Base Heterojunction Bipolar Transistors Research Articles

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.

Design and simulation of integration of vertical cavity surface emitting lasers and heterojunction bipolar transistor

Vertical cavity surface emitting lasers (VCSELs) are widely used in the field of short-range optical communication and optical interconnection because of their advantages such as low threshold current, large modulation bandwidth, easy two-dimensional integration, easy coupling with optical fibers and low cost. The VCSELs and heterojunction bipolar transistor (HBT) are longitudinal current devices, so they can be well integrated on the same wafer, and the output light power can be modulated by the HBT base current. Integration of VCSELs and HBT are designed in this paper. The VCSELs and PNP InGaP/GaAs HBT form a direct series structure. The reflectivity of DBR is 99.72% at a resonant wavelength of 850 nm and 99.57% after adding HBT separately. Therefore, the addition of HBT has little influence on the reflectivity of DBR at the resonant wavelength. The electro-optical characteristics of the integrated structure are simulated by using PICS3D software. An interim collector is added into the model in order to ensure that the simulation can be carried out smoothly. Firstly, HBT is conducted and the current flows from the emitter to the interim collector. Then, the voltage across the interim collector and the N-type electrode is increased to make VCSELs conducted and the current of the transition collector drop to zero. The interim collector can be removed from the actual structure because the current is zero. The simulation results show that the current gain coefficient is 400, and the maximum modulation rate of the base current to the output light power rises up to 280 mW/mA. The maximum temperature in the active region increases with the base current increasing, and the output light power first increases and then tends to be saturated. The ac optical gain characteristics of the integrated structure is simulated by PICS3D, and the simulation result shows that cutoff frequency exceeds 1 GHz. The addition of HBT limits the modulation rate of the integrated structure, and further optimization of HBT structure parameters and geometric dimension are needed to improve the modulation rate. The integrated structure and simulation method established in this paper can also be used to integrate LED, LD, DFB or other luminescent devices with HBT.

An Integrated Linearization Technique for GaAs Bipolar WCDMA Power Amplifier

Here we report a novel linearization and efficiency improvement technique for heterojunction bipolar transistor (HBT)-based Wireless Code Division Multiple Access (WCDMA) power amplifier. A process solution is proposed where a tantalum nitride (TaN) layer is strapped to the HBT base metal layer that resolves the current hogging issue. This is known as the strap ballasting technique. The resistance introduced by TaN improves the linear output power without trading-off its power added efficiency. At supply voltage of 4 V, the strap ballasting methodology improves the adjacent channel leakage ratio by 4.5 dB compared to the conventional base ballasting technique at output power of 28 dBm. The corresponding improvement in power added efficiency is 4%. The maximum output power delivered by power amplifier is 36 dBm. The proposed technique can be employed in the WCDMA power amplifier to minimize the fundamental trade-off issue between linear output power and efficiency.

Boron atomic-scale mapping in advanced microelectronics by atom probe tomography

Two types of industrial transistor technologies have been studied using atom probe tomography (APT). Both 14 nm node high-K metal-oxide-semiconductor field effect transistors (MOSFETs) on ultrathin body and buried oxide and 320 GHz Ft Si/SiGe Heterojunction Bipolar Transistors (HBT) embedded in a 55-nm BiCMOS chip have been analysed and their atomic distribution has been mapped. Due to the limitations of routine characterisation techniques, boron can remain invisible in such nanometric sized structures. Also, size effects can induce differences between the actual device and larger test zones used for monitoring these technologies. This paper presents results obtained by APT from two advanced nodes, in contrast to complementary techniques. Using different methodologies, including specific APT-friendly test structures and multiple-impact data filtering, the dopant behaviour in these structures can be better understood. An unexpected boron distribution in both the MOSFET source/drain and HBT base regions has been highlighted.

Two-zone SiGe base heterojunction bipolar charge plasma transistor for next generation analog and RF applications

For next generation terahertz applications, heterojunction bipolar transistor (HBT) with reduced dimensions and charge plasma (CP) can be a potential candidate due to simplified and inexpensive process. In this paper, a symmetric lateral two-zone SiGe base heterojunction bipolar charge plasma transistor (HBCPT) with an extruded (extended) base is proposed and its performance at circuit level is studied. The linearly graded electric field in the proposed HBCPT provides improved self gain (β) and cut-off frequency (fT). Two-dimensional (2-D) TCAD and small-signal model based simulations of the proposed HBCPT demonstrates high self gain β 35–172.93 and fT of 1–4 THz for different device parameters. Moreover, fT of 1104.9 GHz and β of 35 can be achieved by decreasing Nb up to 8.2×1017cm−3. Although, fT of 2 THz and 4 THz can also be achieved by reducing the base resistance up to 10 Ω and increasing the emitter/collector length up to 63 nm, respectively. The small-signal analysis of common-emitter amplifier based on the proposed HBCPT demonstrate high voltage gain of 50.11 as compared to conventional HBT (18.1).

