Emitter-base Space-charge Region Research Articles

A comprehensive semi-analytical model of the polysilicon emitter contact in bipolar transistors

This paper aims to develop a comprehensive physical model for a bipolar transistor’s polysilicon-contacted emitter. Poisson’s equation is solved numerically in the emitter–base space charge region to specify the boundary conditions and the excess minority carriers injected from base to emitter. The continuity and current transport equations are also solved numerically to obtain the minority carrier current in the emitter region. The polysilicon along with the interface layers is modeled by using an effective value for the lifetime. In this model, all the technological parameters of different emitter regions are taken into consideration. Also, the heavy doping effects and the built-in electric field in the shallow non-homogeneous doped single crystalline layer are also included. Such a systematic model does not exist in the literature. The results of the analytical model are numerically evaluated using MATLAB. The trends provided by the model are validated against published experimental results whenever possible and found to be in good agreement with them.

Improved Model for Increased Surface Recombination Current in Irradiated Bipolar Junction Transistors

Current gain degradation in irradiated bipolar junction transistors is primarily due to excess base current caused by enhanced carrier recombination in the emitter-base space-charge region (SCR). Radiation-induced traps at the interface between silicon and the bipolar base oxide facilitate the recombination process primarily above the sensitive emitter-base junction. This leads to an increase in surface recombination current in the SCR, which is a non-ideal component of the BJT's base current characteristic under active bias conditions. In this paper, we derive a precise analytical model for surface recombination current that captures bias dependencies typically omitted from traditional models. This improved model is validated by comparisons to these traditional approaches.

Transconductance Degradation in Near-THz InP Double-Heterojunction Bipolar Transistors

We examine the relationship between transconductance gm and emitter current density JE for InP/InGaAs/InP abrupt emitter-base (EB) double-heterojunction bipolar transistors operating at high JE. High JE is needed to increase gm for reduced C/gm delays. We observe a significant degradation in measured gm below qIE/kT with increased JE. This degradation primarily results from the Fermi-Dirac statistics governing current injection at high current densities and from quantum-mechanical reflection at the EB junction arising from changes in the electron effective mass and in the conduction band potential. Transconductance is further reduced by gradients in the quasi-Fermi level in the EB space-charge region and by modulation of the heterointerface energy barrier by the applied bias.

Temperature dependence of the current gain in power 4H-SiC NPN BJTs

For 1-kV 30-A 4H-SiC epitaxial emitter n-p-n bipolar junction transistors, the dependences of the common-emitter current gain /spl beta//sub CE/ on the collector current I/sub C/ were measured at elevated temperatures. The collector-emitter voltage was fixed (at 100 V) to provide an active operation mode at all collector currents varying in a wide range from 150 mA to 40 A (current densities of 24-6350 A/cm/sup 2/). The maximum current gain was measured to be /spl beta//sub CEmax/=40(I/sub C/=7 A) at room temperature and /spl beta//sub CEmax/=32(I/sub C/=10 A) at 250/spl deg/C. The /spl beta//sub CE/-I/sub C/ dependences were simulated in terms of a model that takes into account the main processes affecting the current gain: 1) recombination in the emitter-base space charge region; 2) surface recombination; 3) crowding of the emitter current; 4) decrease in the emitter-injection coefficient at high-level injection; and 5) ionization of deep acceptors. The minority carrier lifetimes and surface recombination velocity were obtained by means of this simulation.

Burn‐in effects in GaInP/GaAs/GaAs HBTs

The burn-in effect in GaInP/GaAs HBTs has been associated to processes involving hydrogen contained in the GaAs carbon doped base region of this transistor. Looking for a better understanding of this effect we have realised a detailed study of the evolution of the charge transport properties during the burn-in effect and during the step by step thermal restoration to its initial condition. Our results indicate that hydrogen processes taking place in the emitter-base space charge region and in the neutral emitter region are the main mechanisms of the burn-in. Finally, solutions for reducing this burn-in effect are discussed.

