High-performance Heterojunction Bipolar Transistors Research Articles

Contactless device characterization of transistor structures in silicon using electro optical frequency mapping (EOFM)

Electro-optical frequency mapping (EOFM) is sensitive to carrier densities in electronic devices. Here, parametric measurements of FET and bipolar transistor structures have been performed with EOFM. The Metal-Insulator-Semiconductor (MIS) system of the FET could be characterized for strong inversion and accumulation for estimations of flatband and threshold voltage. Driving the MIS into accumulation also turns on the source/drain pn junctions into forward bias. The carrier profile of the corresponding parasitic bipolar structure is also measured with EOFM showing the different operation modes of a parasitic bipolar junction transistor and the limited performance of the unwanted bipolar parasitic under the FET. For comparison, EOFM results for a vertical high-performance heterojunction bipolar transistors (HBT) in SiGe:C BiCMOS technology are included, showing clearly distinguishable results of a golden and a faulty SiGe:C HBT.

Substitutional C effect on generation lifetime in MBE-grown SiGeC layers

This paper investigates the effect of substitutional carbon on generation lifetime in pseudomorphically strained Si1−x−yGexCy (x ∼ 0.1, y < 0.006) layers grown by gas source molecular beam epitaxy (GSMBE). The electrical assessment consists of capacitance–voltage and capacitance transient techniques on metal-oxide-semiconductor (MOS) capacitors based on Si/SiGeC heterostructures. The carrier lifetime study shows microsecond range lifetimes for substitutional carbon contents, [Cs] < 0.5%. The charge confinement in the potential well, formed at the SiGeC/Si heterointerface, is found to have less effect on generation lifetime than the substitutional to interstitial carbon ratio in these alloys. Samples with substitutional carbon concentrations above 0.5% exhibit a sharp decrease of generation lifetime into the nanosecond range, attributed to an increased ratio of interstitial carbon. Si0.8826Ge0.116C0.0014 and Si0.8809Ge0.116C0.0031 layers grown by GSMBE can be used in high-performance heterojunction bipolar transistors (HBTs).

Electrical and materials characterization of GSMBE grown Si1−x−yGexCy layers for heterojunction bipolar transistor applications

This paper reports on detailed materials and electrical characterization of strain-compensated Si1−x−yGexCy (x ⩽ 11%, y ⩽ 1%) layers synthesized by gas source molecular beam epitaxy (GSMBE). Materials assessment included the application of spectroscopic ellipsometry (SE), x-ray diffraction (XRD) and high-resolution cross-section transmission electron microscopy (HRXTEM). An almost fully strain-compensated Si0.88Ge0.11C0.01 alloy with high substitutional carbon incorporation (XRD) and well-defined structure with coherent interfaces (HRXTEM) was produced. An increase of C content (up to 1.6%) resulted in a tensile strained SiGeC alloy and degraded electrical characteristics. The minority-carrier lifetime is a parameter strongly dependent on mid-gap energy band levels, which determine the leakage currents in heterojunction bipolar transistors, and it was assessed here using the capacitance-transient technique. Metal–oxide–semiconductor (MOS) test capacitors were fabricated on Si/SiGe/Si and Si/SiGeC/Si structures using low-temperature plasma oxidation for dielectric growth. The strain-free Si0.88Ge0.11C0.01 layers exhibit a minority-carrier lifetime, τg, of ∼0.1–0.2 µs which constitutes degradation of the carrier lifetime by two orders of magnitude compared to control SiGe layers. The obtained range of τg is nevertheless sufficient for the successful application of epitaxial strain-compensated SiGeC layers in high performance heterojunction bipolar transistors (HBTs).

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.

Comprehensive study of InGaP/AlxGa1−xAs/GaAs heterojunction bipolar transistors with different doping concentrations of AlxGa1−xAs graded layers

The performances of InGaP/AlxGa1−xAs/GaAs heterojunction bipolar transistors (HBTs) with different doping concentrations of AlxGa1−xAs graded layers are theoretically studied. The use of the AlxGa1−xAs graded layer plays a key role in affecting the direct current and radio frequency performances of the studied HBTs. It is found that the studied devices with suitable doping concentrations of AlxGa1−xAs graded layers exhibit lower offset voltages, saturation voltages, and base and collector current ideality factors. Furthermore, due to the use of proper doping concentrations of AlxGa1−xAs graded layers, the studied devices show high values of the unity current gain cut-off frequency (fT) and maximum oscillation frequency (fmax). It is known that, from the theoretical analysis, the appropriate doping concentration of the AlxGa1−xAs graded layer is 1 × 1016 to 1 × 1018 cm−3. Consequently, this work is promising for device engineers to design high-performance HBT structures.