Investigation of emitter size effect in InP/InGaAsSb/InGaAs double heterojunction bipolar transistors

The emitter size effect of a series InGaAsSb base series of InP/InGaAsSb/InGaAs heterojunction bipolar transistors (HBTs) with different emitter sizes is investigated. Compared to the InGaAs base HBTs, these devices exhibit much lower base surface recombination current. This is attributed to the surface Fermi level pinning near the valence band of the antimonide base. The effect of Sb composition and doping concentration of the base on the surface recombination current is well explained by the postulate.

Low Surface Recombination in InAlAs/InGaAsSb/InGaAs Double Heterojunction Bipolar Transistors

The surface recombination behavior of a series of InAlAs/InGaAsSb/InGaAs heterojunction bipolar transistors (HBTs) is investigated. It is found that the InGaAsSb base HBTs have lower emitter periphery surface recombination current density (K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SURF</sub> ) than the HBT with an InGaAs base. It is attributed to the type-I band lineup at the B-E junction and the surface pining of the antimonide base layer. A lower S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is deduced for the DHBT with a higher Sb content in the InGaAsSb base.

Comparative Study on Reliability of InP/InGaAs Heterojunction Bipolar Transistors with Highly Zn- and C-Doped Base Layers

AbstractThe reliability of InP/InGaAs heterojunction bipolar transistors (HBTs) with highly carbon-doped and zinc-doped InGaAs base layers grown by metal-organic vapor phase epitaxy has been investigated. The Raman spectroscopy reveals that the post-growth annealing for the carbon-doped InGaAs base improves the crystallinity to become as good as that of the zinc-doped InGaAs base. However, the photoluminescence intensity remains lower than that of the zinc-doped InGaAs even after the post-growth annealing. The current gains of the carbon- and zinc-doped base InP/InGaAs HBTs are 63 and 75, respectively, and they are affected by the base crystallinity. After the 15-min current stress test, the current gains decreased by 40 and 3% from the initial current gains for zinc- and carbon-doped base HBTs, respectively, are observed. These results indicate that the carbon-doped base HBT is much more reliable than that of zinc-doped base HBT, though it has a lower current gain.

The Versatile Use of SiGe for High-Performance Devices

Apart from the widely used bandgap engineering in the base of Hetero-junction Bipolar Transistors (HBTs) and strain engineering in CMOS SiGe has many other possible application in Si technology. This paper will discuss several examples, both within and outside (Bi)CMOS technology. For instance, SiGe bandgap engineering can be used in power diodes. Other possibilities make use of the fact that SiGe can be etched selectively towards Si. This allows, for example, creation of cavities in the silicon substrate. These can be used for, e.g., reduction of parasitic capacitance, device isolation, or electric field shaping.

Elimination of Burn-in Effect in Metal–Organic Chemical Vapor Deposition Grown InGaP/GaAs Heterojunction Bipolar Transistors by Multistep In situ Annealing

The burn-in effect in InGaP/GaAs heterojunction bipolar transistors (HBTs) grown by metal–organic chemical vapor deposition was eliminated by a multistep in situ annealing process. The presence of hydrogen in the HBT base region is the origin of the burn-in effect. An in situ annealing process was conducted during the growth of the emitter cap to reduce the concentration of hydrogen incorporated in the base region. However, diffusion may cause hydrogen to be retrapped in the base and emitter regions during the growth of the residual structure after annealing. In order to eliminate the burn-in effect, a multistep in situ annealing process was evaluated for its ability to overcome the indiffusion of hydrogen. In this investigation, a burn-in effect of less than 5% for metal–organic chemical vapor deposition grown HBTs was achieved without noticeably degrading the device performance by four-step in situ annealing.

