Base Collector Research Articles

High breakdown field of pnp GaN/InGaN/AlGaN DHBTs with AlGaN collector

We have investigated the characteristics of pnp GaN/InGaN/(Al)GaN double heterojunction bipolar transistors (DHBTs) grown by metalorganic vapour phase epitaxy. In GaN-collector DHBTs, the collector–base breakdown voltage under the open-emitter condition increased with increasing collector thickness. By fitting the experimental results to the theoretical model of the punch-through diode, we obtained the breakdown field of 3.1 MV/cm and the residual carrier concentration of 8.5 × 1016 cm–3 for the GaN collector layer. On the other hand, in a 500 nm thick AlGaN collector, the breakdown voltage was 190 V. Assuming that the bias was uniformly applied in the AlGaN collector, the corresponding breakdown field was calculated to be 3.8 MV/cm. The results indicate that the use of an AlGaN collector in HBTs is a promising way to increase the breakdown voltage without increasing the collector thickness. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Parameter extraction of heterojunction bipolar transistor from low-frequency noise and S-parameter measurements

The extraction of the equivalent circuit parameters for bipolar transistors is undertaken via high-frequency S-parameter and low-frequency noise measurements. The extraction is exhaustively detailed here for a double purpose. First, it highlights the not-so-justified approximations so often made for the extraction of the extrinsic base–collector capacitance. Second, it emphasizes the similarity of the values obtained for series resistances in both the low-frequency and high-frequency ranges.

Enhanced fT and linearity performance of InGaP/GaAs HBTs using a non-uniform doping collector

In this paper, the ac and power linearity characteristics of InGaP/GaAs heterojunction bipolar transistors (HBTs) with collector doping design have been presented. The collector design is to employ a thin high-doping layer inside the low-doping collector. The thickness, doping and location of the inserted layer are systematically studied with consideration of breakdown characteristics and the Kirk effect. The Kirk effect is relieved by redistributed electric fields in the non-uniform doping collector to extend the operational current and thus to increase the cutoff frequency (fT) and output power (Pout). The collector current at the onset of the Kirk effect is extended from 9.6 to 17.3 mA and fT is increased from 36 to 48 GHz for a device size of 4 × 12 × 2 µm2. The corresponding improvement in the saturation output power is 2.2 dB measured at 1.8 GHz. In addition, the impact of the collector doping profile on the base–collector capacitance (CBC) and thus the linearity is simulated, discussed and measured. For an HBT with a thin high-doping layer being inserted 4000 Å from the base–collector junction, the experimental result on output third-order intercept point (OIP3) demonstrates the significant improvement by as large as 10 dB at a frequency of 1.8 GHz.

MBE growth of heavily carbon doped GaAsSb on InP for heterojunction bipolar transistor applications

Carbon doped GaAsSb samples have been grown by solid source molecular beam epitaxy using carbon tetrabromide as carbon source. A maximal hole concentration of 4.5×10 20 cm −3 has been obtained with a corresponding mobility of 19 cm 2/V s. For hole concentrations in the 4–8×10 19 cm −3 range, typically used in HBTs, the hole mobility is 45–50 cm 2/V s. We show that 450 °C is the optimal growth temperature at which both high hole concentration and mobility are obtained. An InP/GaAsSb/InP DHBT test structure with a 2×10 19 cm −3 carbon doped base has been realised. The current gain β is about 80 and the emitter–base and base–collector junction ideality factors are 1.15 and 1.05, respectively, indicating good junction properties.

Effects of Mechanical Uniaxial Stress on SiGe HBT Characteristics

The impact of mechanical uniaxial stress on the characteristics of SiGe heterojunction bipolar transistors is reported. A four-point bending apparatus was used to apply a uniaxial stress in the range of . Due to the strain-induced modulation of carrier mobility and energy bandgap, the changes in the collector current , base current , current gain , and breakdown voltage are related to the strain-polarity. The strain-polarity dependence of the collector current was found to oppose that of the breakdown voltage , revealing a trade-off between uniaxial compressive and tensile stress. The response of the cutoff frequency under uniaxial stress was also investigated. Unlike the obvious changes of dc characteristics, a minor change in the maximum could be found for uniaxial stress levels below . Finally, various components of the transit time are analyzed to assess their contribution to the frequency response.

