The realization of a GaAs—Ge wide band gap emitter transistor

The fabrication and properties of an n-GaAs emitter, p-Ge base, n-Ge collector transistor which possesses significant current gain is described, These GaAs wide band gap emitter transistors have shown incremental current gains near 15 when operated at current densities up to 3500 A/cm². The doping level used in the base region was quite high (up to 5×10¹⁹/cm³in order to avoid a spurious Ge p-n junction in this region. The epitaxial deposition of the GaAs emitter region was carried out at a low temperature in order to also avoid a hidden p-n Ge junction. The low deposition temperature resulted in low (about 5×10¹⁵/cm³emitter doping levels. The general nature of the GaAs-Ge heterojunction energy-band diagram permits this high doping in the base or Ge region relative to the GaAs emitter region without reducing the current gain below unity. The observation of gain in this n-p-n heterojunction structure where the emitter is much more lightly doped than the base is considered to be confirmation of the theoretical proposals of Shockley and Kroemer.     

High-low polysilicon-emitter SiGe-base bipolar transistors

Self-aligned heterojunction bipolar transistors with a high-low emitter profile consisting of a heavily doped polysilicon contact on top of a thin epitaxial emitter cap have been fabricated. The low doping in the single-crystal emitter cap allows a very high dopant concentration in the base with low emitter-base reverse leakage and low emitter-base capacitance. The thin emitter cap is contacted by heavily doped polysilicon to reduce the emitter resistance, the base current, and the emitter charge storage. A trapezoidal germanium profile in the base ensures a small base transit time and adequate current gain despite high base doping. The performance potential of this structure was simulated and demonstrated experimentally in transistors with near-ideal characteristics, very small reverse emitter-base leakage current, and 52-GHz peak f_max, and in unloaded ECL and NTL ring oscillators with 24- and 19-ps gate delays, respectively     

Hole-spreading effect on upward current gain in n-p-n transistors

The upward current gain and stored charge for future small-sized n-p-n transistors have been studied. Included is the effect of hole spreading from a narrow base region to a wide emitter region, whether buried layer or substrate. When base dimensions are comparable with or shorter than the hole diffusion length in the emitter, the hole current is concentrated near the fringe of the base so that the base current tends to be proportional to the base length instead of the base area. The same tendency appears regarding the stored charge in the emitter. Hence, the scaling down of lateral dimensions gives rise to a decline in the upward current gain and cutoff frequency. The calculations introduced here are in quite fair agreement with the observed base currents in sidewall base contact structure (SICOS) n-p-n transistors, where dimensions for the emitter-base junction are almost the same as for the collector-base junction.     

A high-current-gain low-temperature pseudo-heterojunction bipolartransistor utilizing sidewall base-contact structure (SICOS)

An experimental pseudo-heterojunction bipolar transistor (HBT) is described. The pseudo-HBT is a homojunction bipolar transistor having a moderately doped emitter and a heavily doped base, providing a bandgap profile similar to those of actual HBTs. Analyses including real constraints, such as a heavily doped emitter region for ohmic contact and the profile tail in the base region, show a significant change in the way they affect injection characteristics between 300 and 77 K. Based on these analyses, an impurity profile is carefully designed for upward mode operation. The electron injection into the external base region. which is thought to be unfavorable for high current gain in the upward mode, is avoided by using the sidewall base-contact structure. The fabricated transistor clearly displays a negative temperature dependence on current gain. The current gain is 107 at 77 K, which is 5 times higher than the room-temperature current gain. In addition, current gain excluding the nonideal effects at 77 K is as high as 25,000. These results not only remove gain degradation at low temperatures, but also verify the pseudo-HBT concept, in which an injection efficiency as high as that of an HBT can be obtained using only homojunctions. Based on these results, the small emitter transit time inherent in pseudo-HBTs is analyzed     

Si hetero-bipolar transistor with a fluorine-doped SiC emitter anda thin, highly doped epitaxial base

The effect of fluorine doping on SiC/Si heterojunction bipolar transistors (HBTs) is studied. The film properties of the fluorine-doped SiC and device characteristics of an HBT using the SiC emitter and a 50-nm-thick, highly doped epitaxial base (10¹⁹/cm³) are presented. The current gain is improved from 15 to 80 by doping with fluorine. The current gain is four times larger than that of a conventional poly-Si emitter homo-transistor with the same base structure. In spite of the very thin base, the Early voltage is over 100 V. Forward-bias tunneling current was hardly seen at the emitter-base junction. The fluorine appears to terminate the dangling bonds. The results show the possibility of fabricating transistors with a very thin, highly doped base     

