High-gain Al0.48In0.52As/Ga0.53As vertical n-p-n heterojunction bipolar transistors grown by molecular-beam epitaxy

We report the first vertical n-p-n heterojunction bipolar transistors formed in the (Al,In)As/(Ga,In)As alloy system. The structures grown by molecular-beam epitaxy (MBE) use a wide band-gap (Eg = 1.44 eV) Al0.48In0.52As emitter on a lower band gap (Eg = 0.73 eV) Ga0.47In0.53As base 2500 Å in width. Transistors with both abrupt and graded heterojunction emitters were demonstrated with dc current gains of 140 and 280, respectively, at a collector current of 15 mA. The (Al,In)As/(Ga,In)As heterojunction transistors offer the attractive possibility of optical integration with long wavelength lasers and photodetectors.     

Measurement of avalanche multiplication of injected holds in germanium p-n-p diffused-base transistors

A three-terminal dc measurement is made of avalanche multiplication of holes injected from the emitter into the collector junction of diffused-base germanium p-n-p transistors. The method is applicable to asymmetric thin-base transistors at currents at which dc base current is independent of base width when emitter to base voltage (VEB) is held constant. Injected collector current and VEBare measured at constant emitter current as a function of collector to base voltage (V). VEBis measured to take base narrowing into account. Collector reverse current is balanced out. The advantages of this method over two-terminal measurements are that surface, space-charge recombination and internal field emission currents are balanced out and no a priori assumption of the form of the multiplication factorM(V)is needed. For collector barriers which are nearly stepn^{+}pjunctions with the p-type resistivity in the neighborhood of 1 ohm-cm, the multiplication data, which cover the range1.05 siml M siml 2.0, fit the Miller equationM^{-1} = 1 - (V/V_{B})^{n} n = 3.2 pm 0.2in agreement with Miller's two-terminal measurements using alloy and grown-junction Ge transistors. The parameters VBfrom extrapolation of the present data agree within experimental error of five per cent with the collector diode breakdowns BVCBOmeasured by Miller. However, in the present measurements on diffused-base transistors, appreciable surface current multiplication occurs with the result that BVCBOmeasured at 1 ma is approximately 18 per cent less than VB. The absence of microplasma noise in the multiplied injected hole current indicates that multiplication occurs uniformly in the collector barrier under the emitter at least toM = 2.     

Avalanche injection and second breakdown in transistors

A rapid type of second breakdown observed in silicon n⁺-p-n-n⁺transistors is shown to be due to avalanche injection at the collector n-n⁺junction. Localized thermal effects, which are usually associated With second breakdown, are shown to play a minor role in the initiation of the transition to the low voltage state. A useful tool in the analysis of avalanche injection is the n⁺-n-n⁺diode, which exhibits negative resistance at a critical voltage and current. A close correspondence between the behavior of the diode and the transistor (open base) is established both theoretically and experimentally. Qualitative agreement with the proposed model is obtained for both directions of base current flow. It is shown that transistors having thin, lightly doped collector regions are particularly susceptible to avalanche injection, which suggests that some compromise may be necessary in the design of high-frequency power transistors.     

Pseudomorphic AlInP/InP heterojunction bipolar transistors

Novel InP-based heterojunction bipolar transistors (HBTs) using an AlInP pseudomorphic emitter, together with an InP base and collector, have been fabricated. By using InP as both base and collector, the advantage of high electron velocity and high breakdown field of InP collectors are obtained without the problem associated with the energy barrier between the more standard InGaAs/InP base and collector heterojunction. Epitaxial layers were grown by gas-source molecular beam epitaxy (GSMBE). The 200 Å pseudomorphic emitter had an aluminium fraction of 15%, sufficiently suppressing hole injection from the base. The DC gain for 40×40 μm² devices reached 18. The breakdown voltage BV_CEO of 10 V is an improvement over devices with InGaAs base and collector layers     

Leakage current mechanisms in SiGe HBTs fabricated using selectiveand nonselective epitaxy

