Heterostructure Bipolar Transistors Research Articles

Current gain increase by SiN x passivation in self-aligned InGaAs/InP heterostructure bipolar transistor with compositionally graded base

Room-temperature deposited SiN x passivation of InGaAs/InP heterostructure bipolar transistors (HBTs) with graded base was investigated. The passivation indicated that the surface recombination significantly affected the HBT performances. Both the base current and the base current ideality factor were found to decrease drastically after the SiN x passivation. The passivation also resulted in the increase of the current gain. The ideality factor of the surface recombination current of 2 was identified by the analyses of emitter–base diodes with different sizes. The emitter-size-dependent current gain showed that the improvement was due to the reduction of surface recombination. The SiN x passivation also suppressed the dependence on the perimeter-to-area ratio of the emitter junction.

Surface Recombination Mechanism in Graded-Base InGaAs–InP HBTs

The surface recombination at extrinsic base region in graded-base InGaAs-InP heterostructure bipolar transistors (HBTs) is studied. The (NH/sub 4/)/sub 2/S treatment eliminates the surface recombination, seen from the size-independent current gain. The surface recombination currents of the selfand nonself-aligned HBTs have different behavior. The mechanism of it is discussed.

Growth of Be-doped GaInP/GaAs heterostructure bipolar transistor by all solid-source multiwafer production molecular beam epitaxy

The rapid setup of the molecular beam epitaxy (MBE) growth process for GaInP/GaAs heterostructure bipolar transistor (HBT) in an all solid-source multiwafer production MBE system is reported. The first 100×100 μm2 large area test structures enable determination of the static gain, which is above 200. rf measurements on 8×10 μm2 devices exhibit a cut-off frequency fT of 34 GHz. Although most publications concerning production of such devices are related to metalorganic chemical vapor deposition grown material, we intend to demonstrate that MBE is a very efficient method for high volume production of GaInP/GaAs HBT. In particular we emphasize the fact that MBE growth for production of benchmark HBT devices is a high yield process that is attainable in a very short time. In order to optimize device characteristics further work focuses on metallurgical and electrical emitter/base interface studies by means of photoluminescence and I/V measurements on processed PN junctions.

Sulfur and low-temperature SiNx passivation of self-aligned graded-base InGaAs/InP heterostructure bipolar transistors

The passivation of self-aligned InGaAs/InP heterostructure bipolar transistors (HBTs) with graded base by the combination of S and low-temperature deposited SiNx was investigated. Base current was found to decrease after the passivation. Collector current significantly increases at low base-emitter voltages. The increase is attributed to the leakage current of the base-collector diode. The current gain was found to increase. When annealing was performed at 300 °C for 5 min, the base current decreases further and the collector current decreases. The leakage of collector current was found to be suppressed. The current gain was further improved and the leakage current can affect the Gummel plots. The leakage source was identified to be the interface between the semiconductor and the SiNx layer. The leakage current can be decreased by the annealing process.

Characteristics of integrated QWIP-HBT-LED up-converter

In this paper, we evaluate characteristics of a novel device based on the integration of a quantum well IR photodetector (QWIP), a heterostructure bipolar transistor (HBT), and a light-emitting diode (LED) for up-conversion of middle infrared (IR) into near IR (visible) radiation. Its operation is associated with intersubband absorption of middle IR radiation in the QWIP, amplification of the QWIP output electric signal in the HBT, and emission of near IR or visible radiation from the LED fed by the current injected from the HBT. This device allows us to use Si sensors for IR and thermal imaging and significantly surpass the one based on integration of a QWIP and a LED. If the HBT current gain is high enough, the total noise of the IR camera with the up-converter can be comparable to that of the QWIP electrically connected to a read-out circuit. The major sources of the excess noise are shot noise and 1/f noise of the HBT and shot noise of the charge coupled device (CCD) sensor. The criteria to reduce their influence are established. The QWIP-HBT-LED up-converter becomes most advantageous for megapixel focal plane arrays and high read-out rate cameras.

