Molecular beam epitaxial GaAs/AlGaAs heterojunction bipolartransistors on (311)A GaAs substrates with all-silicon doping

The authors have investigated the characteristics and reproducibility of Si-doped p-type (311)A GaAs layers for application to heterojunction bipolar transistors (HBTs) grown by molecular beam epitaxy (MBE). The authors obtained p=2.2×10¹⁹ cm^-3 in a layer grown at 670°C. They have used all-Si doping to grow n-p-n transistors. These devices exhibit excellent DC characteristics with β=230 in a device with base doping of p=4×10¹⁸ cm^-3     

GaN HBT: toward an RF device

This paper reviews efforts to develop growth and fabrication technology for the GaN HBT. Conventional devices are grown by plasma assisted MBE on MOCVD GaN templates on sapphire. HBTs were fabricated on LEO material identifying threading dislocations as the primary source of collector-emitter leakage which was reduced by four orders of magnitude for devices on nondislocated material. Base doping studies show that the mechanism of this leakage is localized punch-through caused by compensation near the dislocation. High contact and lateral resistance in the base cause large parasitic common emitter offset voltages (from 1 to 5 V) in GaN HBTs. The effect of this voltage drop on common emitter characteristics is discussed. The combination of this voltage drop and the emitter collector leakage make Gummel and common base characteristics unreliable without verification with common emitter characteristics. The selectively regrown emitter bipolar transistor is presented with a DC current gain of 6 and early voltage greater than 400 V. The transistor operated to voltages over 70 V. This device design reduces base contact resistance, and circumvented difficulties associated with the emitter mesa etch process. The Mg memory effect in MOCVD grown GaN HBTs is discussed, and MBE grown device layers are shown to produce sharp doping profiles. The low current gain of these devices is discussed, and an HBT with a compositionally graded base is presented, as well as simulations predicting further current gain improvements with base grading     

InP基MOCVD及MBE外延生长HBT的制备与分析

设计并研制了用于光电集成(OEIC)的InP基异质结双极晶体管(HBT),介绍了工艺流程及器件结构。分别采用金属有机化学气相沉积(MOCVD)及分子束外延(MBE)生长的外延片,并在外延结构、工艺流程相同的条件下,对两种生长机制的HBT直流及高频参数进行和分析。结果表明,采用MOCVD生长的InP基HBT,直流增益为30倍,截止频率约为38GHz;MBE生长的HBT,直流增益达到100倍,截止频率约为40GHz。这表明,MBE生长的HBT外延层质量更高,在相同光刻条件下,所对应的HBT器件的性能更好。     

Heterojunction bipolar transistors with SiGe base grown bymolecular beam epitaxy

High-quality SiGe heterojunction bipolar transistors (HBTs) have been fabricated using material grown by molecular beam epitaxy (MBE). The height of parasitic barriers in the conduction band varied over the wafer, and the influence of these barriers on controller current, early voltage, and cutoff frequency were studied by experiments and simulations. Temperature-dependent measurements were performed to study the influence of the barriers on the effective bandgap narrowing in the base and to obtain an expression for the collector-current enhancement. From temperature-dependent measurements, the authors demonstrate that the collector-current enhancement of the HBTs can be described by a single exponential function with a temperature-independent prefactor     

Resolving degradation mechanisms in ultra-high performance N-p-nheterojunction bipolar transistors

The conduction band barrier caused by base dopant outdiffusion is investigated for MBE-grown Al_xGa_1-xAs/GaAs N-p-n linearly graded heterojunction bipolar transistors (HBTs). The change of the B-E heterojunction conduction band barrier can be directly revealed by a novel technique based on the diffusion-thermionic emission current model. This is accomplished by measuring the inverted collector current ratio at two different heterojunction reverse biases. In addition, this ratio is found to correlate with an anomalous base current component measured at low temperature. The heterojunction potential barrier and the anomalous base current component are attributed to beryllium redistribution during MBE growth and forward current stress. This suggests that the diffusion and incorporation of beryllium dictate V _BE uniformity and long-term reliability of HBTs     