Random telegraph signal noise in SiGe heterojunction bipolar transistors

In this work, random telegraph signal (RTS) noise in SiGe heterojunction bipolar transistors (HBTs) was characterized both as a function of bias voltage and temperature. The RTS amplitudes were found to scale with the total base current, and the characteristic times in the higher and lower RTS state were found to decrease rapidly with bias voltage, approximately as 1/exp(qVBE/kT) or stronger. The RTS amplitudes were explained by a model based on voltage barrier height fluctuations across the base–emitter junction induced by trapped carriers in the space charge region. It was shown that the relative RTS amplitudes ΔIB/IB decrease exponentially with temperature in this model, which also was verified by measurements. The trapping/detrapping mechanism was suggested to be electron and hole capture, where the hole capture process occurs by tunneling. The characteristic times in both the lower and higher RTS state were in some cases found to decrease exponentially with temperature, characteristic for a thermally activated process, and in some cases found to be only weakly temperature dependent. The former behavior was explained by a multiphonon process with thermally activated capture cross sections, and an activation energy of 0.39 eV was extracted. RTS amplitudes proportional to the nonideal base current component or weaker were also found, originating from traps at the Si/SiO2 interface at the emitter periphery. The trapped carriers affect the recombination rate in the base–emitter space charge region, probably by changing the number of carriers. In this case, ΔIB/IB only showed a weak temperature dependence, which correlates well with this model. Characteristic times that decreased exponentially with temperature were observed, originating from a multiphonon process in the SiO2 with an activation energy for the capture cross section of 0.29 eV.

Orientation and dielectric overlayer effects in InGaP/GaAs heterojunction bipolar transistors

Emitter orientation effects in AlGaAs/GaAs heterojunction bipolar transistors (HBTs) in which DC current gain was greater for the [0 1 1] emitter orientation compared to the [0 1 1 ̄ ] orientation were previously attributed to the piezoelectric effect, however no effects of dielectric overlayers were examined. In this work, we establish that for InGaP/GaAs HBTs, dielectric passivation effects can be as important as the piezoelectric effect. Non-self-aligned InGaP/GaAs HBTs with ECR SiON dielectric deposited at 25 °C exhibited greater stress and showed less current gain difference and lower base current ideality factors for these orientations than identical HBTs that were passivated with plasma enhanced chemical vapor deposition (PECVD) SiN dielectric. Non-self-aligned HBTs with ECR SiON passivation also showed slightly better rf performance for the [0 1 1] orientation with f t of 48 GHz compared to f t of 44 GHz for the [0 1 1 ̄ ] orientation. These differences in dielectrics are attributed to higher dielectric-induced damage in emitter–base space-charge region for the PECVD SiN film.

Low-frequency noise in TFSOI lateral n-p-n bipolar transistors

Low-frequency (1/f) noise is characterized as a function of base current density (J/sub B/) on thin-film-silicon-on-insulator (TFSOI) lateral bipolar transistors. In the low injection region of operation, the noise power spectral density was proportional to J/sub B//sup 1.8/ for J/sub B/ 0.4 /spl mu/m/sup 2/), the noise bias dependence shifts to J/sub B//sup 1.2/, indicating current crowding effects, alter the contribution of noise sources near the extrinsic base link region of the device. In addition to the expected 1/f noise and shot noise, we have observed a bias dependent generation-recombination (Gm) noise source in some of the devices. This G/R noise is correlated to random-telegraph-signal (RTS) noise resulting from single trapping centers, located at or near the spacer oxide and/or the Si to SIMOX interface, which modulate the emitter-base space charge region.

Low-frequency noise in single–poly bipolar transistors at low base current density

This paper deals with the low-frequency noise of single–poly bipolar transistors (SBJT) biased at low base current density. From Gummel plots and low-frequency noise measurements performed on many SBJTs made using the same process, it is clearly demonstrated that the low-frequency noise is related to generation–recombination mechanisms (GR) that occur at the periphery of the emitter–base space charge region. A model which takes into account both the diffusion noise and the GR noise is proposed. A comparison with some measurements performed on another BiCMOS technology and with some results of the literature validates this analysis.