A theory and some characterisitcs of power transistors at high-level conditions

The concentration of injected carriers is large compared with the impurity doping concentration in the base region of a power transistor operating at high level. Carrier concentration and characteristics of a two-dimensional transistor model are calculated for this case. Emitter and base contacts are in the form of strips. Most of the injected emitter current reaches the collector while the remainder recombines in the slightly doped base region under the emitter, resulting in a current in the highly doped base contact. In addition, a recombination current generated in the base region under the base contact is added to this base current and results in a decrease of current gain. In order to analyze the base recombination current, a special transistor with divided collector contacts was prepared. In this way, the collector current due to the region under the emitter contact can be separated from the collector current due to the region under the base contact. The presented theory could be verified. Additional corrections are necessary, however, in the direction of the current in the slightly doped base region.     

Simulation analysis of the DC current gain in an n-p-n a-Si:H/SiGe/Si heterojunction bipolar transistor

This paper presents the results of a simulation study focused on the evaluation of the DC characteristics of an n-p-n SiGe-based heterojunction bipolar transistor (HBT) performing an extremely thin n⁺ hydrogenated amorphous silicon (a-Si:H) emitter. The a-Si:H(n)/SiGe(p) structure exhibits an energy gap difference of approximately 0.8 eV mostly located at the valence band side and this results in an optimal configuration for the emitter/base junction to improve the emitter injection efficiency and thus the device performance.Considering a 20% Ge uniform concentration profile in the base region, simulations indicate that the DC characteristics of an a-Si:H/SiGe HBT are strictly dependent on two essential geometrical parameters, namely the emitter width and the base width. In particular, the emitter thickness degrades device characteristics in terms of current handling capabilities whereas higher current gains are obtained for progressively thinner base regions. A DC current gain exceeding 9000 can be predicted for an optimized device with a thin emitter and a 10 nm-thick, doped base.     

The effects of distributed base potential on emitter-current injection density and effective base resistance for stripe transistor geometries

Simple analytic expressions are derived for the variation of base current, emitter current and emitter-base voltage along the emitter junction of a transistor structure due to the flow of dc base current parallel to the emitter junction. The effective width of the emitter region is discussed, and it is shown that as the emitter width is increased, the effective emitter width approaches a constant value. The change in base resistance with emitter current is also calculated. The use of base contacts on each side of an emitter region is discussed and the analysis is shown to be applicable to this problem for equal or unequal division of base current between the two contacts. The results are applied to a planar transistor structure.     

Determination of lifetimes and recombination currents in p-n junction solar cells, diodes, and transistors

New methods are presented and illustrated that enable the accurate determination of the diffusion length of minority carriers in the narrow regions of a solar cell or a diode. Other methods now available are inaccurate for the desired case in which the width of the region is less than the diffusion length. Once the diffusion length is determined by the new methods, this result can be combined with measured dark I-V characteristics and with small-signal admittance characteristics to enable determination of the recombination currents in each quasi-neutral region of the cell-for example, in the emitter, low-doped base, and high-doped base regions of the BSF (back-surface-field) cell. This approach leads to values for the effective surface recombination velocity of the high-low junction forming the back-surface field of BSF cells or the high-low emitter junction of HLE cells. These methods are also applicable for measuring the minority-carrier lifetime in thin epitaxial layers grown on substrates with opposite conductivity type.     

Effect of emitter contact materials on high-performance verticalp-n-p transistors

Ion-implant doped polysilicon, in situ doped polysilicon, and in situ doped ultrahigh vacuum chemical vapor deposition (UHV/CVD) low-temperature epitaxial silicon emitter contacts were used to fabricate shallow junction vertical p-n-p transistors. The effect of these materials on emitter junction depth and on device characteristics is reported. A DC current gain as high as 45 was measured on polysilicon emitter devices. Regardless of emitter contact material, all devices showed sufficiently high breakdown voltages for circuit applications. However, only for ion-implant doped polysilicon emitter devices was the narrow-emitter effect observed through the emitter-collector punchthrough voltage, emitter resistance, and current gain measurements     