Rapid isothermal processing for fabrication of GaAs-basedelectronic devices [HBTs]

The use of rapid isothermal processing (RIP) is detailed for each of the three annealing steps in the fabrication of heterostructure-based devices such as heterojunction bipolar transistors (HBTs). RIP can be used for the alloying of ohmic metal contacts, annealing of ion-bombarded regions for device isolation or parasitic capacitance reduction, and for conventional implant activation annealing. High-speed (f_T=65 GHz) HBTs were achieved using RIP for all of the required heating steps. The authors compare the use of several types of silicon carbide-coated graphite susceptors for eliminating slip formation on 2- and 3-in-diameter GaAs wafers during high-temperature implant activation annealing     

Heterojunction bipolar transistors using Si-Ge alloys

Advanced epitaxial growth techniques permit the use of pseudomorphic Si_1-xGe_x alloys in silicon technology. The smaller bandgap of these alloys allows for a variety of novel band-engineered structures that promise to enhance silicon-based technology significantly. The authors discuss the growth and properties of pseudomorphic Si_1-xGe_x structures and then focus on their applications, especially the Si_1-xGe_x -base heterojunction bipolar transistor (HBT). They show that HBTs in the Si_1-xGe_x system allow for the decoupling of current gain and intrinsic base resistance. Such devices can be made by using a variety of techniques, including molecular-beam epitaxy and chemical vapor deposition. The authors describe the evolution of fabrication schemes for such HBTs and describe the DC and AC results obtained. They show that optimally designed HBTs coupled with advanced bipolar structures can provide performance leverage     

High-speed GaAlAs-GaAs heterojunction bipolar transistors with near-ballistic operation

GaAlAs-GaAs heterojunction bipolar transistors (HBTs) with an abrupt emitter-base interface have been realised by molecular-beam epitaxy (MBE) on semi-insulating substrates. A gain-bandwidth product FT of 15 GHz has been measured for Ic = 20 mA and VCE = 8 V. These results are the best reported so far for HBTs and are very promising for high-speed logic.     

InP/InGaAs HBTs with n+-InP contacting layers grown byMOMBE using SiBr4

InP/InGaAs heterojunction bipolar transistors (HBTs) with low resistance, nonalloyed TiPtAu contacts on n⁺-InP emitter and collector contacting layers have been demonstrated with excellent DC characteristics. A specific contact resistance of 5.42×10^-8 Ω·cm², which, to the best of our knowledge, is the lowest reported for TiPtAu on n-InP, has been measured on InP doped n=6.0×10¹⁹ cm^-3 using SiBr₄. This low contact resistance makes TiPtAu contacts on n-InP viable for InP/InGaAs HBTs     

Submicron scaling of HBTs

The variation of heterojunction bipolar transistor (HBT) bandwidth with scaling is reviewed. High bandwidths are obtained by thinning the base and collector layers, increasing emitter current density, decreasing emitter contact resistivity, and reducing the emitter and collector junction widths. In mesa HBTs, minimum dimensions required for the base contact impose a minimum width for the collector junction, frustrating device scaling. Narrow collector junctions can be obtained by using substrate transfer or collector-undercut processes or, if contact resistivity is greatly reduced, by reducing the width of the base ohmic contacts in a mesa structure. HBTs with submicron collector junctions exhibit extremely high f_max and high gains in mm-wave ICs. Transferred-substrate HBTs have obtained 21 dB unilateral power gain at 100 GHz. If extrapolated at -20 dB/decade, the power gain cutoff frequency f_max is 1.1 THz. f_max will be less than 1 THz if unmodeled electron transport physics produce a >20 dB/decade variation in power gain at frequencies above 110 GHz. Transferred-substrate HBTs have obtained 295 GHz f_T. The substrate transfer process provides microstrip interconnects on a low-ϵ_r polymer dielectric with a electroplated gold ground plane. Important wiring parasitics, including wiring capacitance, and ground via inductance are substantially reduced. Demonstrated ICs include lumped and distributed amplifiers with bandwidths to 85 GHz and per-stage gain-bandwidth products over 400 GHz, and master-slave latches operating at 75 GHz     

