Emitter-base Region Research Articles

Current Gain Degradation in 4H-SiC Power BJTs

SiC BJTs are very attractive for high power application, but long term stability is still problematic and it could prohibit commercial production of these devices. The aim of this paper is to investigate the current gain degradation in BJTs with no significant degradation of the on-resistance. Electrical measurements and simulations have been used to characterize the behavior of the BJT during the stress test. Current gain degradation occurs, the gain drops from 58 before stress to 43 after 40 hours, and, moreover, the knee current shows fluctuations in its value during the first 20 hours. Current gain degradation has been attributed to increased interface traps or reduced lifetime in the base-emitter region or small stacking faults in the base-emitter region, while fluctuations of the knee current might be due to stacking faults in the collector region.

On the assessment of few design proposals for 4H-SiC BJTs

A theoretical design analysis using two-dimensional numerical computer aided design tool (TCAD) is presented for 4H-SiC BJTs. Compared to conventional design, a high p-doped thin layer between the base–emitter region is effective to suppress the surface recombination between the base–emitter region. Using a surface recombination layer (SR layer of 60nm thickness and acceptor doping of 1.0×1019cm−3) between base–emitter region, a peak gain of 410 was obtained at 100A/cm2. While a second epitaxy growth run is needed to utilize the advantage of this design, an alternate design with thin high doped (i.e., delta layer design) layer at the base–emitter junction is also interesting from the point of view of improving current gain with reduced on-resistance. A delta layer design thus produces a peak current gain of 300 at 100A/cm2 and on-resistance value of 4mΩ-cm2 at 300K. Compared to the experimental data of 4H-SiC BJTs, present simulated analysis provides superior performance in terms of current gain.

Degradation of Current Gain in SiC BJTs

AbstractThe reduction in the current gain of SiC BJTs has been observed after operating the devices for as little as 15 minutes. It is accompanied by an increase in the on-resistance of the BJT. The origin of the current gain degradation in the BJTs is investigated. Two possible mechanisms, which may be simultaneously present in the device, are thought to be responsible: (a) increase in the surface recombination particularly in the region between the emitter and the base implant, and (b) bulk recombination in the base due to the generation and growth of stacking faults. Initial observation reveals the presence of stacking fault in the base-emitter region when the device is forward-biased. At the same time, minimizing the effect of recombination from the surface using improved passivation helped in the suppression of the current gain degradation in SiC BJTs.

Nondestructive determination of layers producing Franz-Keldysh oscillations appearing in photoreflectance spectra of heterojunction bipolar transistor structures based on their line-shape analysis

We report on the demonstration that layers producing Franz-Keldysh oscillations (FKOs) in photoreflectance (PR) spectra of multiple-layer structures can be nondestructively determined through the line-shape analysis. At first, we discuss PR spectra of two kinds of AlGaAs∕GaAs heterojunction bipolar transistor (HBT) structures with a difference in the base-layer thickness. We have found that the phase of FKOs is sensitive to the total layer thickness over the interface associated with the FKOs. In order to analyze the FKO phase, we have derived a calculation model for the line shape of the FKOs, taking account of the probe-light interference effect. The phase of the calculated FKO profile agrees with that of the measured spectra, which supports the appropriateness of the present model. The calculation model has been also applied to the analysis of FKOs from an InGaP∕GaAs HBT structure. The calculation model well reproduces the phase of the FKOs from the base-emitter region as well as that of the FKOs from the base-collector region. In addition, we have demonstrated that the band-gap energy of the InGaP emitter layer can be precisely estimated from the FKOs with the use of the phase factor taking account of the interference effect.

Investigation of an InGaP/GaAs heterostructure-emitter bipolar transistor (HEBT) with reduced potential spike

In this work, an improved InGaP/GaAs heterostructure-emitter bipolar transistor (HEBT) with an InGaP wide-bandgap collector is investigated. In the emitter–base region, the thin narrow bandgap n-type GaAs layer is sandwiched between a wide-bandgap N-type InGaP confinement layer and a narrow-bandgap p-type GaAs base layer. In the collector–base structure, an undoped 30 Å-thick GaAs spacer and a heavily doped 30 Å-thick GaAs are inserted between the base and collector. Due to the absence of a potential spike both at the base–emitter and base–collector junctions, the studied device shows lower offset and saturation voltages. In addition, not only are the excellent current–voltage characteristics observed, but also the undesired effects, e.g. the electron blocking effect, are completely eliminated.

Breakdown characteristics of MOVPE grown Si-doped GaAs Schottky diodes

The breakdown characteristics of Au/n-GaAs Schottky contacts on metal-organic vapor-phase epitaxy grown Si-doped n-GaAs were measured in the doping range of 6×10 15–1.5×10 18 cm −3. These results are compared with the experimentally measured breakdown voltages by several workers and also with the theoretical calculation predicted by Sze and Gibbons [Sze SM, Gibbons G. Appl. Phys. Lett. 1966;8:111]. Good agreement was observed between the measured data and the breakdown voltages by Sze and Gibbons in the high doping concentrations. The maximum depletion layer width is found to be in good agreement with the theoretical analysis by Sze and Gibbons. The breakdown voltage at higher doping concentration will be useful for the design and development of GaAs switching devices and the emitter-base region of bipolar transistors.

