analytical solution of the temperature distribution in the substrate. The influence of the nonplanar and multilayer device structure and the effect of metal airbridges on the HBT performance is determined using this model. The phenomenon of thermal runaway in common emitter configuration is explained.

Influence of quantum-mechanical reflection at the emitter-base spike on the base transit time through abrupt heterojunction bipolar transistors

A model to simulate electron transport through the base region of abrupt heterojunction bipolar transistors has been developed taking into account the finite probability for electrons in the base to tunnel through the emitter-base spike back into the emitter. The average base transit time is calculated as a function of the emitter-base voltage using the impulse response technique. For all biases, the average base transit time is found to be smaller than its value computed while neglecting the finite probability for electrons with energy below the emitter-base spike to tunnel back into the emitter. For the case of an AlGaAs/GaAs structure, the average base transit time is found to increase with the forward emitter-base voltage.

Limit of validity of the thermionic-field-emission treatment of electron injection across emitter-base junctions in abrupt heterojunction bipolar transistors

A hybrid model is developed to simulate electron transport through the emitter-base heterojunction and the base region of abrupt heterojunction bipolar transistors. The energy distribution of the injected electron flux through the emitter-base junction is calculated using a rigorous quantum-mechanical treatment of electron tunneling and thermionic emission across the spike at the emitter-base junction. The results are compared with those predicted by the conventional thermionic-field-emission model. For both models, the electron fluxes injected across the emitter-base junction are used as initial energy distributions in a regional Monte Carlo calculation to model electron transport through the base. The average base transit times are calculated using the impulse response technique as a function of the emitter-base voltage. The differences between the thermionic-field-emission model and the rigorous quantum-mechanical approaches to model electron transport through abrupt heterojunction bipolar transistors are pointed out.

Early voltage in double heterojunction bipolar transistors

The Early voltage for abrupt double heterojunction bipolar transistors (DHBTs) has been calculated by using an effective junction velocity (S/sub c/) at the base-collector heterojunction. S/sub c/ is obtained by self-consistently partitioning thermionic and quantum mechanical tunneling currents. Unlike single heterojunction bipolar transistors (SHBTs), the Early voltage varies very rapidly at low reverse bias and approaches the SHBT-limit at sufficiently high reverse bias. This is attributed to the presence of an energy barrier at the b-c heterojunction.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Cascaded collector current formulations of abrupt heterojunction bipolar transistors and their applications to graded HBTs with base dopant outdiffusion

A cascaded thermionic emission-diffusion current model is presented to analyze the forward and inverted collector current in graded HBTs with base dopant outdiffusion. In addition to a reduction of forward I C in HBTs with dopant diffusion, a bias dependent non-ideality factor of I C is also predicted by the cascaded model. Furthermore, the variation of non-ideality factor observed in HBTs with different amounts of dopant outdiffusion can be explained by the formation of a potential barrier and the dominance of thermionic emission current. Based on the inverted collector current formulation, the ratio of currents measured at two heterojunction biases is proposed as an indicator of the integrity of a graded heterojunction. This is based on the fact that when base dopants diffuse into the graded layer, a potential barrier is formed in the conduction band, whose height can be modulated by heterojunction applied reverse bias, resulting in a bias dependent current. As the base dopant outdiffusion affects collector current uniformity across the wafer and device long term reliability, the design of emitter-base heterojunctions is discussed in the context of aluminum composition and grading length.

Numerical study on the injection performance of AlGaAs/GaAs abrupt emitter heterojunction bipolar transistors

The injection performance of abrupt emitter HBT's and related effects on the device characteristics are studied by taking an Npn Al/sub 0.25/Ga/sub 0.75/As/GaAs/GaAs HBT as an example. In order to take into account the coupled transport phenomena of drift-diffusion and tunneling-emission processes across the abrupt heterojunction in a single coupled formulation, a numerical technique based on the boundary condition approach is employed. Compared to previous numerical investigations relying on either a drift-diffusion or a tunneling-emission scheme, more complete and accurate characterization of abrupt emitter HBT's has been achieved in this study. It is demonstrated that the presence of abrupt discontinuities of the conduction and valence bands at the emitter-base junction brings several different features to the injection efficiency and recombination characteristics of abrupt emitter HBT's compared to graded emitter HBT's. Based on investigations of the emitter doping effects on the current drive capability and device gain, an optimum emitter doping density is determined for a given structure. When the emitter-base p-n junction of the abrupt emitter HBT is slightly displaced with respect to the heterojunction, significant changes in the electrical characteristics are observed. A small displacement of the p-n junction into the narrow bandgap semiconductor is found to be very attractive for the performance optimization of abrupt emitter HBT's. >

Effect of hot-electron injection of high-frequency characteristics of abrupt In/sub 0.52/(Ga/sub 1-x/Al/sub x/)/sub 0.48/As/InGaAs HBT's

The effect of hot-electron injection energy (E/sub i/) into the base on the high-frequency characteristics of In/sub 0/52/(Ga/sub 1-x/Al/sub x/)/sub 0.48/As/InGaAs abrupt heterojunction bipolar transistors (HBTs) is investigated by changing the composition of the emitter. There exists an optimum E/sub i/ at which a maximum current gain cutoff frequency (ft) is obtained. Analysis of hot-electron transport in the base and collector by Monte Carlo simulation is carried out to understand the above phenomenon. The collector transit time ( tau /sub c/) increases with E/sub i/, because electrons with higher energy transfer from the Gamma valley into the upper L and X valleys. At first, the base transit time ( tau /sub b/) decreases with E/sub i/ at the low E/sub i/ region. However, tau /sub b/ does not decrease monotically with E/sub i/, because of the nonparabolicity in the energy-band structure of InGaAs. Consequently, there exists a minimum in the sum of tau /sub b/ and tau /sub c/, in other words a maximum f/sub t/, at an intermediate value of E/sub i/. >

Space-charge region recombination in heterojunction bipolar transistors

A new comprehensive model for space-charge region (SCR) recombination current in abrupt and graded energy gap heterojunction bipolar transistors (HBTs) is derived. It is shown that if a spike is present in one of the bands at the heterojunction interface, the SCR recombination current becomes interrelated with the collector current. A previously proposed charge control model for the HBT is modified to include the SCR recombination current. The model is used to study SCR recombination characteristics in HBTs. >