Ohmic Contacts to p-Type III-V Semiconductors for the Base of Heterojunction Bipolar Transistors

Ohmic contacts to p-type In0.25Ga0.75Sb, InAs, and InAs0.8P0.2 have been investigated for the base of heterojunction bipolar transistors. On all these semiconductors, using Pd and/or Pt as the first layer in the metallization stack provided the lowest contact resistances. Specific contact resistances as low as 3 x 10-7 Ω-cm2 were measured for Au/W/Pd/p-InGaSb. Thin reactive layers of Pd or Pt were used beneath an unreactive W barrier to keep the contacts shallow, but contact resistance and thermal stability were sacrificed when 2 nm of Pd was used instead of 5 nm. The lowest contact resistance we measured on p-InAs was obtained with Pd/Pt/Au, but Pd/W/Au exhibited better thermal stability at 250 °C. For p-type InAsP, Pd/Ru/Au provided a specific contact resistance of 4 x 10-6 Ω-cm2 as deposited. The contact consumed only 4 {plus minus} 2 nm of InAsP after 3 days at 250 °C, and its resistance remained low.

C-doped GaAsSb base HBT without hydrogen passivation grown by MOVPE

Hydrogen passivation of C-doped GaAsSb during both metal-organic vapor phase epitaxy growth and annealing under hydride atmosphere was investigated in detail. It is well known that C accepters are easily passivated by hydrogen in III-V materials. We have found that hardly any hydrogen atoms are incorporated into C-doped GaAsSb if the annealing is performed under a hydride atmosphere. We have also found that AsH 3 exposure yields a lower rate of hydrogen passivation of the C-doped GaAsSb than during C-doped GaAsSb growth. These results are very advantageous for fabricating C-doped GaAsSb base heterojunction bipolar transistors (HBTs) without hydrogen passivation. In fact, we have successfully fabricated an InP/GaAsSb/InP D-HBT whose hydrogen concentration in the base ([C]: 8×10 19 cm −3, thickness: 20 nm) was under the detection limit of our SIMS measurement equipment. We demonstrate a relatively high-performance HBT ( β∼40, f T∼300 GHz, f max∼200 GHz, and BV CEO∼6 V) without any dehydrogenation process.

Three-terminal Si-based negative differential resistance circuit element with adjustable peak-to-valley current ratios using a monolithic vertical integration

Si-based resonant bipolar transistors are demonstrated by the monolithic vertical integration of Si-based resonant interband tunnel diodes atop the emitter of Si/SiGe heterojunction bipolar transistors (HBTs) on a silicon substrate. In the common emitter configuration, IC versus VCE shows negative differential resistance characteristics. The resulting characteristics are adjustable peak-to-valley current ratios, including infinite and negative values, and tailorable peak current densities by the control of the HBT base current under room temperature operation. With the integrated RITD-HBT combination, latching properties which are the key operating principle for high-speed mixed-signal, memory, and logic circuitry, are experimentally demonstrated.

Photoreflectance spectroscopy for the study of GaAsSb/InP heterojunction bipolar transistors

This work is devoted to a photoreflectance (PR) study of single GaAsSb layers grown pseudomorphically on InP. Such antimonide alloys included in the base of heterojunction bipolar transistors (HBT) are expected to lead to high cut-off frequency transistors. However, many important parameters such as the nature of the InP/GaAsSb interface, the conduction band offset and the Fermi level pinning at GaAsSb surface are not well known in this system. In this study, they are determined from optical measurements performed between 8 and 300 K. Unusual evolution of the photovoltage as a function of temperature is reported in GaAsSb. Finally, a PR study of a complete HBT structure shows the potential of this technique to non-destructively characterize complex epitaxial layers, and further on to study fully processed devices under operation.

Simulations of InGaN-base heterojunction bipolar transistors

GaN/InGaN heterojunction bipolar transistors (HBTs) are very promising for high speed, high power density applications at elevated temperatures. In this paper we report on simulations of the dc performance of GaN/In 0.2Ga 0.8N HBTs as a function of the layer design and doping levels. The conductivity of p-InGaN is significantly lower than p-GaN, reducing the deleterious effects of high ohmic contact resistance. Predicted dc current gains are given as a function of base doping and thickness and are in excess of several hundred even for aggressive layer designs. Advantages with respect to GaN base HBTs are also discussed.

Heterojunction bipolar transistors implemented with GaInNAs materials

Use of GaInNAs in the base of heterojunction bipolar transistors (HBTs) on GaAs substrates allows a reduction of the turn-on voltage, Vbe,on, of the devices, facilitating their use in applications with low power supply voltage (particularly battery operated power amplifiers for mobile communications). Using GaInNAs with N content below 2% and In content of 1–20%, HBTs have been demonstrated with Vbe,on values lower by 25–400 mV than those of conventional GaAs-based HBTs. The GaInNAs base regions exhibit lower diffusion length than conventional GaAs bases, which reduces current gain and detracts from high-frequency performance, as well as higher base sheet resistance. These adverse effects can be mitigated by proper design tradeoffs of base thickness and nitrogen composition, as well as by compositional grading in the base to provide a built-in quasi-electric field to assist electron transport.