DLTS study of deep level defects in Li-ion irradiated bipolar junction transistor

Commercial npn transistor (2N 2219A) irradiated with 50MeV Li3+-ions with fluences ranging from 3.1×1013ionscm−2 to 12.5×1013ionscm−2, is studied for radiation induced gain degradation and minority carrier trap levels or recombination centers. The properties such as activation energy, trap concentration and capture cross section of induced deep levels are studied by deep level transient spectroscopy (DLTS) technique. Minority carrier trap levels with energies ranging from 0.237eV to 0.591eV were observed in the base–collector junction of the transistor. In situ I–V measurements were made to study the gain degradation as a function of ion fluence. Ion induced energy levels result in increase in the base current through Shockley Read Hall (SRH) or multi-phonon recombination and subsequent transistor gain degradation.

Performance analysis of new-design solar air collectors for drying applications

This paper presents a performance analysis of four types of air heating flat plate solar collectors: a finned collector with an angle of 75°, a finned collector with an angle of 70°, a collector with tubes, and a base collector. In this study, the first and second laws of efficiencies were determined for the collectors and comparisons were made among them. The results showed that the efficiency depends on the solar radiation and the construction of the solar air collectors. The temperature rise varied almost linearly with the incident radiation. The first law of efficiency changed between 26% and 80% for collector-I, between 26% and 42% for collector-II, between 70% and 60% for collector-III, and between 26% and 64% for collector-IV. The values of second law efficiency varied from 0.27 to 0.64 for all collectors? The highest collector efficiency and air temperature rise were achieved by the finned collector with angle of 75°, whereas the lowest values were obtained for the base collector. The effectiveness order of the collectors was determined as the finned collector with angle of 75°, the finned collector with angle of 70°, the collector with tubes, and the base collector.

A monolithic GaAs-based short wavelength smart pixel based on fabricated p–i–n PD, aDHBT and a VCSEL

For the first time a smart pixel that can function as a building block for an optical transceiverwith circular dimensions is fabricated using an InGaP/GaAs double heterojunction bipolartransistor (DHBT) and a GaAs/AlGaAs vertical cavity surface emitting laser (VCSEL). Anovel InGaP DHBT structure based on an abrupt base–collector heterojunction, theδ+-layer composite collector structure, is employed in this work. The DHBTshowed a breakdown voltage of 13 V and a cut-off frequency of 32 GHz atIc = 30 mA. Also a high performance 850 nm infrared VCSEL exhibited athreshold current of 3.5 mA with a maximum optical output of 4.8 mW atIc = 20 mA and forward voltage of 1.8 V. The detector, comprising the base-to-collectorjunction of the DHBT as a p–i–n photodiode (PD), produced a photocurrent of180 µA for a given input power of 0.3 mW.

Fabrication of high-speed InP/InGaAs/InP DHBT with a new self-aligned metallization technique for reduced base resistance

A new self-aligned emitter–base metallization (SAEBM) technique with wet etch is developed for high-speed heterojunction bipolar transistors (HBTs) by reducing extrinsic base resistance. After mesa etch of the base layer using a photo-resist mask, the base and emitter metals are evaporated simultaneously to reduce the emitter–base gap (SEB) and base gap resistance (RGAP). The InP/InGaAs/InP double heterojunction bipolar transistor (DHBT) fabricated using the technique has a reduced RGAP, from 16.48Ω to 4.62Ω comparing with the DHBT fabricated by conventional self-aligned base metallization (SABM) process. Furthermore, we adopt a novel collector undercut technique using selective etching nature of InP and InGaAs to reduce collector–base capacitance (CCB). Due to the reduced RGAP, the maximum oscillation frequency (fmax) for a 0.5μm-emitter HBT is improved from 205GHz to 295GHz, while the cutoff frequency (fT) is maintained at around 300GHz.

Characterization of silicon–germanium heterojunction bipolar transistors degradation in silicon–germanium BiCMOS technologies

The degradations of SiGe heterojunction bipolar transistors (HBTs) caused by the integration of the HBTs with a CMOS process were investigated. SiGe HBTs were fabricated using two sorts of BiCMOS technology, the HBT-during-CMOS (HDC) process by which SiGe HBTs and Si CMOS transistors are generated simultaneously, and the HBT-after-CMOS (HAC) process whereby SiGe HBTs are prepared after the CMOS element formation. While the current gain, the cut-off frequency ( f t), and the maximum oscillation frequency ( f max) decreased by the HDC process, the SiGe HBTs prepared by the HAC process exhibited lower f max and breakdown voltage than the control HBTs made by a SiGe HBT process. These specific deteriorations originated from the doping concentration variations in the SiGe base for the HDC process and in the surface of collector for the HAC process, respectively. The thermal heat of the source–drain anneal broadened the boron profile in the SiGe base and induced the location change of emitter–base and collector–base junctions in the HDC process. The surface concentration of the collector increased due to the phosphorus evaporated from the gate poly-silicon during the HAC process. The device performances could be improved to the level of the control HBTs by modifying the BiCMOS processes either by decreasing the source–drain anneal temperature to suppress the boron diffusion or overlaying capping layers on the gate poly-silicon to prevent the auto-doping.