Finite Element Modelling of Flip Chip Gold-Gold Thermocompression Bonding

The gold-gold thermocompression process for power heterostructure bipolar transistor (HBT) flip chip has been modelled and simulated by finite element method. A model for plated gold creep has been determined on the basis of empirical data and model comparison. This model seems suited for gold-gold thermocompression process optimization.

The enhancement of Q factor in RPCVD SiGe varactors by the structural modification of the base-collector junction

We designed two silicon germanium (SiGe) varactors enhanced in Q factor through a structural modification by using a cost-effective SiGe heterostructure bipolar transistor (HBT) process, a conventional reduced-pressure chemical vapor deposition (RPCVD). As a result, the suggested structures showed a superiority in Q factor (160/GHz/pF at 2.5 GHz) to the conventional one (70/GHz/pF), even with neither a change in process nor an additional mask. We attributed the enhancement of Q factor to the structural feature of the varactors and quantitatively analyzed it with a lumped element model.

Investigation of interface charges at the heterojunction discontinuity in HBT devices

In this paper we investigate the impact of interface charges at heterojunctions on the performance of heterostructure bipolar transistors (HBT). Interface charges can modify the limiting process for the carrier transport in a device. Therefore, intentional interface charges introduced by δ-doped layers are basic tools for interface engineering. An accurate modelling of heterointerfaces which includes thermionic-field emission, surface charges, and surface dipoles allows to analyse the electrical performance of some modern devices based on band gap and interface engineering. It is demonstrated that the limiting transport process at an abrupt heterojunction can be shifted from thermionic emission towards drift-diffusion due to the presence of interface charges. We will also show that controlling the number and polarity of interface charges enables to improve HBT device performances.

Transient enhanced diffusion of boron in Si

On annealing a boron implanted Si sample at ∼800 °C, boron in the tail of the implanted profile diffuses very fast, faster than the normal thermal diffusion by a factor 100 or more. After annealing for a sufficiently long time, the enhanced diffusion saturates. The enhanced diffusion is temporary, on annealing the sample a second time after saturation, enhanced diffusion does not occur. It is therefore designated as transient enhanced diffusion (TED). The high concentration peak of the implanted boron profile, which is electrically inactive, does not diffuse. TED makes it difficult to fabricate modern Si based devices, in particular TED produces the parasitic barriers which degrade the performance of the SiGe heterostructure bipolar transistors and TED can limit the fabrication of shallow junctions required for sub-100 nm complementary metal–oxide–semiconductor technology. The mechanisms of TED have been elucidated recently. A Si interstitial “kicks out” the substitutional boron atom to an interstitial position where it can diffuse easily. Alternatively the interstitials and boron atoms form highly mobile pairs. In both cases Si interstitials are required for the diffusion of boron. Therefore the enhanced boron diffusivity is proportional to the concentration of the excess Si interstitials. The interstitials are injected during implantation with Si or dopant ions. The interstitials are also injected during oxidation of the Si surface. Therefore the diffusivity increases temporarily in both cases. Even at relatively low annealing temperatures (∼800 °C) the mobility of the interstitials is high. The TED at this temperature lasts for more than 1 h. This large TED time can be explained by the presence of interstitial clusters and interstitial–boron clusters. The interstitial clusters are the {311} extended defects and dislocation loops. The precise structure of interstitial–boron clusters is not yet known though several models have been proposed. The clusters are the reservoirs of the interstitials. When the supersaturation of interstitials becomes low, the clusters dissolve and emit interstitials. The interstitials emitted from the clusters sustain the TED. Many groups have suggested that the rate of emission of interstitials is determined by Ostwald ripening of the clusters. However, recently TED evolution has also been explained without invoking Ostwald ripening of the {311} defects. The evidence of Ostwald ripening of dislocation loops is more direct. In this case the Ostwald ripening has been confirmed by the measurements of the size distributions of the dislocation loops at different times and temperatures of annealing. At higher temperatures the extended clusters are not stable and coupling between the interstitials and boron atoms is reduced. Therefore at high temperatures TED lasts only for a short time. At high temperatures the displacement during TED is also small. This suggests that if rapid thermal annealing with high ramp rates is used, TED should be suppressed. Currently high ramp rates, 300–400 °C/s are being tried to suppress TED.

