Submicron transferred-substrate heterojunction bipolar transistors

We report submicron transferred-substrate AlInAs/GaInAs heterojunction bipolar transistors (HBT's). Devices with 0.4-μm emitter and 0.4-μm collector widths have 17.5 dB unilateral gain at 110 GHz. Extrapolating at -20 dB/decade, the power gain cutoff frequency f_max is 820 GHz. The high f_max, results from the scaling of HBT's junction widths, from elimination of collector series resistance through the use of a Schottky collector contact, and from partial screening of the collector-base capacitance by the collector space charge     

Scaling `ballistic' heterojunction bipolar transistors

Reducing length scales in npn heterojunction bipolar transistors leads to unexpected changes in the fundamental limits of device performance. Very high p-type carrier concentrations in the base result in a reduced inelastic electron scattering rate. In addition, there exists a maximum base/collector bias above which ballistic collector transport is not possible, and correct scaling requires the n-type collector contact to be unusually heavily doped     

GaAsSb for heterojunction bipolar transistors

The advantages of using GaAsSb in heterojunction bipolar transistors (HBT) are discussed with emphasis on two recent experimental results in the AlGaAs/GaAsSb material system. The performances of a prototype n-p-n AlGaAs/GaAsSb/GaAs double HBT (DHBT) that exhibits stable current gain with maximum collector current density of 5×10 ⁴ A/cm², and a p-n-p AlGaAs/GaAs HBT with a superlattice GaAsSb emitter ohmic contact which has a specific contact resistivity of 5±1×10^-7 Ω-cm² across the sample, are examined     

Self-aligned AlInAs-GaInAs heterojunction bipolar transistors andcircuits

AlInAs-GaInAs heterojunction bipolar transistors (HBTs) and static flip-flop frequency dividers have been fabricated. An f_t and an f_max of 49 and 62 GHz, respectively, have been achieved in a device with a 2×5-μm² emitter. Current-mode logic (CML) was used to implement static divide-by-two and divide-by-four circuits. The divide-by-two circuit operated at 15 GHz with 82-mW power dissipation for the single flip-flop. The divide-by-four circuit operated at 14.5 GHz with a total chip power dissipation of 444 mW     

Early voltage in double heterojunction bipolar transistors

The Early voltage for abrupt double heterojunction bipolar transistors (DHBTs) has been calculated by using an effective junction velocity (S_c) at the base-collector heterojunction. S_c is obtained by self-consistently partitioning thermionic and quantum mechanical tunneling currents. Unlike single heterojunction bipolar transistors (SHBTs), the Early voltage varies very rapidly at low reverse bias and approaches the SHBT-limit at sufficiently high reverse bias. This is attributed to the presence of an energy barrier at the b-c heterojunction     

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

Si/Si_1-xGe_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 LN₂ temperature resulting in current gains of 1600 for the SiGe-base transistors despite base sheet resistances as low as 7.5 kΩ/□     

Dynamic Early effect in heterojunction bipolar transistors

A theory of base transport in an abrupt-junction heterostructure bipolar transistor (HBT) is developed in the diffusion limit. The theory is valid for a continuous range of emitter injection energies Δ and accounts for the Early effect in both the static and the high-frequency limits. Small-signal network parameters strongly depend on Δ and differ from those in a graded emitter-base junction HBT     

p-n-p heterojunction bipolar transistors with buried subcollectorlayers

The buried-layer technology was applied to the fabrication of high-speed p-n-p AlGaAs/GaAs heterojunction bipolar transistors (HBTs). The subcollector layer was selectively implanted prior to the epitaxial growth of the rest of the device structure thereby eliminating the need for deep mesa isolation. Devices with 2×10-μm² emitter fingers and 100-nm base thickness had common-emitter current gains of 15 and cutoff frequencies of 17 GHz     

High-speed InP/InGaAs heterojunction bipolar transistors

Very-high-performance common-emitter InP/InGaAs single heterojunction bipolar transistors (HBTs) grown by metalorganic molecular beam epitaxy (MOMBE) are reported. They exhibit a maximum oscillation frequency (f_T) of 180 GHz at a current density of 1×10⁵ A/cm². this corresponds to an (R_BC_BC)_eff=f _T/(8πf²_max) delay time of 0.12 ps, which is the smallest value every reported for common-emitter InP/InGaAs HBTs. The devices have 11 μm² total emitter area and exhibit current gain values up to 100 at zero base-collector bias voltage. The breakdown voltage of these devices is high with measured BV_CEO and BV_CEO of 8 and 17 V, respectively     

High-frequency characteristics of inverted-mode heterojunction bipolar transistors

It is shown that in emitter-down heterojunction bipolar transistors (HBT's), parasitics can be reduced sufficiently that intrinsic transit-time delays become the dominant limitations to high-frequency performance. In this situation it is found that the dependence of the unilateral gain on frequency can be significantly different from the simple 6-dB/octave decrease usually assumed. It is found that the device exhibits negative output conductance over certain bands of frequencies, and that when this occurs a series of resonances are observed in the gain versus frequency characteristics. Explanations of this behavior are given in terms of the phase delay of the common-base current gain. The generality and relevance of these observations to other types of transistors, and the utilization of the negative output conductance to enhance high-frequency operation are also discussed.     