High‐speed self‐aligned SiGe HBT and application to optical‐fiber‐link ICs

AbstractSiGe base heterojunction bipolar transistor (HBT) has high‐frequency and high‐speed performance comparable to compound semiconductor devices, and also has high cost because the existing Si process is used. Hence, this HBT is expected to be a key device for high‐speed optical transmission systems and microwave/millimeter‐wave wireless systems. The self‐aligned SiGe HBT developed here has a PASS structure with selective epitaxial growth by UHV/CVD and buffered polycrystalline Si in the lower layer of the external base polycrystalline Si. In this way, high‐speed operation of the intrinsic transistor and reduction of the parasitic capacitance and resistance are realized at the same time. The cutoff frequency and maximum oscillation frequency of this SiGe HBT are 122 and 163 GHz, and the gate delay time of the ECL circuit is an extremely fast 5.5 ps. Further, an IC to form a 40 Gbit/s optical receiver is fabricated. A preamplifier with a 45‐GHz bandwidth, a limit amplifier with a gain of 32 dB, a frequency divider of 60‐GHz operations, and a 1:4 DEMUX with a decision circuit operating at 40 Gbit/s are realized. It is shown that the 40 Gbit/s optical receiver IC chip set with the present SiGe HBTs reached a practical level. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 85(7): 66–76, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.10004

An analytical model of the inverse base width modulation effect in SiGe graded heterojunction bipolar transistors

The analytical model of the collector current ideality factor degradation at high V be in modern SiGe graded base heterojunction bipolar transistors (HBTs) has been developed for the first time. It is subsequently used for the analysis of the inverse base width modulation (IBWM) effects in SiGe HBTs with respect to collector current degradation, current gain premature roll-off and SiGe base transit time. Comparing the IBWM effects calculated for HBTs with various base impurity concentration doping profiles and the Ge grading, we have found that the careful optimization of SiGe base parameters may substantially minimize negative influence of the IBMW effects on the HBT electrical parameters.

Annealing study of InGaP/GaAs heterojunction bipolar transistor and carbon-doped p +GaAs base layers

In this work we have examined the effect of RF annealing (450–750°C, 5–30 min) upon both InGaP/GaAs-based hetero-junction bipolar transistor (HBT) structures, fabricated by metalorganic vapour phase epitaxy (MOVPE), as well as thick carbon (C)-doped p +GaAs HBT base layers with varying layer thickness, dopant level and type (intrinsic and extrinsic C precursors) and co-doping (In) strain compensation. Anneal-induced changes in the p +GaAs layer lattice strain, Hall carrier concentration and mobility were compared with non-radiative losses, determined from photoluminescence (PL) intensity data. Majority and minority carrier property differences were also compared with IR reflection, Raman backscattering and photoreflectance (PR) data and correlated with changes in MOVPE hydrogen background concentration as determined by secondary-ion-mass-spectroscopy (SIMS). Thick base layer (1.3 μm) HBT structures were also examined for different anneal temperatures and time, showing significant changes in the PR emitter(InGaP)/base (p +GaAs) and base/collector (n-GaAs) interface regions for the 650°C anneal condition, as correlated with both PL and SIMS hydrogen concentration data.

A nonfundamental theory of low-frequency noise in semiconductor devices

A general low-frequency noise theory based on the fluctuation in the number of carriers is presented. In this theory, the low-frequency noise is attributed to the traps within the bandgap of a semiconductor, which are the sources of the generation-recombination noise. The cumulative effect of the generation-recombination noise from each trap center generates a 1/f type noise. It is shown that in fact, 1/f noise may have any frequency dependence between 1/f/sup 0/-1/f/sup 2/. If not masked by thermal noise, the low-frequency noise generated from these traps becomes 1/f/sup 2/ at very high frequencies. Also, if the lifetime of the carriers in the semiconductor under nonequilibrium condition is finite, at very low frequencies, the noise spectral density reaches a plateau. While this theory can be applied to any semiconductor device, only heterojunction bipolar transistors (HBTs) were considered in detail. Based on this theory, a model for low-frequency noise in the base of HBTs is derived. Frequency and current dependence of low-frequency noise are modeled. Results of the base noise measurements in AlGaAs/GaAs HBTs were found to agree with the noise theory presented here. This significant theory, for the first time, proves the possibility of the number fluctuation model as a general 1/f noise cause without a need for specific and nonrealistic carrier lifetime probability functions.