A fundamental limitation for bipolar transistor scaling

A microscopic model of minority-carrier diffusion in a heavily doped emitter is proposed. Monte Carlo simulation demonstrates that statistical fluctuation in the base current is one of the fundamental limitations in high-speed applications of scaled bipolar transistors. For the transistor presently investigated, with 5.0-μm² emitter area, 0.1-μm junction depth, 8.5-ps measurement time, and 0.75-V emitter/base bias, the base current deviation is 43%. This sets up the maximum operating frequency for the transistor. More lightly doped emitters (such as for heterojunction bipolar transistors) will relax this limitation, but at a cost of increased contact resistance, especially when poly-emitters are utilized. Increasing the emitter/base bias will also make the base current rate more deterministic, but the other limitations such as power dissipation and contact resistance will become more obvious     

Bipolar transistor: Two-dimensional effects on current gain and base transit time

Two-dimensional effects in bipolar devices with small dimensions are calculated analytically if simplifying assumptions are made e.g. uniformly doped regions and zero recombination. In this paper we treat effects of the emitter sidewalls on the current gain and the base transit time.

The current flows through the base and emitter are calculated for rectangular as well as curved emitter-base junctions. Current gain and base transit time are analysed as local variables, i.e. functions that vary along the 2-D emitter-base junction. In this way it can be seen where and how several parameters influence the overall performance of the device.

The total current gain is modified by the sidewalls; it increases if the distance between the emitter-base junction and the emitter metallisation is increased. However, this distance cannot be increased indefinitely because of finite recombination lifetimes.

The diffusion capacitance is strongly affected by the sidewalls. Because the sidewalls carry a considerable part of the current the transit time is less affected. However, the influence of the sidewalls is still noticeable. It is expected that in a transistor with a 1 μm wide and 0.15 μm deep emitter the transition frequency is already negatively influenced by the emitter sidewalls.     

High emitter efficiency in InP/GaInAs HBT's with ultra high basedoping levels

The emitter efficiency of InP/GaInAs heterojunction bipolar transistors is calculated taking into account bandgap narrowing in the base, quantum mechanical tunneling, and the exact doping profile in the base. It is found that the emitter efficiency is high and does not limit the current gain of practical devices, up to a base doping level of 1×10²⁰ cm^-3, and up to 400°K . It is shown that the base emitter junction saturation current can be controlled over two orders of magnitude by a proper small displacement of the doped layer in the base     

具有超高掺杂基区的GaAs赝HBT分析

超高掺杂GaAs具有明显的禁带变窄(BGN)效应,因此采用超高掺杂基区的GaAs同质晶体管也可获得HBT的效果.故称之为赝HBT(p—HBT)。本文根据实验结果讨论了超高掺杂情况下GaAs的BGN效应及其对有效本征载流子浓度的影响,并对np~+n型结构GaAs p—HBT的发射极注入效率和共发射极电流增益进行了理论分析。结果表明,当基区掺杂浓度高于1×10~(20)/cm~3时可以获得较好的器件特性。     

射频晶体管3DG2714基极电阻R_(bb’)的分析与研究

射频晶体管具有高的特征频率fT,高的功率增益GP。为此在工程上多采用浓硼扩散形成嫁接基区。尽可能减少基极电阻Rbb’,同时采用梳状结构电极,浅结扩散,小的结面积等工艺,提高fT,从而双方面提高功率增益GP。文章以该公司生产的射频晶体管3DG2714为例,分析了发射结下基区部分电流流动状态,合理计算了这部分的基极电阻Rb1,分析了发射极与基极之间淡硼扩散区的电流流动状态,合理计算了这部分的基极电阻Rb2,忽略了两个影响极小的电阻,计算了总的基极电阻Rbb’。为减小基极电阻提高功率增益的射频晶体管设计制造提供了依据。     

Monte Carlo analysis of nonequilibrium electron transport inInAlGaAs/InGaAs HBT's

A Monte Carlo analysis of nonequilibrium electron transport in InAlGaAs/InGaAs HBTs is discussed. The discussion focuses on hot electron transport in a heavily doped p-type base from the viewpoint of high-speed transistor performance. Simulation shows that a hybrid base structure which incorporates an abrupt emitter/base junction and a graded base layer has a shorter base transit time than conventional uniform base structures with an abrupt emitter or fully graded base structures. The short base transit time is due to the high initial electron velocity provided by the abrupt emitter/base junction and the acceleration of energy-relaxed electrons (which cause a lowering of average electron velocity in the base region) by a built-in electric field in the graded base layer. The influence of hot electron injection into the collector is also considered     

