Extrinsic Base Surface Recombination Research Articles

Extrinsic Base Surface Passivation in High Speed “Type-II" GaAsSb/InP DHBTs Using an InGaAsP Ledge Structure

Type-II GaAsSb/InP DHBTs with selectively-etched InGaAsP ledge structures are fabricated and characterized for the first time. The novel InGaAsP/GaAsSb/InP DHBTs with a 20 nm lattice-matched GaAsSb base and a 75 nm InP collector have a dc current gain improvement by a factor of 2 and a cutoff frequency fT of 190 GHz. The InGaAsP ledge design provides a simple but effective approach to suppress the extrinsic base surface recombination and enable GaAsSb/InP DHBTs to further increase the operating frequencies and integration levels for millimeter wave applications.

Emitter-Size Effects and Ultimate Scalability of InP:GaInP/GaAsSb/InP DHBTs

Emitter-size effects (ESEs) are experimentally investigated in GaAsSb-based double heterojunction bipolar transistors (DHBTs) with a new InP:GaInP composite emitter. This letter reveals that both the extrinsic base surface recombination and the intrinsic space charge recombination directly under the emitter are effectively suppressed over a wide range of collector current densities by eliminating the type-II conduction band at the emitter-base heterojunction. The reduction of ESEs enables aggressive scaling and suggests that the GaInP/GaAsSb heterojunction could become a key enabling element for sub-100-nm terahertz bandwidth InP/GaAsSb DHBTs.

Study of direct current characteristics of carbon-doped GaInP/GaAs heterojunction bipolar transistor grown by solid source molecular beam epitaxy

This article reports the dc characteristics of a GaInP/GaAs heterojunction bipolar transistor (HBT) with carbon-doped GaAs base layer grown by solid source molecular beam epitaxy using carbon tetrabromide (CBr4) as p-type dopant precursor. Hydrofluoric acid (HF) was used to passivate the GaInP/GaAs HBTs. At base bias voltages below 0.9 V in the Gummel plot, the base current of large area devices after HF passivation, was greatly reduced indicating that the extrinsic base surface recombination current was significantly reduced. After HF passivation, detailed dc characterization of the device performance within the temperature range of 300–380 K was carried out, and the carrier transport properties were investigated. The base current and collector current ideality factors at 300 K were 1.12 and 1.01, respectively. This indicates that a space charge region recombination current is insignificant in the base. From the temperature dependent Gummel plot, the activation energies of the collector current and base current were obtained. For the collector current, the activation energy is 1.4 eV, which is close to the band gap of the GaAs base. This indicates that the collector current is determined by the drift-diffusion process. For the base current, the activation energy is also 1.4 eV, indicating that band-to-band recombination plays a dominant role in the base current. No trap-related recombination was observed in the base and collector currents, which further indicates the good base material quality in the HBT structure. The current gain versus collector current characteristics at different temperatures were also investigated and analyzed.

Base–collector leakage currents in circular geometry InGaP/GaAs double heterojunction bipolar transistors

Circular geometry InGaP/GaAs double heterojunction bipolar transistors with spacings of 94.5, 44.5, 19, 10.5 and 5.5 µm between the base metal and emitter mesa were fabricated and characterized. The measured Gummel plots showed a current gain of 50 for the 94.5 µm device. As the spacing decreases to 5.5 µm, the base current increases due to the extrinsic base surface recombination current at VBE = 0.9 V thus reducing the current gain to a value of 10. It was found that the use of Ti/Au over Au/Zn/Au base metallization schemes eliminates this recombination current component of the base thereby improving the current gain significantly to 100. With a novel scheme employed here termed the δ+-layer composite collector structure, the collector leakage current was investigated.

Elevated Temperature Characteristics of Carbon-Doped GaInP/GaAs Heterojunction Bipolar Transistor Grown by Solid Source Molecular Beam Epitaxy

ABSTRACTThis paper reports the characteristics of GaInP/GaAs heterojunction bipolar transistor (HBT) with carbon-doped GaAs base layer grown by solid source molecular beam epitaxy (SSMBE) using carbon tetrabromide (CBr4) as p-type dopant precursor. Hydrofluoric acid (HF) was used to passivate the GaInP/GaAs HBTs. At base bias voltages below 0.8V in the Gummel plot, the base current of large-area devices after HF treatment was greatly reduced. This indicates that the extrinsic base surface recombination current was greatly reduced. After HF treatment, detailed DC characterization of the device performance from 300K to 380K was carried out and the carrier transport properties were investigated. The base current and collector current ideality factors at 300K were 1.12 and 1.01, respectively. This indicates that the space- charge region recombination current in the base is insignificant. From the temperature- dependent Gummel plot, the activation energies of collector current and base current were obtained. For the collector current, the activation energy is 1.4eV, which is close to the bandgap of the GaAs base. This indicates that the collector current is determined by the drift-diffusion process, in which an energy barrier of the same magnitude as the base bandgap is to be overcome by electrons before they reach the collector. For the base current, the activation energy is also 1.4eV, which is close to the bandgap of GaAs, indicating that band-to-band recombination plays a dominant role in the base current. No trap-related recombination was observed for the base and collector currents, which further indicates the high quality carbon-doped GaAs base material for the HBT structures.