Metallurgical stability of ohmic contacts on thin baseInP/InGaAs/InP HBT's

The metallurgical stability of ohmic contacts: Pt, Pt/Ti, Au/Ti, Au/Pt/Ti, and Au/Pt/Ti/W, on a 500 Å thick p⁺-InGaAs base of InP/InGaAs/InP HBTs have been investigated as a function of anneal temperature. All contacts were stable after a 300°C-30 s anneal. Pt contact failed at 350°C whereas Pt/Ti, Au/Ti, and Au/Pt/Ti contacts failed at 400°C. The failure mechanism was a collector leakage short owing to the penetration of Pt or Ti through the thin base. Only HBTs with Au/Pt/Ti/W contact were still functional after a 400°C anneal with no apparent shift in the turn-on voltage for the emitter and collector junctions     

MBE-grown Si/SiGe HBTs with high β, fT,and fmax

Si/SiGe heterojunction bipolar transistors (HBTs) were fabricated by growing the complete layer structure with molecular beam epitaxy (MBE). The typical base doping of 2×10¹⁹ cm^-3 largely exceeded the emitter impurity level and led to sheet resistances of about 1 kΩ/□. The devices exhibited a 500-V Early voltage and a maximum room-temperature current gain of 550, rising to 13000 at 77 K. Devices built on buried-layer substrates had an f_max of 40 GHz. The transit frequency reached 42 GHz     

Reliability characteristics of ohmic contacts for AlGaAs/GaAs HBTs

The thermal stability of ohmic contacts for AlGaAs/GaAs HBTs is presented. Ti/Pt/Au, Ti/Pt/Au and Cr/Pt/Au, and AuGe/Ni/Ti/Pt/Au were investigated for emitter, base, and collector contacts, respectively. As a result of 200 degrees C storage tests, it was found that these contacts did not limit the reliability of AlGaAs/GaAs HBTs.<>     

Reliability characteristics of GaAs and InP-based heterojunction bipolar transistors

The reliability characteristics of Heterojunction Bipolar Transistor made on GaAs and InP substrates are reviewed and ways of improving them by design, growth and processing are described. Materials for HBTs are grown by a variety of techniques (MBE, MOCVD, CBE). Their choice is made based on considerations such as satisfaction of design requirements and manufacturing but also dopant incorporation without risk of diffusion, as well as, minimization of hydrogen incorporation which may result in device degradation. The minority carrier lifetime in the base is influenced by hydrogen incorporation in C-InGaAs. Conventional alloyed and non-alloyed, as well as, refractory metallization schemes are considered for best reliability performance. The dielectric deposition scheme used for passivation plays a major role on device reliability. Good reliability performance is reported for GaAs but also for InP-based HBTs. A correlation is finally reported to exist between the low-frequency noise properties of HBTs and their reliability characteristics.     

Monolithic HEMT-HBT integration by selective MBE

We have achieved successful monolithic integration of high electron mobility transistors and heterojunction bipolar transistors in the same microwave circuit. We have used selective molecular beam epitaxy and a novel merged processing technology to fabricate monolithic microwave integrated circuits that incorporate both 0.2 μm gate-length pseudomorphic InGaAs-GaAs HEMTs and 2 μm emitter-width GaAs-AlGaAs HBTs. The HEMT and HBT devices produced by selective MBE and fabricated using our merged HEMT-HBT process exhibited performance equivalent to devices fabricated using normal MBE and our baseline single-technology processes. The selective MBE process yielded 0.2 μm HEMT devices with g_m=600 mS/mm and f_T=70 GHz, while 2×10 μm² HBT devices achieved β>50 and f_T=21.4 GHz at J_c=2×10⁴ A/cm². The performance of both a 5-10 GHz HEMT LNA with active on-chip HBT regulation and a 20 GHz Darlington HBT amplifier are shown to be equivalent whether fabricated using normal or selective MBE     