Local electrostatic potential and process-induced boron redistribution in patterned Si/SiGe/Si heterostructures

We present microscopic maps of the electrostatic potential in Si/SiGe/Si patterned structures of the type used in heterostructure bipolar transistors. By obtaining such maps before and after anneals typically used in device processing, we directly reveal the “vertical” and “lateral” redistribution of boron during device fabrication. Such data can be compared with the results of process simulation to extract the fundamental parameters for dopant diffusion in complex device structures.

Process Control of Epi-Layers for SiGe:C Hetero-Structure Bipolar Transistors

ABSTRACTThe SiGe:C hetero-structure bipolar transistor (HBT) has turned into a key technology for wireless communication. This paper describes the metrology tools for SiGe epitaxy process control. Two types of analysis are critical, (1) routine control of SiGe base and Si cap thicknesses, location and thickness of the doping layer, doping dose, Ge composition profile, and their uniformity across the wafer; and (2) root-cause analysis on non-routine problems. This is achieved by developing a transmission electron microscopy (TEM) technique allowing a thickness measurement with a reproducibility better than 3 Å. Charge-compensated low-energy secondary ion mass spectrometry (SIMS) using an optical conductivity enhancement (OCE) allows a Ge composition measurement to a required precision of 0.5 at. %.

High power compact 16 GHz integrated oscillator design using active reactances

This paper reports on results obtained using an oscillator incorporating active inductance and active capacitance concepts. The only passive reactance used is MIM capacitor. With the goal of high output power and low area consumption, the analyzed circuit was realized in a 300×300 μm 2 compact area. A commercially available SiGe heterostructure bipolar transistor MMIC technology with relaxed lateral scaling of 1.2 μm has been used. This oscillator design provides a load independent oscillation condition provided by a cascode buffer stage. An output power of +12.5 dBm at 15.6 GHz is achieved.

Silicon carbide and silicon carbide:germanium heterostructure bipolar transistors

In this letter, we report on heterostructure bipolar transistors (HBTs) based on silicon carbide (SiC) and a silicon carbide:germanium (SiC:Ge) alloy. The SiC:Ge base alloy was formed by the ion implantation of Ge into p-type 4H–SiC and subsequent annealing. HBT mesa structures were fabricated using a reactive ion etching process. The incorporation of Ge was found to increase the gain and the Early voltage of the devices. A common-emitter current gain (β) of greater than 3 was measured for the SiC:Ge HBTs. Homojunction SiC transistors were fabricated as a reference using the same process (except no Ge in the base region) and exhibited a β of 2.2. The transistors exhibited high breakdown voltages (>50 V without passivation), that typify SiC-based devices. These results indicate that SiC:Ge is a promising material for use in SiC-based heterostructure devices.

InAlAs/InGaAs HBTs with simultaneously high values of F/sub /spl tau// and F/sub max/ for mixed analog/digital applications

We report the design, fabrication, and measurement of InAlAs/InGaAs heterostructure bipolar transistors (HBTs) designed for high speed digital circuits. At 0.96 V V/sub CE/ the current gain cutoff frequency, f/sub /spl tau//, is 300 GHz and the maximum frequency of oscillation, f/sub max/, is 235 GHz. This value of f/sub /spl tau//, is the highest reported for bipolar transistors. At a slightly higher V/sub CE/ bias, a high value of 295 GHz for f/sub /spl tau// and f/sub max/ were obtained simultaneously.

Effect of UV-ozone oxidation on the device characteristics of InP-based heterostructure bipolar transistors

In this letter, we report the effect of UV-ozone treatments on the electrical characteristics of InGaAs/InP heterostructure bipolar transistors (HBTs). For treatments of less than 10 min, the HBT’s current gain increased with the UV-ozone exposure. This improvement is attributed to a passivation of the extrinsic base. For exposures longer than 10 min, the current gain is reduced. An increase of the base collector leakage current, leading to a degradation of the HBT’s breakdown voltage, was observed after only about 2 min of UV-ozone treatment.

