Si/Si1-xGex heterojunction bipolartransistors produced by limited reaction processing

Si/Si_1-xGe_x heterojunction transistors (HBTs) fabricated by a chemical vapor deposition (CVD) technique are reported. A rapid thermal CVD limited-reaction processing (LRP) technique was used for the in situ growth of all three device layers, including a 20-mm Si_1-xGe_x layer in the base. The highest current gains observed (β=400) were for a Si/Si_1-x Ge_x HBT with a base doping of 7×10¹⁸ cm^-3 near the junction and a shallow arsenic implant to form ohmic contacts and increase current gain. Ideal base currents were observed for over six decades of current and the collector current remained ideal for nearly nine current decades starting at 1 pA. The bandgap difference between a p-type Si layer doped to 5×10¹⁷ cm^-3 and the Si_1-xGe_x(x=0.31) base measured 0.27 eV. This value was deduced from the measurements of the temperature dependence of the base current and is in good agreement with published calculations for strained Si_1-xGe_x layers on Si     

Effect of exponentially graded base doping on the performance ofGaAs/AlGaAs heterojunction bipolar transistors

The authors have fabricated n-p-n GaAs/AlGaAs heterojunction bipolar transistors (HBTs) with base doping graded exponentially from 5×10¹⁹ cm^-3 at the emitter edge to 5×10¹⁸ cm^-3 at the collector edge. The built-in field due to the exponentially graded doping profile significantly reduces base transit time, despite bandgap narrowing associated with high base doping. Compared to devices with the same base thickness and uniform base doping of 1×10¹⁹ cm^-3 , the cutoff frequency is increased from 22 to 31 GHz and maximum frequency of oscillation is increased from 40 to 58 GHz. Exponentially graded base doping also results ill consistently higher common-emitter current gain than uniform base doping, even though the Gummel number is twice as high and the base resistance is reduced by 40%     

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     

High reliability InGaP/GaAs HBT

Excellent long term reliability InGaP/GaAs heterojunction bipolar transistors (HBT) grown by metalorganic chemical vapor deposition (MOCVD) are demonstrated. There were no device failures (T=10000 h) in a sample lot of ten devices (L=6.4 μm ×20 μm) under moderate current densities and high-temperature testing (J_c=25 kA/cm ², V_ce=2.0 V, Junction Temp =264°C). The dc current gain for large area devices (L=75 μm ×75 μm) at 1 kA/cm² at a base sheet resistance of 240 ohms/sq (4×10 ¹⁹ cm^-3@700 Å) was over 100. The dc current gain before reliability testing (L=6.4 μm ×10 μm) at 0.8 kA/cm² was 62. The dc current gain (0.8 kA/cm²) decreased to 57 after 10000 h of reliability testing. The devices showed an f_T=61 GHz and f_max=103 GHz. The reliability results are the highest ever achieved for InGaP/GaAs HBT and these results indicate the great potential of InGaP/GaAs HBT for numerous low- and high-frequency microwave circuit applications. The reliability improvements are probably due to the initial low base current at low current densities which result from the low surface recombination of InGaP and the high valence band discontinuity between InGaP and GaAs     

Small offset-voltage In0.49Ga0.51P/GaAsdouble-barrier bipolar transistor

In_0.49Ga_0.51P/GaAs double-barrier bipolar transistors (DBBTs) grown by gas-source molecular beam epitaxy (GSMBE) have been fabricated and measured. This structure has two InGaP barrier layers (100 Å in thickness): one is inserted between the emitter-base (e-b) junction and the other between the base-collector (b-c) junction. An offset voltage of 26 mV and a differential current gain of 120 at room temperature were obtained with a heavily doped p⁺ (2×10¹⁹ cm^-3) base (500 Å in thickness). The small offset voltage was attributed to the similar structure of the e-b and b-c junctions and to the suppression of the hole injection current into the collector by the InGaP hole barrier at the b-c junction     

Drain-avalanche and hole-trapping induced gate leakage inthin-oxide MOS devices

Leakage current components due to band-to-band tunneling and avalanche breakdown in thin-oxide (90-160 Å) gated-diode structures are discussed. Experimental results show that while the band-to-band tunneling current is not sensitive to channel doping concentration, the avalanche current is sensitive to channel doping concentration in the range of 10¹⁶ to 10¹⁷ cm^-3. For oxides thicker than 110 Å, the gate current is found to be dominated by hot-hole injection and for oxides thinner than 110 Å the gate current is dominated by Fowler-Nordheim electron tunneling. After hot-hole injection, the gate oxide exhibits significant low-level leakage, which is explained by the barrier-lowering effect caused by the trapped holes in the oxide     

Planarization of emitter-base structure of heterojunction bipolartransistors by doping selective base contact and nonalloyed emittercontact