Effects of intermediate semiconductor layers on carrier transport mechanisms through p-ZnSe/metals interfaces

The effects of formation of intermediate semiconductor layers at p-ZnSe/metals interfaces on carrier transport mechanisms were studied by comparing contacts prepared by the deposition and annealing (DA) technique or the molecular beam epitaxy (MBE) technique. Current density vs voltage (J-V) curves of the MBE contact with a p-ZnSe/p-ZnTe superlattice intermediate layer showed ohmic behavior. However, J-V curves of the DA contact with a ZnTe intermediate layer showed rectifying behavior. The difference of the electrical properties between these two contacts was due to existence of a highly resistive intermediate layer with highly dense defects in the DA contact and a low resistance p-type conductive intermediate layer with relatively small densities of crystalline defects in the MBE contacts. From the present results, it was concluded that formation of the highly resistive semiconductor layer with dense crystalline defects prevented the DA contact to transit from non-ohmic J-V behavior to ohmic.     

High gain AllnAs/GaAsSb/AllnAs NpN HBTs on InP

We report the first growth and characterization of high gain double heterojunction NpN HBTs on InP with a lattice-matched GaAs₅Sb₅ base layer. This AllnAs/GaAsSb heterojunction has almost no discontinuity in the conduction band edge, eliminating the need to grade the emitter-to-base heterojunction to achieve optimal carrier injection. The layers were grown in a solid source MBE system, using tetramer As₄ and Sb₄ sources. Be is an efficient acceptor in the GaAsSb, but the mobility is about half that measured inp type GaAs on GaAs substrates. The HBTs fabricated were large area mesa isolated transistors, with a beta of 80 at a current density of 2 kA/cm², and the gain remained high at lower current densities. The turnon voltage,V _be, is only 0.45 V at a current density of 2 A/cm².     

Bidirectional bistability in n-p-nSi/Si1-xGex/Si structures

Several structures of n-Si/p-Si_1-xGe_x/n-Si double heterojunction bipolar transistors (DHBTs) with strained thin base, fabricated by molecular beam epitaxy (MBE), are described. Negative differential resistance (NDR) phenomena-a strong and symmetric bidirectional bistability modulated by base bias, together with a multistep characteristic in collector current versus emitter-collector bias voltage in the devices with very thin base-were observed at room temperature. The physical origins are analyzed. The results are compared with the characteristics of n-Ga_1-xA1_xAs/p-GaAs/n-GaAs single HBTs (SHBTs)     

Integrated npn/pnp GaAs/AlGaAs HBTs grown by selective MBE

npn and pnp GaAs/AlGaAs heterojunction bipolar transistors have been successfully fabricated on the same GaAs substrate using selective molecular beam epitaxy and a new merged HBT processing technology. The DC and microwave characteristics of the transistors are equivalent to those of similar HBTs grown by conventional MBE on separate GaAs substrates.<>     

Symmetric P-n-P InAlAs/InGaAs double-heterojunction bipolartransistors fabricated with Si-ion implantation

The authors have successfully fabricated symmetric P-n-P InAlAs/InGaAs double-heterojunction bipolar transistors (DHBTs) using self-aligned Si-ion implantation and refractory emitter contacts with current gains of 115 and 30 in the emitter-up and the emitter-down configurations, respectively. Two thin Be-doped In_0.53Ga_0.47As layers inserted on both sides of base lead to the excellent I-V characteristics. The authors have shown that hole injection from the external portions of the emitter should be suppressed by a factor of 10^-5 to 10^-3 at a collector current density of about 10³ A/cm² , which is much smaller than that of N-p-n GaAs/AlGaAs HBTs and DHBTs are promising devices for applications to circuits with low power dissipation