G.L. Patton

75-GHz f/sub T/ SiGe-base heterojunction bipolar transistors

The fabrication of silicon heterojunction bipolar transistors which have a record unity-current-gain cutoff frequency (f/sub T/) of 75 GHz for a collector-base bias of 1 V, an intrinsic base sheet resistance (R/sub bi/) of 17 k Omega / Square Operator , and an emitter width of 0.9 mu m is discussed. This performance level, which represents an increase by almost a factor of 2 in the speed of a Si bipolar transistor, was achieved in a poly-emitter bipolar process by using SiGe for the base material. The germanium was graded in the 45-nm base to create a drift field of approximately 20 kV/cm, resulting in an intrinsic transit time of only 1.9 ps.<<ETX>>

Heterojunction bipolar transistors using Si-Ge alloys

Advanced epitaxial growth techniques permit the use of pseudomorphic Si/sub 1-x/Ge/sub 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/sub 1-x/Ge/sub x/ structures and then focus on their applications, especially the Si/sub 1-x/Ge/sub x/-base heterojunction bipolar transistor (HBT). They show that HBTs in the Si/sub 1-x/Ge/sub 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. >

Graded-SiGe-base, poly-emitter heterojunction bipolar transistors

Si/Si/sub 1-x/Ge/sub x/ heterojunction bipolar transistors (HBTs) fabricated using a low-temperature epitaxial technique to form the SiGe graded-bandgap base layer are discussed. These devices were fabricated on patterned substrates and subjected to annealing cycles used in advanced bipolar processing. These devices, which have base widths under 75 mm, were found to have excellent junction qualities. Due to the small bandgap of SiGe, the collector current at low bias is ten times higher than that for Si-base devices that have a pinched base resistance. This collector current ratio increases to more than 40 at LN/sub 2/ temperature resulting in current gains of 1600 for the SiGe-base transistors despite base sheet resistances as low as 7.5 k Omega / Square Operator .<<ETX>>

On the profile design and optimization of epitaxial Si- and SiGe-base bipolar technology for 77 K applications. I. Transistor DC design considerations

The DC design considerations associated with optimizing epitaxial Si- and SiGe-base bipolar transistors for the 77-K environment are examined in detail. Transistors and circuits were fabricated using four different vertical profiles, three with a graded-bandgap SiGe base, and one with a Si base for comparison. All four epitaxial-base profiles yield transistors with DC properties suitable for high-speed logic applications in the 77-K environment. The differences between the low-temperature DC characteristics of Si and SiGe transistors are highlighted both theoretically and experimentally. A performance tradeoff associated with the use of an intrinsic spacer layer to reduce parasitic leakage at low temperatures and the consequent base resistance degradation due to enhanced carrier freeze-out is identified. Evidence that a collector-base heterojunction barrier effect severely degrades the current drive and transconductance of SiGe-base transistors operating at low temperatures is provided. >

Silicon-germanium base heterojunction bipolar transistors by molecular beam epitaxy

The devices were fabricated using molecular-beam epitaxy (MBE), low-temperature processing, and germanium concentrations of 0, 6%, and 12%. The transistors demonstrate current gain, and show the expected increase in collector current as a result of reduced bandgap due to Ge incorporation in the base. For a 1000-AA base device containing 12% Ge, a six-times increase in collector current was measured at room temperature, while a 1000-times increase was observed to 90 K. The temperature dependence of the collector current of the Si/sub 0.88/Ge/sub 0.12/ base transistor is consistent with a bandgap shrinkage in the base of 50 meV. For the homojunction transistors, base widths as thin as 800 AA were grown, corresponding to a neutral base width of no more than 400 AA.<<ETX>>

Profile leverage in self-aligned epitaxial Si or SiGe base bipolar technology

The authors have developed a planar, self-aligned, epitaxial Si or SiGe-base bipolar technology and explored intrinsic profile design leverage for high-performance devices in three distinct areas: transit time reduction, collector-base (CB) junction engineering, and emitter-base (EB) junction engineering. High f/sub T/ Si (30-50 GHz) and SiGe (50-70 GHz) epi-base devices were integrated with trench isolation and polysilicon load resistors to evaluate ECL (emitter coupled logic) circuit performance. A 15% enhancement in ECL circuit performance was observed for SiGe relative to Si devices with similar base doping profiles in a given device layout. Minimum SiGe-base ECL gate delays of 24.6 ps (8 mW) were obtained. Lightly doped spacers were positioned in both the EB and CB junctions to tailor junction characteristics (leakage, tunneling, and avalanche breakdown), reduce junction capacitances, and thereby obtain an overall performance improvement.<<ETX>>

SiGe-base heterojunction bipolar transistors: physics and design issues

It has been shown that SiGe technology has the capability to extend the performance of Si bipolar transistors at both high and low current levels. The ability to tailor the bandgap, independently of the doping profile design, provides considerable flexibility for optimizing cutoff frequency, intrinsic base resistance, and junction capacitances for a given application. It is concluded that, when combined with a self-aligned process, SiGe can significantly improve the speed of Si bipolar circuits.<<ETX>>

63-75 GHz f T SiGe-base heterojunction bipolar technology

Experimental results for maximum cut-off frequency (fT) values of 75 and 52 GHz were achieved for SiGe-base and Si-base bipolar transistors with intrinsic base sheet resistances in the 10-17 k&Omega;/square range. These results extend the speed of silicon bipolar devices into a regime previously reserved to GaAs and other compound semiconductor technologies. Excellent junction characteristics were also obtained for devices as large as 100000 &mu;m2 and for current densities as high as 106 A/cm2. The performance levels obtained for the SiGe transistors, which contain less than 10% germanium in the base, represent almost a factor of two increase in the speed of a Si bipolar transistor. These results demonstrate the significant performance advantage offered by SiGe heterojunction technology

Current gain rolloff in graded-base SiGe heterojunction bipolar transistors

The authors report the experimental observation of a novel effect in SiGe heterojunction bipolar transistors (HBTs) with graded bases which results in a significant emitter-base bias dependence of the current gain. The nonideal collector current is caused by the interaction of the bias dependence of the emitter-base space-charge region width and the exponential dependence of the collector current on the germanium concentration at the edge of the space-charge region. The resulting current gain rolloff must be taken into account for accurate modeling of bipolar transistors with bandgap grading in the base.<<ETX>>

Sub-30-ps ECL circuit operation at liquid-nitrogen temperature using self-aligned epitaxial SiGe-base bipolar transistors

The authors report the operation of emitter coupled logic (ECL) circuits at liquid-nitrogen temperature using self-aligned epitaxial SiGe-base bipolar transistors. A minimum ECL gate delay of 28.1 ps at 84 K was measured; this is essentially unchanged from the room-temperature value of 28.8 ps at 310 K. This delay number was achieved under full logic-swing (500-mV) conditions and represents an improvement of greater than a factor of 2 over the best reported value for 84 K operation. Lower-power ECL circuits have switching speeds as fast as 51 ps at 2.2 mW (112-fJ power-delay product) at 84 K. These results suggest that silicon-based bipolar technology is suitable for very-high-speed applications in cryogenic computer systems.<<ETX>>