SiGe heterojunction bipolar transistors (HBTs) have been fabricated using selective epitaxy for the Si collector, followed in the same growth step by nonselective epitaxy for the p⁺ SiGe base and n-Si emitter cap. DC electrical characteristics are compared with cross-section TEM images to identify the mechanisms and origins of leakage currents associated with the epitaxy in two different types of transistor. In the first type, the polysilicon emitter is smaller than the collector active area, so that the extrinsic base implant penetrates into the single-crystal Si and SiGe around the perimeter of the emitter and the polycrystalline Si and SiGe extrinsic base. In these transistors, the Gummel plots are near-ideal and there is no evidence of emitter/collector leakage. In the second type, the collector active area is smaller than the polysilicon emitter, so the extrinsic base implant only penetrates into the polysilicon extrinsic base. In these transistors, the leakage currents observed depend on the base doping level. In transistors with a low doped base, emitter/collector and emitter/base leakage is observed, whereas in transistors with a high doped base only emitter/base leakage is observed. The emitter/collector leakage is explained by punch through of the base caused by thinning of the SiGe base at the emitter perimeter. The emitter/base leakage is shown to be due to a Poole-Frenkel mechanism and is explained by penetration of the emitter/base depletion region into the p⁺ polysilicon extrinsic base at the emitter periphery. Variable collector/base reverse leakage currents are observed and a variety of mechanisms are observed, including Shockley-Read-Hall recombination, trap assisted tunneling, Poole-Frenkel and band to band tunneling. These results are explained by the presence of polysilicon grains on the sidewalls of the field oxide at the collector perimeter     

High-temperature DC characteristics of AlxGa0.52-xIn0.48P/GaAs heterojunction bipolar transistors grownby metal organic vapor phase epitaxy

A series of Al_xGa_0.52-xIn_0.48P/GaAs heterojunction bipolar transistors (HBT's) with x=0 to x=0.52 showed ideality factors close to unity for both base current and collector current and small variation in gain with temperature up to at least T=623 K across the whole range of x composition. Hole current injection from the base into the emitter in these devices was shown to be negligible. The current gain, β, which is temperature insensitive was thought to be limited by bulk base recombination for x⩽0.3 and recombination at the graded emitter region for x>0.3. The optimum emitter composition (highest β, and good β stability with collector current and temperature) was found to be x=0.18-0.30. Useful transistor action with very high gain and output resistance is possible up to at least T=623 K, limited only by the thermal performance of the unoptimized ohmic contacts employed in the devices     

Complementary MOS—Bipolar transistor structure

Bipolar transistors can be used to increase the driving capabilities of complementary MOS transistors while retaining the low power dissipation feature. The fabrication of n-p-n bipolar transistors is compatible with the fabrication of the complementary MOS transistors in a monolithic structure. Common collector n-p-n transistors can be fabricated using a diffused n⁺source-drain region as emitter, a diffused p^-isolation region as base and an n^-substrate as collector with a hfegreater than 100. Lateral n-p-n transistors can be fabricated using a diffused n⁺source-drain region as emitter and collector, and p^-isolation region as base with a hfegreater than 10.     

Determination of bandgap narrowing and parasitic energy barriers inSiGe HBT's integrated in a bipolar technology

This paper describes a method for characterizing the bandgap narrowing and parasitic energy barrier in SiGe heterojunction bipolar transistors (HBTs), fabricated using a single-polysilicon self-aligned bipolar process. From a comprehensive study of the temperature dependence of the collector current, the bandgap narrowing in the base due to germanium has been dissociated from that due to the heavy dopant concentration. The same approach has been used to characterize the height and width of parasitic energy barriers which appear when boron out-diffusion from the SiGe base is present. The method has been applied to SiGe heterojunction bipolar transistors fabricated using a single polysilicon, self-aligned, bipolar process, as well as mesa transistors. The experimental results show that small geometry transistors have degraded collector currents due to boron out-diffusion around the perimeter of the emitter. This behavior has been explained by accelerated boron diffusion due to point defects generated during the extrinsic base implant. The values of undoped SiGe spacer thickness needed to suppress the parasitic energy barrier are described. Finally, high-frequency results are reported, which correlate the frequency transition to these parasitic energy barriers     

In0.52Al0.48As/In0.53Ga0.47As double-heterojunction p-n-p bipolar transistors grown bymolecular beam epitaxy

P-n-p In_0.52Al_0.48As/In_0.53Ga_0.47 As double-heterojunction bipolar transistors with a p⁺-InAs emitter cap layer grown by molecular-beam epitaxy have been realized and tested. A five-period 15-Å-thick In_0.53Ga_0.47As/InAs superlattice was incorporated between the In_0.53Ga_0.47As and InAs cap layer to smooth out the valence-band discontinuity. Specific contact resistance of 1×10^-5 and 2×10^-6 Ω-cm² were measured for nonalloyed emitter and base contacts, respectively. A maximum common emitter current gain of 70 has been measured for a 1500-Å-thick base transistor at a collector current density of 1.2×10³ A/cm². Typical current gains of devices with 50×50-μm² emitter areas were around 50 with ideality factors of 1.4     

Effect of hot-electron injection of high-frequency characteristicsof abrupt In0.52(Ga1-xAlx)0.48As/InGaAs HBT's