Characteristics and performance of silicon solar cells between low-and high-level injection—Part II: Results of the study

Applying the extension of the transport velocity transformation method described in Part I of this paper [1], the I-V characteristics of several types of solar-cell structures have been studied in the range between the validity of the low-level or of the high-level injection assumptions. The structures investigated were a more heavily doped "wide-base diode," a lowly doped "narrow-base diode," and a structure with a high-low junction in its base. Various effects have been seen to dominate in different ranges of the I-V characteristic, causing observable changes in its slope. Depending on the particular structure, the major effects are: changes in the majority-carrier distribution, ohmic effects and changes in conductivity modulation, changes in minority-carrier lifetime, and reduction of the high-low junction barrier height, both of the latter resulting from increasing carrier concentrations. In addition, the influence of the impurity concentration in the more lowly doped layer of the base in solar cells with a high-low junction, on the conversion efficiency and on its dependence on the light intensity was investigated for optical concentration ratios up to 1000. In these studies, the doping range of 5E15-5E17/cm³was seen to yield a broad efficiency maximum, with the higher doping more favorable except for its limitation by the onset of the heavy doping effects. It has also been seen that the collection efficiency is increased or decreased by some of the effects investigated. In consequence, the collection efficiency can no longer be rigorously considered as independent of the light intensity. In some cases, this dependency occurs even when the low-level injection condition is still fulfilled.     

Ultra high-speed InP/InGaAs DHBTs with ft of 203 GHz

InP/InGaAs/InP double heterojunction bipolar transistors (DHBTs) were designed for wide band digital and analog circuits, and fabricated using a conventional mesa structure with benzocyclobutene (BCB) passivation and planarization process techniques. Our devices exhibit a maximum ft of 203 GHz, which is the highest ft for DHBTs in mainland China. The emitter size is 1.0×20 μm2. The DC current gain β is 166, and BVCEO=4.34 V. The devices reported here employ a 40 nm highly doped InGaAs base region and a 203 nm InGaAsP composite structure. They are suitable for high speed and intermediate power applications.     

On the operation configuration of SiGe HBTs based on power gain analysis

The power gain difference, under different device stability conditions, between common-emitter (CE) and common-base (CB) bipolar junction transistors (BJT) is analyzed comprehensively. The analysis reveals that the CB configuration offers higher maximum available power gain than the CE configuration in the device's high operation frequency range, while the inverse relation holds in the very low frequency range. In the intermediate frequency range, the base resistance value, mainly affected by the base doping concentration, determines which configuration offers higher maximum stable power gain (MSG). These analyses have explicit implications on the operation configurations of SiGe heterojunction bipolar transistors (HBTs). Employing a typical doping profile of Si bipolar junction transistors with a trapezoidal Ge profile in SiGe HBTs usually results in a larger base resistance than the emitter resistance. For these devices, the CE configuration exhibits higher MSG than the CB configuration. Employing a higher base doping concentration than the emitter with a box-type Ge profile considerably reduces the base resistance and thus favors the CB configuration for power amplification in this frequency range. The analysis are quantitatively verified with simulation and measurement results from SiGe HBTs of representative Ge and base doping profiles.     

Self-aligned SiGe-base heterojunction bipolar transistor byselective epitaxy emitter window (SEEW) technology

In the device a SiGe epitaxial base is integrated in a structure which uses in situ doped epitaxial lateral overgrowth for the formation of the emitter window and the extrinsic base contact. Nearly ideal I -V characteristics have been achieved for a base width of 60 nm with an intrinsic base resistance of 4.6 kΩ/□ and for emitter widths down to 0.4 μm. A DC collector current enhancement factor of 3.1 was obtained relative to a Si homojunction transistor with a 1.25 times higher intrinsic base resistance. The breakdown voltage BV_CBO is identical for both Si and SiGe devices, even though the collector-base depletion region is partly overlapped with the reduced-bandgap SiGe strained layer. The lower BV_CEO, measured for the SiGe-base transistor, is due to the higher current gain. Based on these results the fabrication of high-speed bipolar circuits that take advantage of SiGe-base bandgap engineering seems possible using selective epitaxy emitter window (SEEW) technology