Thermal and chemical loading effects in non selective Si/SiGe epitaxy

Rapid thermal reduced pressure chemical vapour deposition of non-selective Si and SiGe epitaxy is used for the base formation in the fabrication of high performance heterojunction bipolar transistors. The performance of the device is very sensitive to the base stack (thickness, Ge composition…). Thus, kinetics must be carefully controlled. This paper reports on the influence of the substrate on the kinetics of the epitaxial deposition. This so-called loading effect is found to be due to two significant effects, thermal and chemical. The thermal effect comes from a differential heating, depending on the emissivity of the substrate and on the deposition tool. This effect can induce a variation in Si and SiGe growth rates when the growth is carried out in the surface-kinetics limited regime, where the process is very sensitive to the temperature. The chemical effect depends on the polycrystalline silicon/total surface ratio and on the pad size. For the first time, these two effects are experimentally and unambiguously separated and quantified. We show that in our conditions, the thermal effect is the most important but chemical effect, which is usually even not considered, also plays a significant role.

A 70-GHz- fT double-polysilicon SiGe HBT using a non selective epitaxial growth

This paper describes a high performance heterojunction bipolar transistor using a low complexity double polysilicon architecture. Non selective Si/SiGe epitaxy was selected for the base formation, allowing higher manufacturability than for a selective process. Low spread in statistical results confirms very good control and reproducibility of the Si/SiGe stack deposition and epitaxially aligned emitter fabrication. Static and high frequency measurements analysis shows excellent set of electrical parameters: 70-GHz- f T and 90-GHz- f max have been measured for 2.5 BV CE0 devices.

Modeling and algorithm development for automated optical endpointing of an HBT emitter etch

This paper discusses the development of a high-accuracy endpointing algorithm for the emitter etch of a heterojunction bipolar transistor (HBT). Fabrication of high-performance HBTs using self-aligned base-emitter processes requires etching through the emitter layer and stopping with very high accuracy on the base layer. The lack of selectivity in dry etching coupled with the high etch rates possible in high density plasmas render the use of a standard timed overetch impractical, especially as device layers continue to become thinner. The etch process under study requires the complete removal of an AlInAs emitter while etching no more than 5 nm of the underlying GaInAs base layer. Etch products are monitored using optical emission spectroscopy (OES) to determine etch endpoint. The process under study relies on the intensity of the 417.2 nm Ga emission line. The detection of the Ga line indicates that the etch has reached the GaInAs layer. However, the presence of a time-varying Ga baseline signal before endpoint and significant noise in the OES signal necessitate more than a simple threshold scheme for critical endpoint detection. The algorithm presented here is based on a generalized likelihood ratio with a signature function. This algorithm is robust to variance in the optical gains of the measurement equipment and is applicable to other etch processes. Experimental results of automated endpointing using this algorithm are presented in the form of pre- and post-etch ex situ film thickness measurements.

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.

Pulsed-jet epitaxy: Application to device processes

We report recent activities in the application of pulsed-jet epitaxy (PJE), our new type of atomic layer epitaxy, to device processes. We discuss three major factors of PJE; excellent uniformity of thickness and interface controllability, low-temperature growth and heavy doping, and selective epitaxy. To study the degree of thickness control, we compare the thickness profile of the GaAs epitaxial layer between PJE and MOVPE. The inherent ability of PJE to make a uniform layer is clarified. We find good thickness and interface control of PJE also in the successful fabrication of InAs/InP and GaAs/GaP short-period strained-layer superlattices. Making use of the low-temperature growth and high-concentration Be doping into PJE-grown InGaAs, we fabricate the InGaAs/InP heterojunction bipolar transistor (HBT) structures using a novel PJE/MOVPE hybrid growth process. We show that PJE is a key technology used for making a high-performance HBT. The selective growth of PJE-GaAs shows ideal features in both morphology and thickness uniformity. As an application, we report the formation of a nonalloyed contact layer using selective regrowth of GaAs on a processed substrate.