The effect of hot-electron injection energy (E_i) into the base on the high-frequency characteristics of In_0/52(Ga_1-xAl_x)_0.48 As/InGaAs abrupt heterojunction bipolar transistors (HBTs) is investigated by changing the composition of the emitter. There exists an optimum E_i at which a maximum current gain cutoff frequency (ft) is obtained. Analysis of hot-electron transport in the base and collector by Monte Carlo simulation is carried out to understand the above phenomenon. The collector transit time (τ_c ) increases with E_i, because electrons with higher energy transfer from the Γ valley into the upper L and X valleys. At first, the base transit time (τ_b ) decreases with E_i at the low E_i region. However, τ_b does not decrease monotically with E_i, because of the nonparabolicity in the energy-band structure of InGaAs. Consequently, there exists a minimum in the sum of τ_b and τ_c , in other words a maximum f_t, at an intermediate value of E_i     

Second breakdown of double epitaxial transistors

This paper deals with the second breakdown of transistors with epitaxial collector, epitaxial base, and diffused emitter. Transistors were fabricated with base width WBin the range of 2 to 18 µ and resistivity in the range of 0.1 to 10 ohm . cm. The optimum values of the resistivity and the thickness of these regions were calculated by computer techniques. The devices were mounted onto a TO-63 header and the base and the emitter leads were bonded onto the device ultrasonically. The electrical characteristics, including the frequency response ftand secondary breakdownS/Bcapability, were tested. For the measurement of second breakdown current IM, forward bias condition was used. It was found that for fixed collector and emitter parameters, IMwas controlled by the product of base resistivity ρBand base width WB. The value of IMwas found to increase withrho_{B}W_{B}. However, for a specified device characteristic, an optimum value ofrho_{B}W_{B}was found to exist. For transistors withV_{CEO} =150volts,f_{t}=20mHz andh_{FE}=20, the optimum value ofrho_{B}W_{B}was found to be 6 × 10^-4ohm . cm².     

Location of 1/f noise sources in BJT's and HBJT's—I. theory

The most general case of1/fnoise in transistors can be described by three independent noise current generators: ibebetween base and emitter, ibcbetween base and collector, and iecbetween emitter and collector. By short-circuiting the base and the collector to ground and comparing the base and collector noise spectraS_{IB}(f)andS_{IC}(f)for the case of zero feedback from the emitter with the base and collector noise spectraS'_{IB}(f)andS'_{IC}(f)for the case of strong feedback from the emitter, one can evaluate the relative strength of the three noise sources. By measuring the current dependence ofS_{IB}(f),S_{IC}(f),S'_{IB}(f), andS'_{IC}(f), one can assign physical processes to the current generators ibc, ibe, and iec. It is the aim of this paper to demonstrate theoretically a simple method for locating1/fnoise sources in BJT's and HBJT's by comparing the base and collector1/fnoise for the cases without and with strong emitter feedback. In later papers we shall demonstrate experimentally how this method is applied to practical situations.     

High-Current-Gain InP/GaInP/GaAsSb/InP DHBTs With fT = 436 GHz

Combining a pseudomorphically strained (Ga,In)P emitter with a GaAs_0.6Sb_0.4 base effectively eliminates the emitter heterojunction type-II conduction band offset in InP/GaAsSb double heterojunction bipolar transistors (DHBTs). A peak f_T of 436 GHz at J_C = 10 mA/mum², with BV_CEO = 3.8 V, is achieved with 0.6 times 5 mum² InP/GalnP/GaAsSb DHBTs with a 75-nm InP collector. Compared to a binary InP emitter, the (Ga,In)P emitter doubles the DC current gain from 166 to 338 for otherwise identical devices. These are the highest DC current gain and cutoff frequencies to date in uniform base GaAsSb DHBTs. The gain improvement reported here will greatly facilitate device design tradeoffs that are encountered while scaling InP/GaAsSb DHBTs toward higher frequencies by allowing higher base doping levels and smaller emitter geometries.     

Lateral PNP GaAs bipolar transistor with minimized substrate current

Lateral pnp bipolar transistors have been fabricated using Be implantation to define the emitter and collector areas. The base area (1 - 2 µm wide) has been protected against Be ions during implantation by SiO2and photoresist. The lateral straggling and diffusion during the anneling process reduces the base width, which can be adjusted with the annealing temperature and time. Between the active n-GaAs layer and substrate, a n-Ga0.7Al0.3As layer is deposited. The Be ions penetrating the GaAs/GaAlAs interface form a pn junction in the GaAlAs layer below the emitter and collector area. This reduces the current by several orders of magnitude through the parasitic emitter-substrate (base) diode compared to a GaAs pn junction, due to the higher band gap. For these devices with an effective base width of 0.5 µm, a current gain of 10 in common emitter configuration has been obtained.     