Experimental evaluation of separate contributions to ideality factor for the base surface recombination current in heterojunction bipolar transistors

In this paper it is shown experimentally, for the first time, that the ideality factor n for the extrinsic base surface recombination current in AlGaAs/GaAs heterojunction bipolar transistors (HBTs) is unity and is bias independent. The devices with various emitter radii have been fabricated and the n = 1 and n = 2 current components of the emitter-base junction current were calculated. It was observed experimentally that the n = 1 current component was proportional to the total area of the emitter-base junction plus the exposed extrinsic base surface, both at lower biases such as emitter-base voltage V BE = 0.60 V (when the n = 2 current component dominates), and at higher biases such as V BE = 1.0 V (when the n = 1 current component dominates). The ideality factor value for the diffusion current is 1. Therefore, these results suggest that the ideality factor of the extrinsic base surface recombination current in HBTs is unity and is independent of the emitter-base bias.

Critical passivation ledge thickness in AlGaAs/GaAs heterojunction bipolar transistors

Npn AlGaAs/GaAs heterojunction bipolar transistors (HBTs) are fabricated to examine the critical passivation ledge thickness needed to prevent current gain degradation due to extrinsic base surface recombination current. The current gains of these molecular-beam epitaxially grown HBTs with Be-doped base layers are measured as the passivation ledge is gradually etched away. Experimental results indicate that partial passivation is achieved with a ledge thickness of 300 Å and a maximum current gain is measured with a ledge thickness of 900 Å. The ideality factor values of the base currents at various etch depths are also examined and compared with published results.

Effective lateral diffusion length in Npn AlGaAs/GaAs heterojunction bipolar transistors

The effective lateral diffusion length for the base minority carriers in HBT is calculated. This effective lateral diffusion length is a measure of the spreading distance toward the base contacts for the base minority carriers injected from the emitter before reaching the collector. With knowledge of this effective lateral diffusion length, the extrinsic base surface recombination current for any device geometries, epitaxial designs and operating conditions is easily determined. This quantitative determination of surface recombination current allows one to directly correlate one's experimental results on extrinsic base surface recombination with theoretical values.

A High-Gain AlGaAs/GaAs Heterojunction Bipolar Transistor Grown on Silicon Substrate

High gain AlGaAs/GaAs heterojunction bipolar transistors (HBTs) grown on silicon substrate have been fabricated. These HBTs-on-Si incorporate depleted AlGaAs passivation ledge to reduce extrinsic base surface recombination current and base quasi-electric field to reduce base bulk recombination current. Measured current gain of 100 is obtained for devices with an emitter area of 4×10 µm2. To our knowledge, this is the highest current gain reported for AlGaAs/GaAs HBTs grown on Si substrates. The dominant base current components is determined. The leakage currents of the base-collector junctions are also examined, and are compared to similar devices grown on GaAs substrate. Implications of these observed junction characteristics of HBTs-on-Si for power applications are discussed.

Parasitic conduction current in the passivation ledge of AlGaAs/GaAs heterojunction bipolar transistors

Passivating the extrinsic base surfaces of AlGaAs/GaAs heterojunction bipolar transistors with a thin AlGaAs emitter ledge is widely utilized to reduce the extrinsic base surface recombination current. The effect on current gain of this ledge being only partially depleted is analyzed. This analysis examines the ledge parasitic conduction current as a function of device geometry. Results indicate that even with an undepleted portion of the ledge as thin as 50 Å, the elimination of base contact recombination current requires a passivation ledge length 6 times longer than from devices with a fully-depleted passivation ledge. The effect of a base quasi-electric field on the current gain as a function of passivation ledge distance is also discussed. This theoretical analysis is compared with published experimental work.

Ideality factor of extrinsic base surface recombination current in AlGaAs/GaAs heterojunction bipolar transistors

Various structures of Npn AlGaAs/GaAs heterojunction bipolar transistor have been fabricated to examine the ideality factor for extrinsic base surface recombination. Comparison of the measured results indicates that the base surface recombination current increases exponentially with the base-emitter voltage with an ideality factor of 1, rather than 2. This finding agrees with a published theoretical analysis.

Low-frequency noise properties of N-p-n AlGaAs/GaAs heterojunction bipolar transistors

The low-frequency noise characteristics of N-p-n Al/sub x/Ga/sub 1-x/As/GaAs heterojunction bipolar transistors (HBTs) have been investigated as a function of bias current, device geometry, extrinsic-base-surface condition, Al mole fraction in the emitter, and temperature in order to identify the dominant noise mechanisms. These measurements show the existence of three distinct regions in the noise spectra: a 1/f noise line shape, a Lorentzian spectrum (noise 'bump'), and a white-noise region. The 1/f noise is attributed to fluctuations in the extrinsic-base surface recombination current. The noise bump is generated by an AlGaAs trap in the emitter-base junction. The DX center was identified as a possible candidate for this trap. It is shown that for 4- mu m*10- mu m emitter AlGaAs/GaAs HBTs, the use of a depleted, AlGaAs passivation ledge over the extrinsic-base surface typically reduced the 1/f base noise current by a factor of 10, and the reduction of the Al mole fraction from 0.3 to 0.2 decreased the magnitude of the noise bump by a factor of 3. >