Analytical modeling of pnp InP/InGaAs heterojunction bipolar transistors

Pnp InP/InGaAs heterojunction bipolar transistors have been modeled using a modified Gummel–Poon model, and the results are compared with experimental measurements and results from a commercial simulator. The model provides a good description of the transistor’s high frequency performance and describes the falloff in device performance at high current densities. The model overestimates the current gain at low current densities by neglecting hole recombination in the base side of the emitter–base space charge region.

Emitter-base bias dependence of the collector current ideality factor in abrupt Pnp AlGaAs/GaAs heterojunction bipolar transistors

Starting with the 4×4 Luttinger–Kohn Hamiltonian and making use of the axial approximation, we calculate the emitter current as a function of the applied forward emitter-base bias for a typical Pnp AlGaAs/GaAs single heterojunction bipolar transistor (HBT). While including the effects of emitter series resistance and recombination in the quasi-neutral base and emitter-base space-charge region, we then calculate the collector current density versus emitter to base bias and find it to be in excellent agreement with the experimental results for a Al0.4Ga0.6As/GaAs Pnp HBT recently reported in the literature. For that structure, the collector current ideality factor is found to increase from 1.1 at low forward bias VEB to 3.0 at large applied emitter-base forward bias approaching the built-in potential. Experimental values are equal to 1.2 and 2.25 at low and large VEB, respectively.

Electron irradiation effects in AlGaAs/GaAs single heterojunction bipolar transistors

The effects of high-energy electron irradiation on the dc characteristics of polyimide passivated AlGaAs/GaAs HBTs of different base thicknesses and different emitter sizes are investigated. The devices show gain degradation for doses greater than 10/sup 15/ e/cm/sup 2/. The gain degradation of the passivated devices decreases (1) with increase in the base thickness, (2) with increase in the perimeter to area ratio of the emitter, and (3) with increase in the base current. Our results suggest that the observed gain degradation due to electron irradiation is mainly caused by an increase in the emitter-base junction space charge region recombination.

Space-charge recombination currents and their influence on the dc current gain of AlGaAs/GaAs Pnp heterojunction bipolar transistors

The effects of Shockley–Read–Hall, Auger, radiative, and intrinsic surface base recombination processes in the emitter-base space-charge region on the current gain of Pnp AlGaAs/GaAs heterojunction bipolar transistors are analyzed. At low forward emitter-base bias, the current gain of a typical Pnp AlGaAs/GaAs heterojunction bipolar transistor is shown to be reduced substantially below its value calculated while neglecting recombination currents in the emitter-base space-charge region. Excellent agreement between theory and experiment is found for the current gain variation versus collector current density for a Pnp device recently reported by Slater et al. [IEEE Electron Device Lett. 15, 91 (1994)].

The microwave performance of SiGe HBTs and its amplifiers

In this paper, the microwave performances of SiGe/Si heterojunction bipolar transistors (HBTs) and its amplifier have been analyzed and simulated based on a physical model considering the following influences. They are (1) the emitter and collector delay time, when the base width is small, (2) the resistance and capacitance of the low doping level emitter layer, (3) and the free carrier injection into the emitter-base space charge region. SiGe/Si HBTs suitable for integration and with width of 3 μm of emitter have been fabricated. The current gain is about 100 (at 300 K) and 6300 (at 77 K). The cut off frequency f T is 10 GHz, and the maximum oscillation frequency f MAX is 5 GHz. A single stage amplifier has been optimized and fabricated on the alumina material. The linear gain G P is 9.5 dB, while the output power P −1 is 8 dB m. The 1 dB bandwidth f L–H is from 0.6 to 1.2 GHz.

Hole transport and quasi-Fermi level splitting at the emitter-base junction in Pnp heterojunction bipolar transistors

Hole transport across the emitter-base heterojunction in Pnp heterojunction bipolar transistors is considered and the amount of hole quasi-Fermi level splitting at the interface is analyzed. Hole injection at the emitter is shown to significantly limit the emitter current to a level more than an order of magnitude less than that predicted by the hole-base diffusion model. Due to the low hole mobility, hole drift diffusion across the emitter-base space charge region is shown to be of comparable importance to thermionic-tunneling emission in controlling hole injection into the base.