Analytical model for base transit time of a bipolar transistor with Gaussian-doped base

The present analysis for base transit time τ b of a modern high-speed npn bipolar transistor is done for Gaussian-doped base considering doping dependence of mobility, bandgap narrowing effect and carrier velocity saturation effect at base–collector junction. First the minority carrier current density and electron density equations incorporating all the effects are analytically solved. The collector current density J c and base stored charge Q b are then separately expressed as a function of the injected electron density n(0) in the base in order to find base transit time. The modeling of J c, Q b and τ b are essential for the design of high-speed bipolar transistor. The equations are applicable for low level of injection. The base transit time calculated analytically is compared with numerical results in order to demonstrate the validity of the assumptions made in deriving the equations. The closed form equations for collector current density and base transit time offer a physical insight into device operation and are a useful tool in device design and optimization.

75 GHz InP HBT distributed amplifier with record figures of merit and low power dissipation

We present an InP heterojunction bipolar transistor (HBT) distributed amplifier with a bandwidth of 75 GHz, gain of 14 dB, and a record low power consumption of 78 mW. The HBTs had a 600-nm-thick collector, and hence a relatively low f/sub T/ and f/sub max/ of 84 and 150 GHz, respectively. The thick collector is a tradeoff required in optoelectronic integrated receivers, in which the PIN diode layers are the same as the base collector layers. To obtain high PIN diode responsivity, the collector layer needs to be thicker than in optimized HBTs. The amplifier topology comprises an emitter follower at the input and a cascode stage, with a resistor and inductance at the emitter follower output, and a peaking line between the HBTs in the cascode stage. The amplifier exhibits matching at the input better than -10 dB up to 85 GHz. The chip contains 16 HBTs and its size is 1.7/spl times/0.9 mm/sup 2/.

Very High On/Off Ratio in Vertical-Type Metal-Base Organic Transistors

ABSTRACTThe on/off ratio of the vertical-type metal-base organic transistors was drastically improved by heat treatment in air. The heat treatment after deposition of the collector layer and base electrode reduced the leakage current between the base and collector, resulting in remarkable suppression of the off current. As a result, in addition to the advantage of low voltage and high current operation based on the vertical structure, very high on/off ratio exceeding 105 was achieved.

Extraction of small-signal equivalent circuit model parameters for Si/SiGe HBT using S-parameters measurements and one geometrical information

A physical and simple method is proposed to extract the hybrid-π small-signal equivalent circuit model of Si/SiGe heterojunction bipolar transistor (HBT). In this method, we use test (dummy) structures to extract by means of fitting techniques the extrinsic bias-independent parameters representing the contact pads plus the transmission line connections to the core of the active device. All intrinsic bias-dependent parameters are calculated analytically from S-parameters only. The ratio of the area of the emitter contact to base area is used to solve the base–collector feedback problem due to the distributed nature of the base. Using this physical (geometry) constraint instead of the measured direct current (DC) information helps to get more reliable parameters and easier calculations. When we applied this methodology, a good agreement is obtained between the modelled S-parameters with the corresponding measured ones over the broad band from 40 MHz to 20.02 GHz. The error for three different bias points was less than 1.2%.

Evidence for hot electron magnetocurrent in a double barrier tunnel junction device

Hot electron transport has been studied in three terminal Ta∕TaOx∕Co∕AlOx∕Ni81Fe19 structures fabricated by magnetron sputtering through shadow masks. With the Co base and Ta collector connected together via a small resistor, the collector current contains contributions first from hot electrons injected from the Ni81Fe19 emitter, and second from a geometrical artifact that leads to tunneling from the Fermi level in the base. Both sources of collector current lead to a room temperature magnetocurrent effect. The hot electron contribution begins to dominate as the emitter-base voltage −Veb exceeds 0.3 V.