Single-event dynamics of high-performance HBTs and GaAs MESFETs

Picosecond charge-collection transients measured for GaAs/AlGaAs HBTs (heterojunction bipolar transistors) following 3.0-MeV alpha -particle and 620-nm picosecond laser excitation reveal charge-collection efficiencies up to 28 times smaller than for GaAs MESFETs, with approximately 90% of the charge collected within 75 ps of the ionizing event. The small charge-collection efficiency of the HBTs is a consequence of the ultrafast charge-collection dynamics in these devices. It is shown that picosecond laser excitation reproduces the ion-induced transients nicely providing a valuable tool for the investigation of charge-collection and SEU (single event upset) phenomena in these devices. >

High performance heterojunction bipolar transistors grown by molecular-beam epitaxy using novel growth method

A new molecular-beam epitaxial (MBE) growth method is proposed to study the Be diffusion problem in the base of Npn AlGaAs/GaAs heterojunction bipolar transistors (HBTs). The base layer was grown at As-stabilized conditions under minimum V/III beam equivalent pressure of 8 to enhance Be migration and reduce the concentration of nonradiative recombination centers. For a base doping of 4×1019 cm−3 (Be) and emitter dimensions of 2.0 μm×10 μm, the device demonstrates excellent direct-current as well as microwave performance as a power transistor. The unit current gain cut-off frequency fT is 54 GHz and the maximum frequency of oscillation fmax is above 85 GHz. For power transistors with large emitter periphery (120 μm2), 66% power-added-efficiency and 11 dB power gain have been achieved at 4.5 GHz. After the device was continuously stressed at current densities of 2, 4, and 8×104 A/cm2, its current gain, hfe (measured at stressed current density), turn-on voltage Vbe and junction ideality factor n remain almost identical to the original values. This suggests that heavily Be-doped base AlGaAs/GaAs HBT grown by MBE at low V/III flux ratio may be reliable for small signal-type application in microwave and millimeterwave frequencies.

High-performance MOCVD-grown AlGaAs/GaAs heterojunction bipolar transistors with carbon-doped base

Excellent microwave performance is demonstrated by metalorganic chemical vapor deposition (MOCVD) grown AlGaAs/GaAs heterojunction bipolar transistors (HBTs) with carbon-doped base. These devices achieve a current-gain cutoff frequency of 76 GHz and a maximum frequency of oscillation of 102 GHz. Varying the device structures allows the current gain to reach over 300 in structures with a base doping of 2*10/sup 19 /cm/sup -3/. A static divide-by-four divider implemented with C-doped base HBTs has been operated up to a frequency of 20.4 GHz. These results indicate the suitability of carbon doping for high-performance HBTs. >

Qualification of OMVPE AlGaAs/GaAs HBT structures using nondestructive photoreflectance spectroscopy

Abstract High performance heterojunction bipolar transistor (HBT) integrated circuits are extremely dependent upon the uniformity and quality of the III–V compound heteroepitaxial materials used in their fabrication. HBT requirements include the need for excellent control over layer thickness, uniformity, alloy composition, and intentional impurity concentration over a wide range of carrier concentrations. We have used the contactless technique of photoreflectance (PR) to evaluate HBT structures of various epitaxial layer design and correlated the measured PR results with transistor performance. PR was performed in the range 1.3–2.0 eV which is ideally suited to probing AlGaAs/GaAs HBT structures. The resulting signals gave information about the gap energy of the material probed (and thus its composition) and the built-in DC electric fields within the various epitaxial layers. Measurement of the spectral energy of the observed Franz-Keldysh oscillation extrema near the GaAs and AlGaAs edges, were used to determine the built-in electric field in the collector and the emitter regions. These measured fields were in good agreement with electric field distribution calculations obtained by solving the Poisson and the continuity equations for an HBT structure. Run-to-run uniformity of HBT structures from a multi-wafer OMVPE reactor were correlated to measured transistor characteristics.

Improvement In AlGaAs/GaAs HBT power gains with buried proton-implanted layer

A new structure for AlGaAs/GaAs heterojunction bipolar transistors (HBTs) that employs a buried proton-implanted external collector layer is proposed. This structure has been successfully used to achieve high-performance HBTs with maximum oscillation frequencies fmax up to 51 GHz for a device with 2 μ×5 μm emitter and 4 μm×7 μm collector dimensions.