An investigation of the spatial location of proton-induced traps in SiGe HBTs

The effects of 46 MeV proton irradiation induced trap generation and its impact on the electrical characteristics of silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) from an advanced ultrahigh vacuum/chemical vapor deposition (UHV/CVD) SiGe BiCMOS technology are examined and discussed for the first time. At proton fluences as high as 10/sup 14/ p/cm/sup 2/ the peak current gain of the devices degraded by less than 8% compared to the pre-irradiated samples. The maximum oscillation frequency and cutoff frequency of the SiGe HBTs showed only minor degradation after 10/sup 14/ p/cm/sup 2/. Calibration of 2-D device simulation (MEDICI) to measured data in both forward and inverse modes of operation was used to infer the spatial location of the proton-induced traps. Traps in the collector-base space charge region appear as generation/recombination (G/R) centers in the inverse emitter-base region and are the result of displacement damage. Traps at the emitter-base spacer oxide interface appear as G/R centers in the forward emitter-base space charge region and are the result of ionization damage. Taken together, these results suggest that UHV/CVD SiGe HBT technology is robust to proton fluences at least as high as 10/sup 13/ p/cm/sup 2/ without radiation hardening.

Influence of the oxygen content in SiGe on the parameters of Si/SiGe heterojunction bipolar transistors

We demonstrate peak fT and fmax of 50 GHz for heterojunction bipolar transistors (HBTs) with an oxygen concentration in the epitaxial SiGe base layer of about 1020 cm−3. These fT/fmax values are over 10 GHz higher than for identically processed HBTs with an O content of only 1018 cm−3. This is due to reduced transient enhanced diffusion of boron in the O-rich layers. However, the base carrier lifetimes are reduced by the high oxygen content. We show that ideal base current characteristics and a low 1/fnoise level can be obtained despite this effect by localizing the emitter-base space-charge region outside the O-rich layer.

A thermionic-emission-diffusion model for graded base Pnp heterojunction bipolar transistors

An analytical model which matches thermionic-emission-diffusion of holes across the emitter-base heterojunction with drift-diffusion transport across a graded base has been developed and used to examine the performance capabilities of InP-based Pnp heterojunction bipolar transistors (HBTs). Hole drift-diffusion across the emitter-base space charge region is shown to be of comparable importance to thermionic emission in controlling hole injection into the base. The effects of compositional base grading on the recombination currents is also taken into account. Compositional grading of the base is shown to enhance the device’s current gain by as much as a factor of 10 by reducing recombination in the quasi-neutral base. More importantly, compositional base grading significantly reduces the base transit time which improves the device’s peak cutoff frequency by as much as a factor of 1.5. A cutoff frequency as high as 35 GHz is found to be possible. The analysis indicates that composition grading of the base can be useful in developing high performance Pnp InP-based HBTs.

Low 1/f noise characteristics of AlGaAs/GaAs heterojunction bipolar transistor with electrically abrupt emitter-base junction

It is shown that the use of an electrically abrupt emitter-base junction considerably reduces the 1/f noise of self-aligned AlGaAs/GaAs heterojunction bipolar transistor (HBT). Although this device does not have depleted AlGaAs ledge passivation layer, the low-frequency noise spectra show a very low 1/f noise corner frequency of less than 10 kHz, which is much lower than previously reported value of about 100 kHz from conventional passivated or unpassivated AlGaAs/GaAs HBT's. Except for a residual generation-recombination (g-r) noise component, the noise power is comparable to that of Si BJT. It is also found that the low-frequency noise power of the AlGaAs/GaAs HBT is proportional to the extrinsic GaAs base surface recombination current square. Unlike the other HBT's reported, the noise sources associated with interface state and emitter-base (E-B) space charge region recombination are not significant for our device.

Analytical modeling of the recombination current in the base-emitter space charge region of heterojunction bipolar transistors

An analytical modeling of the recombination current in the base-emitter space charge region of heterojunction bipolar transistors is derived. The recombination rates as functions of distance in the space charge region are calculated for different base doping concentrations. The relations between the space charge recombination current and the applied voltage have been determined.