Improved modeling and parameter-extraction procedure emphasizing on extrinsic inductances and base–collector capacitances of collector-up HBTs

In this paper, an improved modeling and parameter-extraction procedure requiring no special de-embedding test structures, reverse/high-forward-biased measurements, or the use of numerical optimization process has been successfully developed to efficiently determine the equivalent-circuit parameters of collector-up heterojunction bipolar transistors. This new approach, modified from a previous work by our group, emphasizes the ad hoc analytical extraction of extrinsic inductances ( L b, L c, L e) and base–collector capacitances ( C ex, C bc), which are crucial parameters for characterizing RF performances in device modeling. A comprehensive set of practical modeling equations is derived from systematically formulating two-port-network matrices on the basis of measured S-parameters. Physically realistic results are demonstrated under various biasing conditions for the p–n–p InGaAs collector-up heterojunction bipolar transistor with a graded base of 25 nm. The superiority of the improved technique is verified by observing the consistency between calculated and measured S-parameters.

Model of transit time for SiGe HBT collector junction depletion-layer

The transit time through collector junction depletion-layer is an important parameter that influences AC gain and frequency performance. In SiGe heterojunction bipolar transistor (HBT) collector junction, the depletion-layer width is given in three cases. The models of collector depletion-layer transit time, considering the collector current densities and base extension effect, are established and simulated using MATLAB. The influence of the different collector junction bias voltage, collector concentration of As or P dopant and collector width on collector junction transit time is quantitatively studied. When the collector junction bias voltage, collector doping concentration and collector width are large, the transit time is quite long. And, from the results of simulations, the influence of the collector depletion-layer transit time on frequency performance is considerable in SiGe HBT with a thin base, so it could not be ignored.

Comprehension and modelling of heterojunction phototransistors operated in the Gummel-plot and common-emitter modes

Fabrication, characterization and modelling of heterojunction phototransistors (HPTs) are reported. The common-emitter (with current-biased base) and Gummel-plot (with voltage-biased base) modes are employed to characterize and fully comprehend what differences exist between the current- and voltage-biased HPT's performance. The results of further studies include the case when a series of different optical-power injection levels is illuminated at our HPTs. The performance of the current- and voltage-biased HPTs was also compared to that from a newly proposed HPT model and related equivalent circuit with good agreement found. Although an independent voltage source can be used to tune the operating point of a heterojunction bipolar transistor to a higher current level where the dc current gain is larger, the photocurrent generated within the base–collector (B–C) region offers few contributions to the final collector photocurrent. The optical gain obtained from the HPT biased using a high voltage is even smaller than that of the HPT with a floating base. It is concluded that (i) a current-biased HPT's dc base current and photocurrent generated within the B–C region entirely flow into the base–emitter (B–E) junction so that the device's optical gain is enhanced; (ii) however, no enhancement of optical gain for a HPT will be obtained using dc base bias, since the dc current gain is independent of collector (or base) current; (iii) a voltage-biased HPT behaves like a p–i–n photodiode and (iv) electrical base bias using a high external voltage source with a large series resistance is a possible way to enhance the optical gain of a HPT.

Empirical expression for base transit time in bipolar transistors

A new and compact formula for the base transit time, τ b , of a modern high speed npn bipolar transistor with exponential base doping profile is derived considering doping dependence of mobility, bandgap narrowing effect, high injection effect and carrier velocity saturation at the base edge of the collector–base junction. The collector current density, J c , and minority carrier stored charge, Q b , in the base are separately expressed as a function of the injected electron density n o in the base in order to find an empirical expression for τ b . The modelling of J c , Q b and τ b is essential for the design of high-speed bipolar transistor. The expressions are applicable for arbitrary injection before the onset of the Kirk effect and they are simple and straight forward to give insight into device operation. The base transit time calculated analytically is compared with simulation results in order to demonstrate the validity of the assumptions made in deriving the expression. The closed form expressions for collector current density and base transit time offer a physical insight into device operation and are a useful tool in device design and optimization.

Optimization of base-to-emitter spacer thickness to maximize the frequency response of bipolar transistors

The impacts of base-to-emitter spacer thickness on the unity gain frequency ( f T), base resistance ( r B), base collector capacitance ( C BC) and maximum oscillation frequency ( f max) of a bipolar junction transistor (BJT) are studied. Using the extracted Y-parameters from a simulated device with structural parameters calibrated to an actual process, the resulting f T and f max with different spacer thickness is reported. A tradeoff between peak f T and f max is observed and the process window to obtain high f T and f max is proposed.