An investigation of the spatial location of proton-induced traps in SiGe HBTs

The effects of 46 MeV proton irradiation induced trap generation and its impact on the electrical characteristics of silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) from an advanced ultrahigh vacuum/chemical vapor deposition (UHV/CVD) SiGe BiCMOS technology are examined and discussed for the first time. At proton fluences as high as 10/sup 14/ p/cm/sup 2/ the peak current gain of the devices degraded by less than 8% compared to the pre-irradiated samples. The maximum oscillation frequency and cutoff frequency of the SiGe HBTs showed only minor degradation after 10/sup 14/ p/cm/sup 2/. Calibration of 2-D device simulation (MEDICI) to measured data in both forward and inverse modes of operation was used to infer the spatial location of the proton-induced traps. Traps in the collector-base space charge region appear as generation/recombination (G/R) centers in the inverse emitter-base region and are the result of displacement damage. Traps at the emitter-base spacer oxide interface appear as G/R centers in the forward emitter-base space charge region and are the result of ionization damage. Taken together, these results suggest that UHV/CVD SiGe HBT technology is robust to proton fluences at least as high as 10/sup 13/ p/cm/sup 2/ without radiation hardening.

Fabrication of anNpMGaInAs/InP bipolar transistor by a two-step epitaxial process

A two-step liquid-phase-epitaxial process has been used to fabricate widegap-emitter Schottky collector transistors. After the growth of the first two epitaxial layers the sample has been structured and then overgrown in a second run. In that way a pn homojunction in the widegap material (InP) underneath the extrinsic base-emitter region and a pn heterojunction under the collector have been formed. First transistors fabricated show a current gain of 10 in the common-emitter configuration.

Electron-beam fabrication of submicrometer bipolar transistors for high-frequency low-current operation

A submicrometer device technology has been developed for the design and fabrication of bipolar transistors capable of high-frequency operation at low currents. Direct write electron-beam lithography is used with a single-level resist process that is compatible with high dose ion implants and dry etching, and is capable of producing feature sizes to at least the 0.5-µm level. A low temperature local oxidation process is used to minimize parasitic capacitances. Both process and device models are used in the design, which must consider the two-dimensional nature of the base-emitter region, since for these structures, the emitter junction depth is comparable in size to the emitter width. Data are presented and compared on 0.5-, 0.75-, and 1.0-µm devices.

Effect of phosphorus gettering on 1/ƒ noise in bipolar transistors

Phosphorus silicate glass gettering has been applied to silicon npn planar transistors fabricated in IC technology in order to reduce 1/ƒ noise by removing metal impurities out of the base-emitter region. It is found that gettered transistors show significantly reduced 1/ƒ noise power as well as lowered recombination current of the emitter-base junction. Furthermore the noise power spectrum becomes structured and is no longer strictly of the pure 1/ƒ type, as is typical for ungettered devices. The results suggest a close relationship between 1/ƒ noise and recombination processes.

L5 System: Ultralinear Transistors

A family of ultralinear npn transistors has been developed for use in the L5 coaxial-carrier system. These 3-GHz devices are characterized by extremely low distortion and noise figure. The transistor comprises an interdigitated base-emitter structure with a heavily doped base grid connected to the peripheral base metal contact. The emitter contact is overlaid on the base-emitter region. Contact metallurgy consists of a platinum silicide, titanium, platinum, and gold system. The transistor is a highly reliable device and meets all the performance requirements of the L5 system.

A distributed model of the junction transistor and its application in the prediction of the emitter-base diode characteristic, base impedance, and pulse response of the device

A distributed model for a junction transistor has been analyzed to include both dc and ac biasing effects in the active base region, with particular emphasis on a small-geometry diffused base planar transistor. For such devices with extremely narrow base width, dc biasing effects cannot be neglected. At high frequencies, the response of these devices is greatly modified by ac biasing effects which are accentuated by the significant dc biasing at large emitter current levels. Two-dimensional current flow under these biasing conditions was studied with a distributed model of the active base region. From such a model, the expressions for emitter-base diode characteristic, small-signal and large-signal base resistance, and complex base impedance valid for high frequencies have been deduced in terms of physical parameters of the devices like the geometry, base resistivity, etc. This equivalent base impedance and an ideal diode with its diffusion and emitter-base transition capacitance constitute the lumped model of the emitter-base region. For any particular frequency, the base impedance can be represented exactly by a parallel RC network. The distributed model can also predict the pulse response of the device more accurately than a lumped model and show the sensitivity of the transient response to the physical parameters mentioned above. Experimental verifications of the theoretical expressions are found to be satisfactory, and limitations of the earlier works are pointed out in regard to present devices.

Accurate noise measurements on transistors

This paper is concerned with a method of noise measurement which permits improved accuracy by circumventing some of the problems of the usual comparison techniques. Results are presented of measurements of I eq and R n showing good agreement between theory and experiment. It is suggested that noise measurements may be a very satisfactory method of determining the effective base resistance for inhomogeneous structures. The theoretical representation of the noise sources, including the effect of generation-recombination in the emitter-base region, is summarized in the appendix.