High-speed InAlAs/InGaAs heterojunction bipolar transistors

InAlAs/InGaAs heterojunction bipolar transistors fabricated from wafers grown by molecular beam epitaxy are discussed. A cutoff frequency of 32 GHz for a collector current of 20 mA is achieved in the emitter area of devices 6×10 μm². The use of heavily doped and nondoped InGaAs layers as the emitter cap and collector, respectively, results in a reduction of the emitter and collector charging times; this, in turn, leads to improved microwave performance     

Ultra-high-speed InP/InGaAs heterojunction bipolar transistors

We report on the microwave performance of InP/In_0.53Ga _0.47As heterojunction bipolar transistors (HBT's) utilizing a carbon-doped base grown by chemical beam epitaxy (CBE). The f_T and f_max of the HBT having two 1.5×10 μm² emitter fingers were 175 GHz and 70 GHz, respectively, at I_C=40 mA and V_CE=1.5 V. To our knowledge, the f _T of this device is the highest of any type of bipolar transistors yet reported. These results indicate the great potential of carbon-doped base InP/InGaAs HBT's for high-speed applications     

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     

Thermal coupling in 2-finger heterojunction bipolar transistors

We have previously analyzed the collapse phenomenon in heterojunction bipolar transistors (HBT's) when the mutual couplings among the transistor fingers are negligible. In this investigation, we derive the collapse loci equations in 2-finger HBT's in the presence of thermal coupling. It is found that the collapse loci equations are closely linked to a thermal instability condition best determined from the transistor regression characteristics. Unlike the previous derivation assuming zero thermal coupling, the collapse loci equations derived here are different depending on whether the HBT is driven by constant base current or constant base voltage bias     

Unilateral gain of heterojunction bipolar transistors at microwavefrequencies

General expressions for the h parameters of small-signal equivalent circuit models for emitter-up and emitter-down heterojunction bipolar transistors (HBTs) are derived. The unilateral gain U is calculated from these h parameters for InGaAs/InAlAs n-p-n HBTs. When the device parasitics are sufficiently reduced, the unilateral gain is found to exhibit a resonance behavior at high frequencies that deviates from the traditionally assumed 6-dB/octave roll-off. This unusual behavior in the unilateral gain is found to be caused by the appearance of negative output conductance of the device in certain bands of frequencies. The occurrence of the negative output conductance is shown to be a transit-time effect, and its implication for the device performance at high frequencies is discussed     

Silicon-germanium base heterojunction bipolar transistors bymolecular 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-Å 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_0.88Ge_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 Å were grown, corresponding to a neutral base width of no more than 400 Å     

Base transit time for SiGe-base heterojunction bipolar transistors

The base transit time expressions for SiGe base heterojunction bipolar transistors are presented including the accelerating field effects due to the base bandgap grading and doping grading, and the retarding field (opposing drift field) effect from the graded boron profile down towards the emitter. It is found that the retarding field exhibits 40-80% contribution to the base transit time, depending on the boron concentration near the emitter. The results of base transit time from these analytic expressions are unambiguously supported by the published simulation data.<>     

Bias dependence of high-frequency noise in heterojunction bipolar transistors

Hawkins' isothermal model developed to study noise in bipolar junction transistors (BJTs) is modified to investigate bias-dependent noise in heterojunction bipolar transistors (HBTs) by incorporating thermal effects. It is shown that the inclusion of thermal effects into the high-frequency noise model of HBTs is necessary as the temperature of the device may become very different from the ambient temperature, especially at high bias current. Calculation of the noise figure by including the thermal effect shows that the isothermal calculation may underestimate the noise figure at high bias current. It is observed that noise at low bias is ideality factor n dependent whereas high bias noise is insensitive to the variation of n. Moreover, the common base current gain plays a major role in the calculation of the minimum noise figure. The excellent fit obtained between the theoretical calculation and the measured data are attributed to the inclusion of the bias-dependent junction heating as well as C/sub De/ and C/sub bc/ into the present calculation.     

Extrapolated fmax of heterojunction bipolar transistors

It is shown that the extrapolated f_max of heterojunction bipolar transistors (HBT's) can be written in the form f _max=√f_T/8π(RC)_eff, where f_T is the common-emitter, unity-current-gain frequency, and where (RC)_eff is a general time constant that includes not only the effects of the base resistance and collector-base junction capacitance, but also the effects of the parasitic emitter and collector resistances, and the dynamic resistance 1/g_m, where g_m is the transconductance. Simple expressions are derived for (RC) _eff, and these are applied to two state-of-the-art devices recently reported in the literature. It is demonstrated that, in modern HBT's, (RC)_eff can differ significantly from the effective base-resistance-collector-capacitance product conventionally assumed to determine f_max     

High-gain InGaAlAs/InGaAs heterojunction bipolar transistors andphototransistors

The characteristics of InGaAlAs/InGaAs heterojunction bipolar transistors (HBTs) grown by molecular beam epitaxy are described. A current gain of 15600 at a current density of ~10⁴ A/cm² and an emitter-base heterojunction ideality factor of 1.02 were measured. Appropriately designed InGaAlAs/InGaAs HBTs, when operated as phototransistors, also had high gains. A current gain of 1000 for a collector current of only 10 μA was obtained for phototransistors. Such high gains are due to low recombination currents as a consequence of the good crystalline quality of the InGaAlAs bulk and InGaAlAs/InGaAs interface