Metalorganic Vapour Phase Epitaxy Growth of InP-based Heterojunction Bipolar Transistors with Carbon Doped InGaAs Base Using Tertiarybutylarsine and Tertiarybutylphosphine in N2 Ambient

A process for growth of heterostructure bipolar transistors (HBT) using tertiarybutylarsine (TBA) and tertiarybutylphosphine (TBP) in N2 ambient is realised, which is compatible with a high temperature overgrowth, thus suitable for the vertical integration of a laser structure on top of an HBT. A high growth temperature for the C–InGaAs base is favourable, to ensure no degradation during subsequent growth. Increasing the growth temperature after the base from 500°C to 680°C within the emitter layer instead of at the base-emitter interface was found to improve the ideality factors, the dc gain and the turn-on voltage.

AlGaN–GaN–AlInGaN induced base transistor

We have simulated an induced base transistor based on n-type GaN–AlGaN–GaN–AlInGaN heterostructure with a thin AlGaN emitter barrier and a thin GaN quantum well base. Our simulations show that such a majority carrier device should have a high collector current density, a low base resistance, a high current gain, and is expected to be a viable competitor to GaN-based heterostructure bipolar junction transistors.

Accumulation hole layer in p-GaN/AlGaN heterostructures

We present the results on piezoelectric and pyroelectric doping in AlGaN-on-GaN and GaN-on-AlGaN heterostructures and demonstrate p-GaN/AlGaN structures with accumulation hole layer. Our results indicate that polarization charge can induce up to 5×1013 cm−2 holes at the AlGaN/GaN heterointerfaces. We show that the transition from three-dimensional (3D) to two-dimensional (2D) hole gas can be only achieved for hole sheet densities on the order of 1013 cm−2 or higher. At lower densities, only 3D-hole accumulation layer may exist. These results suggest that a piezoelectrically induced 2D-hole gas can be used for the reduction of the base spreading resistance in AlGaN/GaN-based heterostructure bipolar transistors.

Two-dimensional hole gas induced by piezoelectric and pyroelectric charges

We present the results of the band structure calculations for a gated AlGaN/GaN heterostructure with undoped AlGaN layer and a lightly doped p-type GaN layer. These calculations are based on the analytical selfconsistent solution of the Poisson and Schrödinger equations at the heterointerface and on the calculations of the spontaneous and piezoelectric polarizations as functions of the lattice mismatch based on the theory of elasticity. The results confirm that piezoelectric and pyroelectric charge can induce the 2D hole gas at the AlGaN/GaN heterointerface. The densities of the 2D hole gas exceeding 10 13 cm −2 can be obtained in a p-type or even in a nominally undoped GaN. (Our calculations show that in an n-type GaN, the hole 2D gas might be very difficult to induce.) The metal/AlGaN/GaN band structures have been calculated for different gate biases with and without accounting for the effects of spontaneous polarization. The results suggest that a piezoelectrically induced 2D hole gas can be used for the reduction of the base spreading resistance in AlGaN/GaN-based Heterostructure Bipolar Transistors.

The values of minority carrier diffusion lengths and lifetimes in GaN and their implications for bipolar devices

The wide bandgap semiconductors GaN and AlGaN show promise as the high voltage standoff layers in high power heterostructure bipolar transistors and thyristors due to their electric breakdown characteristics. Material properties which significantly influence the design and performance of these devices are electron and hole diffusion lengths and recombination lifetimes. We report direct measurements of minority carrier diffusion lengths for both holes and electrons by electron beam induced current. For planar Schottky diodes on unintentionally doped n-type and p-type GaN grown by metal organic vapor phase deposition (MOCVD), the diffusion lengths were found to be (0.28±0.03) μm for holes and (0.2±0.05) μm for electrons. Minority carrier lifetimes of approximately 7 ns for holes and 0.1 ns for electrons were estimated from these measured diffusion lengths and mobilities. In the case of GaN grown by halide vapor phase epitaxy (HVPE) diffusion lengths in the 1–2 μm range were found. We attempt to correlate the measured diffusion lengths and lifetimes with the structural properties of GaN and to explain why linear dislocations might act as a recombination centers. We calculate the performance of nitride based bipolar devices, in particular thyristor switches. The forward voltage drop across standoff layer of the nitride based thyristor switch is shown to significantly depend on the minority carrier (hole) lifetime.