The authors report an n-p-n heterojunction bipolar transistor (HBT) with a planar (and thus passivated) emitter-base structure fabricated using a simple, low-temperature technique. They use a nonalloyed emitter contact facilitated by δ-doping grown at the surface of the sample, so that the cap layer is only 75 Å thick. The base is contacted by depositing Au-Zn or Au-Be on the surface and alloying at 420°C for 10 s, resulting in an ohmic contact with the base and rectifying contact with the emitter. The authors present large-emitter area (50-μm diameter) HBTs with homogeneous-doped bases (gain of 170) and δ-doped bases (10¹⁴ cm^-2, gain of 20). Upon reducing the emitter size of the latter to 3×8 μm the gain increased to 30, demonstrating excellent surface passivation     

GaAs bipolar transistors with a Ga0.5In0.5Phole barrier layer and carbon-doped base grown by MOVPE

GaAs bipolar transistors with a 50-Å-thick lattice matched Ga_0.5In_0.5P layer between the emitter and base acting as a hole repelling potential barrier in the valence band were fabricated from films grown by metalorganic vapor phase epitaxy (MOVPE). The 1000-Å-thick base was doped with carbon to 2×10¹⁹ cm^-3, resulting in a base sheet resistance of 250 Ω/□. Carbon has been chosen because of its low diffusivity. Using the barrier layer as an etch stop the authors fabricated mesa-type broad-area devices. The output characteristics of the devices are ideal with very small offset voltages and infinite Early voltages. Common emitter current gains of up to 70 at 10⁴ A/cm² collector current density were obtained. The current gain is clearly higher than the one calculated for a bipolar junction transistor with the same doping profile because the base-emitter hole current is suppressed by the Ga_0.5In_0.5P potential barrier in the valence band     

Current gain dependence on subcollector and etch-stop doping inInGaP/GaAs HBTs

The DC current gain dependence of InGaP/GaAs heterojunction bipolar transistors (HBTs) on subcollector and etch-stop doping is examined. Samples of InGaP/GaAs HBTs having various combinations of subcollector doping and etch-stop doping are grown, and large area 60 μm×60 (μ) HBTs are then fabricated for DC characterization. It is found that the DC current gain has a strong dependence on the doping concentration in the subcollector and the subcollector etch-stop. Maximum gain is achieved when the subcollector is doped at 6~7×10 ¹⁸ cm^-3 while the subcollector etch-stop is doped either above 6×10¹⁸ cm^-3 (current gain/sheet resistance ratio, β/R_b=0.435 at I_c=1 mA) or below 3.5×10¹⁷ cm^-3 (β/R_b=0.426~0.438 at I_c=1 mA). The data show that it is not necessary to heavily dope the subcollector etch-stop to reduce the conduction barrier and to obtain high current gain. The high current gain obtained with the low InGaP etch-stop doping concentration is attributed to the reduction of the effective energy barrier thickness due to band bending at the heterojunction between the InGaP etch-stop and the GaAs subcollector. These results show that the β/R_b of InGaP/GaAs HBTs can improve as much as 69% with the optimized doping concentration in subcollector and subcollector etch-stop     

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     

A comparison of optoelectronic properties of lattice-matched andstrained quantum-well and quantum-wire structures

The k·p formalism is used to study the absorption spectra, material and differential gain in quantum wires as a function of orientation, built-in strain, and wire dimensions. The results for material and differential gain are compared with those for an optimized quantum-well structure. We find that for quantum wires at 300 K, the gain becomes positive at a carrier density of 1.27·10¹⁸ cm^-3, while in quantum wells this density is calculated to be 1.82·10¹⁸ cm^-3. Incorporating tensile strain in the wires reduces the transparency carrier concentration to 0.96·10¹⁸ cm^-3 while compressive strain allows one to obtain positive gain for densities greater than 1.08·10¹⁸ cm^-3. Orienting the wire along the [111] direction reduces the transparency carrier density to 0.60·10¹⁸ cm^-3. The differential gain in quantum-well structures for injections near the threshold is on the order of 10^-14 cm^-4, while for 50 Å·100-Å quantum wires the differential gain near the threshold is found to be on the order of 10^-13 cm^-4 . The differential gain in wires whose wire axis is parallel to the [111] direction has also been found to be on the order of 10^-13 cm^-4 for carrier injections close to the threshold     