Photo carrier generation in bipolar transistors

Anomalous substrate currents have been observed in SiGe bipolar NPN-transistors, dependent on the collector bias, at high current levels. These currents appear to originate from light that is generated in the collector base junction when it is reverse biased. This light generates electron hole pairs in the n⁺ buried layer-substrate diode, yielding a considerable substrate current. This paper will show that these substrate currents can be used as a useful monitor for the occurrence of avalanche multiplication and high-level injection (Kirk effect) in heterojunction bipolar transistors (HBTs)     

High/fmax collector-up AlGaAs/GaAsheterojunction bipolar transistors with a heavily carbon-doped basefabricated using oxygen-ion implantation

AlGaAs/GaAs collector-up heterojunction bipolar transistors (HBTs) with a heavily carbon-doped base layer were fabricated using oxygen-ion implantation and zinc diffusion. The high resistivity of the oxygen-ion-implanted AlGaAs layer in the external emitter region effectively suppressed electron injection from the emitter, allowing collector current densities to reach values above 10⁵ A/cm ². For a transistor with a 2-μm×10-μm collector, f_T was 70 GHz and f_max was as high as 128 GHz. It was demonstrated by on-wafer measurements that the first power performance of collector-up HBTs resulted in a maximum power-added efficiency of as high as 63.4% at 3 GHz     

A model-based comparison of AlInAs/GaInAs and InP/GaInAs HBT's: aMonte Carlo study

The high-speed performances of AlInAs/GaInAs and InP/GaInAs heterojunction bipolar transistors (HBTs) are investigated using a one-dimensional self-consistent particle simulator. Optimum alloy compositions for a graded-gap base structure are obtained for both transistors through the tradeoff between the emitter-charging time and base transit time. The saturation velocity in the GaInAs n-type collector is found to be smaller than that in InP, which has been attributed to the diffusion of a large number of hot back-scattered Γ-valley electrons in the GaInAs collector. The difference in the collector transit time in p-type collectors is trivial, since the maximum electron velocity was restricted to below 1.2×10⁸ cm/s due to a strong nonparabolicity effect. The cutoff frequency for the former and the latter are estimated to be 2 and 1.5 times higher, respectively, than for AlGaAs/GaAs HBTs. These results are attributed to a larger bandgap difference between the emitter and base, to yield a high base built-in field, rather than a larger Γ-L band separation energy in the collector to enhance the velocity overshoot effect     

Fabrication and characterization of bipolar transistors within-situ doped low-temperature (800°C) epitaxial silicon

The authors report the fabrication of bipolar transistors at a maximum process temperature of 800°C, utilizing in situ doped low-temperature epitaxial silicon deposited by ultralow-pressure chemical vapor deposition (U-LPCVD), and their subsequent characterization. The epitaxial silicon layers form the collector, base, and emitter layers. To attain a high donor concentration in the epitaxial emitter layer, the U-LPCVD process is plasma enhanced. Transistors having excellent DC characteristics down to collector currents of ~10 pA/μm² are obtained, which indicates that the bulk quality of the epitaxial films is good enough for device fabrication     

Application of selective epitaxial silicon and chemo-mechanicalpolishing to bipolar transistors

Successful demonstration of single-polysilicon bipolar transistors fabricated using selective epitaxial growth (SEG) and chemo-mechanical polishing (CMP) is reported. The pedestal structure made possible by the SEG/CMP process combination results in significantly reduced extrinsic-base collector capacitance. Cut-off frequency (f_T) of devices with emitter stripe width of 1 μm, a base width of 110 nm, and a peak base doping of 3×10¹⁸ cm^-3 have been observed to improve from 16 GHz to 22 GHz when the extrinsic-base collector overlap is decreased from 1 μm to 0.2 μm. Leakage current, often a problem for SEG structures, has been reduced to 27 nA/cm² for the area component, and 10 nA/cm for the edge component, by (1) appropriate post-polish processing, including a high-temperature anneal and sacrificial oxidation, (2) aligning the device sidewalls along the 〈100〉 direction, and (3) the presence of the pedestal structure. Base-emitter junction nonideality in these transistors has also been investigated     

Minority carrier injection characteristics of the diffused emitter junction

For the double-diffused transistor, a one-dimensional analysis is presented on the minority carrier injection properties of a diffused emitter junction. This junction is bounded on one side by a reverse biased collector and on the other by an ohmic contact of arbitrary recombination velocity. Furthermore, arbitrary magnitudes of minority carrier lifetime are assumed in both the emitter and base regions of this semiconductor device. Injection efficiency characteristics are graphically illustrated throughout a wide range of physical and geometrical parameters. Assuming, for example, variations in the emitter junction depth, injection properties are demonstrated for transistors exhibiting a fixed collector location and also for transistors exhibiting a fixed base width. A comparison is also shown between the calculated minority carrier injection from this analysis and from other, more approximate, methods.