Heavy doping effects in p-n-p bipolar transistors

This paper presents the heavy doping effects on the injection current characteristics in p-n-p transistors with a heavily doped but thin base region. The results of the present study indicate that 1) at room temperature the hole current injected into heavily doped base is insensitive to the impurity compensation effect, 2) a linear relationship between the base sheet resistance and the collector-current density is observed when the base doping density is under 1 × 10¹⁹cm^-3. This relationship becomes supralinear as the doping density further increases. As a result, useful current gain exists in thin base transistors even when the base doping is greater than 1 × 10¹⁹cm^-3. From the collector-current-base sheet-resistance relationship and the base doping profile, the effective intrinsic carrier density as a function of the doping density is evaluated and found to increase 8.7 times over that of pure silicon, when the average doping density is 5 × 10¹⁹cm^-3(maximum doping density 1 × 10²⁰cm^-3). 3) The collector current and the current gain of the transistors become less sensitive to the temperature as the base doping density increases. We had observed a current gain up to 30 at 77 K for transistors with the maximum base doping density in the 10¹⁸cm^-3range. The transistors with lower base doping suffer much more degradation in current gain when the temperature is lowered to 77 K.     

Comparison of DC and high-frequency performance of zinc-doped andcarbon-doped InP/InGaAs HBTs grown by metalorganic chemical vapordeposition

Zinc and carbon-doped InP/InGaAs heterojunction bipolar transistors (HBTs) with the same design were grown by metalorganic chemical vapor deposition (MOCVD). DC current gain values of 36 and 16 were measured for zinc and carbon-doped HBTs, respectively, and carrier lifetimes were measured by time-resolved photoluminescence to explain the difference. Transmission line model (TLM) analysis of carbon-doped base layers showed excellent sheet-resistance (828 Ω/□ for 600 A base), indicating successful growth of highly carbon-doped base (2×10¹⁹ cm^-3). The reasons for larger contact resistance of carbon than zinc-doped base despite its low sheet resistance were analyzed. f_T and f_max of 72 and 109 GHz were measured for zinc-doped HBTs, while 70-GHz f_T and 102 GHz f_max were measured for carbon-doped devices. While the best performance was similar for the two HBTs, the associated biasing current densities were much different between zinc (4.0×10 ⁴ A/cm²) and carbon-doped HBTs (2.0×10⁵ A/cm²). The bias-dependant high-frequency performance of the HBTs was measured and analyzed to explain the discrepancy     

Identification of a corner tunneling current component in advancedCMOS-compatible bipolar transistors

A corner tunneling current component in the reverse-biased emitter-base junction of advanced CMOS compatible polysilicon self-aligned bipolar transistors has been identified by measuring base current as a function of temperature, bias voltage, and emitter shape. This current is found to be an excess tunneling current caused by an increase in defect density in the corners of the emitter and gives rise to three-dimensional effects in small-geometry devices. The devices used for this study were selected from batches aimed at optimizing the emitter-base system. For this reason, the starting material was n-type (~10¹⁶ cm^-3) and provided the collector regions of the transistors. The intrinsic base and lightly doped extrinsic base regions were both implanted at 30 keV to a dose of 1×10¹³ cm^-2. The activation anneal was performed at 1060°C for 20 s in a rapid thermal annealer. Under such conditions, the emitter-base junction is located about 600 Å below the polysilicon-substrate interface     

Role of neutral base recombination in high gain AlGaAs/GaAs HBT's

Neutral base recombination is a limiting factor controlling the maximum gain of AlGaAs/GaAs HBT's with base sheet resistances between 100 and 350 Ω/□. In this work, we investigate five series of AlGaAs/GaAs HBT growths in which the base thickness was varied between 500 and 1600 Å and the base doping level between 2.9× and 4.7×10¹⁹ cm^-3. The dc current gain of large area devices (L=75 μm×75 μm) varies by as much as a factor of two at high injection levels for a fixed base sheet resistance, depending on the growth optimization. One of these series (Series TA) has the highest current gains ever reported in this base sheet resistance range, with dc current gains over 225 (@ 200 A/cm² ) at a base sheet resistance of 330 Ω/□. A high dc current gain of 220 (@ 10 kA/cm²) was also confirmed in small area devices (L=8 μm×8 μm). High-frequency tests on a separate set of wafers grown under the same conditions indicate these high current gains can be achieved without compromising the RF characteristics: Both high and normal gain devices exhibit an f_t ~68 GHz and f_max~100 GHz. By fitting the base current as a sum of two components, one due to recombination in the neutral base and the other in the space charge region, we conclude that an improvement in the minority carrier lifetime is responsible for the observed increase in dc current gain. Moreover, we observe a thickness-dependent variation in the effective minority carrier lifetime as the gains increase, along with a nonlinear dependence of current gain on base doping. Both phenomena are discussed in terms of an increase in Auger and radiative recombination relative to Hall-Shockley-Read recombination in optimized samples     

Thin oxide thickness extrapolation from capacitance-voltagemeasurements

Five oxide-thickness extrapolation algorithms, all based on the same model (metal gate, negligible interface traps, no quantum effects), are compared to determine their accuracy. Three sets of parameters are used: (acceptor impurity concentration, oxide thickness, and temperature): (10¹⁶ cm^-3, 250 Å, 300 K), (5×10¹⁷ Cm^-3, 250 Å, 300 K), and (5×10¹⁷ cm^-3, 50 Å, 150 K). Demonstration examples show that a new extrapolation method, which includes Fermi-Dirac statistics, gives the most accurate results, while the widely-used C_o≃C_g (measured at the power supply voltage) is the least accurate. The effect of polycrystalline silicon gate is also illustrated     

InP/InGaAs heterojunction bipolar transistors grown by gas-sourcemolecular beam epitaxy with carbon-doped base

The first N-p-n InP/InGaAs heterojunction bipolar transistors (HBTs) with p-type carbon doping in InGaAs are reported. P-type carbon doping in the InGaAs base has been achieved by gas-source molecular beam epitaxy (GSMBE) using carbon tetrachloride (CCl₄) as the dopant source. The resulting hole concentration in the base was 1×10¹⁹ cm^-3. HBTs fabricated using material from this growth method display good I-V characteristics with DC current gain above 500. This verifies the ability to use carbon doping to make a heavily p-type InGaAs base of an N-p-n HBT     

Reliability of AlInAs/GaInAs heterojunction bipolar transistors

The reliability of high-performance AlInAs/GaInAs heterojunction bipolar transistors (HBTs) grown by molecular beam epitaxy (MBE) is discussed. Devices with a base Be doping level of 5×10¹⁹ cm^-3 and a base thickness of approximately 50 nm displayed no sign of Be diffusion under applied bias. Excellent stability in DC current gain, device turn-on voltage, and base-emitter junction characteristics was observed. Accelerated life-test experiments were performed under an applied constant collector current density of 7×10⁴ A/cm² at ambient temperatures of 193, 208, and 328°C. Junction temperature and device thermal resistance were determined experimentally. Degradation of the base-collector junction was used as failure criterion to project a mean time to failure in excess of 10⁷ h at 125°C junction temperature with an associated activation energy of 1.92 eV     

Base profile design for high-performance operation of bipolartransistors at liquid-nitrogen temperature

Measurements of thin epitaxial-base polysilicon-emitter n-p-n transistors with increasing base doping show the effects of bandgap narrowing, mobility changes, and carrier freezeout. At room temperature the collector current at low injection is proportional to the integrated base charge, independent of the impurity distribution. At temperatures below 150 K, however, minority injection is dominated by the peak base doping because of the greater effectiveness of bandgap narrowing. When the peak doping in the base approaches 10¹⁹ cm^-3, the bandgap difference between emitter and base is sufficiently small that the current gain no longer monotonically decreases with lower temperature but instead shows a maximum as low as 180 K. The device design window appears limited at the low-current end by increased base-emitter leakage due to tunneling and by resistance control at the high-current end. Using the measured DC characteristics, circuit delay calculations are made to estimate the performance of an emitter-coupled logic ring oscillator at room and liquid-nitrogen temperatures. It is shown that if the base doping can be raised to 10¹⁹ cm^-3 while keeping the base thickness constant, the minimum delay at liquid-nitrogen temperature can approach the delay of optimized devices at room temperature     

Sub-quarter-micrometer gate-length p-channel MOSFETs with shallowboron counter-doped layer fabricated using channel preamorphization

A technique for forming shallow boron-doped layers for channel doping using preamorphization (channel preamorphization) is described. An extremely shallow boron-doped layer for shallow channel doping has been formed using preamorphization and rapid thermal annealing. Boron peak concentration around the surface is 3.5×10¹⁸ cm ^-3, and the depth at which the boron concentration becomes 10 ¹⁷ cm^-3 is 450 Å. In contrast, the depth is as large as 900 Å for nonpreamorphized samples. It is found that the shallow boron-doped layer formation is made possible because enhanced diffusion arising from ion implantation damage as well as the channeling in boron ion implantation is suppressed by preamorphization. It is also found that preamorphization does not affect MOS capacitor characteristics so long as the amorphous/crystalline interface is sufficiently deep, which allows that channel preamorphization is readily applicable to channel doping in MOSFET fabrication. To substantiate the experimental results, buried-channel p-MOSFETs with a shallow boron counterdoped layer using channel preamorphization have been successfully fabricated. Channel preamorphization did not degrade carrier mobility and improved MOSFET characteristics in the sub-quarter-micrometer-gate-length region suppressing short-channel effects due to the shallower counterdoped boron profile. High-performance 0.2-μm-gate-length p-MOSFETs with good subthreshold characteristics have been fabricated