A low-noise K-Ka band oscillator AlGaAs/GaAsheterojunction bipolar transistors

The design considerations, fabrication process, and performance of the first K-Ka-band oscillator implemented using a self-aligned AlGaAs/GaAs heterojunction bipolar transistor (HBT) are described. A large-signal time-domain-based design approach has been used which applies a SPICE-F simulator for optimization of the oscillator circuit parameters for maximum output power. The oscillator employs a 2×10-μm² emitter AlGaAs/GaAs HBT that was fabricated using a pattern inversion technology. The HBT has a base current 1/f noise power density lower than 1×10^-20 A²/Hz at 1 kHz and lower than 1×10^-22 A/²/Hz at 100 kHz for a collector current of 1 mA. The oscillator, which is composed of only low-Q microstrip transmission lines, has a phase noise of -80 dBc/Hz at 100 kHz off carrier when operated at 26.6 GHz. These results indicate the applicability of the HBTs to low-phase-noise monolithic oscillators at microwave and millimeter-wave frequencies, where both Si bipolar transistors and GaAs FETs are absent     

Reconciliation of methods for estimating fmaxfor microwave heterojunction transistors

An attempt is made to reconcile the various approaches that have recently been used to estimate the maximum frequency of oscillation f_max in high-performance AlGaAs/GaAs HBTs. f_max is computed numerically from the full expression for Mason's invariant gain using y-parameters derived from the different approaches, i.e., the hybrid-π equivalent circuit, the T-equivalent circuit, and the drift-diffusion equations. It is shown that the results for f_max are essentially the same, irrespective of the source of the y-parameters, provided that the phase delays due to transit of carriers across the base and the collector-base depletion region are properly accounted for. It is also shown, for the particular device studied, that the widely used analytical expression for f_max, involving f _T and effective base resistance and collector capacitance, is remarkably accurate for frequencies below those at which transit-time effects become important     

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     

0.33-μm gate-length millimeter-wave InP-channel HEMTs with highf1 and fmax

The fabrication of 0.33-μm gate-length AlInAs/InP high electron mobility transistors (HEMTs) is reported. These InP-channel devices have f_t values as high as 76 GHz, f_max values of 146 GHz, and maximum stable gains of 16.8, 14, and 12 dB at 10, 18, and 30 GHz, respectively. The extrinsic DC transconductances are as high as 610 mS/mm; with drain-source breakdown voltages exceeding 10 V. The effective electron velocity in the InP channel is estimated to be at least 1.8×10⁷ cm/s, while the f_tL_g product is 29 GHz-μm. These results are comparable to the best reported results for similar InGaAs-channel devices     

Near-ideal I-V characteristics of GaInP/GaAsheterojunction bipolar transistors

GaInP/GaAs heterojunction bipolar transistors (HBTs) have been fabricated and these devices exhibit near-ideal I-V characteristics with very small magnitudes of the base-emitter junction space-charge recombination current. Measured current gains in both 6-μm×6-μm and 100-μm×100-μm devices remain constant for five decades of collector current and are greater than unity at ultrasmall current densities on the order of 1×10^-6 A/cm². For the 6-μm×6-μm device, the current gain reaches a high value of 190 at higher current levels. These device characteristics are also compared to published data of an abrupt AlGaAs/GaAs HBT having a base layer with similar doping level and thickness     

High-performance low-base-collector capacitance AlGaAs/GaAsheterojunction bipolar transistors fabricated by deep ion implantation

Low-base-collector capacitance (C_bc) AlGaAs/GaAs HBTs with f_MAX>200 GHz and f_T=52 GHz have been fabricated. With co-implants of high energy, high dose He⁺ and H⁺ ions through the external base layer, part of the heavily doped n⁺ sub-collector was compensated leading to a decrease in the extrinsic portion of C_bc. The implants caused only a slight increase of base resistance. Using this approach in combination with a standard low dose, shallow collector compensating implant, C_bc of double implanted HBT's can be reduced by more than 35%     

Effect of reduced temperature on the fT ofAlGaAs/GaAs heterojunction bipolar transistors

The high-frequency and DC performances of single-heterojunction Al _0.25Ga_0.75As/GaAs heterojunction bipolar transistors (HBTs) have been measured at temperatures between 300 and 110 K. It is found that the maximum unity-current-gain cutoff frequency increases from 26 GHz at 300 K to 34 GHz at 110 K. It is shown that electron diffusion as determined from the majority-carrier mobility does not accurately estimate the base transit time, at least until corrections for degeneracy and minority-carrier mobility enhancement are included. Reasonable agreement is obtained assuming that base transport is limited by the thermal velocity of electrons at reduced temperatures     

Si/a-Si:H heterojunction microwave bipolar transistors with cut-offfrequencies ft above 5 GHz

The fabrication of a silicon heterojunction microwave bipolar transistor with an n⁺ a-Si:H emitter is discussed, and experimental results are given. The device provides a base sheet resistance of 2 kΩ/□ a base width 0.1 μm, a maximum current gain of 21 (V_CE=6 V, I_c=15 mA), and an emitter Gummel number G _E of about 1.4×10¹⁴ Scm^-4. From the measured S parameters, a cutoff frequency f_t of 5.5 GHz and maximum oscillating frequency f_max of 7.5 GHz at V_CE=10 V, I_c=10 mA are obtained     

Fabrication of 200-GHz fmax resonant-tunnelingdiodes for integration circuit and microwave applications

Room-temperature current densities of 1.3×10⁵ A/cm² and peak-to-valley ratios of 2.5 have been achieved for resonant tunneling diodes (RTDs) in the GaAs/AlAs material system. The devices were fabricated in a microwave-compatible process using topside contacts and a semi-insulating substrate to allow device integration. Proton implantation creates a nonconducting surface compatible with high-frequency coplanar transmission lines and other passive microwave structures     

A 6-GHz integrated phase-locked loop using AlGaAs/GaAsheterojunction bipolar transistors

A fully integrated 6-GHz phase-locked-loop (PLL) fabricated using AlGaAs/GaAs heterojunction bipolar transistors (HBTs) is described. The PLL is intended for use in multigigabit-per-second clock recovery circuits for fiber-optic communication systems. The PLL circuit consists of a frequency quadrupling ring voltage-controlled oscillator (VCO), a balanced phase detector, and a lag-lead loop filter. The closed-loop bandwidth is approximately 150 MHz. The tracking range was measured to be greater than 750 MHz at zero steady-state phase error. The nonaided acquisition range is approximately 300 MHz. This circuit is the first monolithic HBT PLL and is the fastest yet reported using a digital output VCO. The minimum emitter area was 3 μm×10 μm with f_t=22 GHz and f_max=30 GHz for a bias current of 2 mA. The speed of the PLL can be doubled by using 1-μm×10-μm emitters in next-generation circuits. The chip occupies a die area of 2-mm×3-mm and dissipates 800 mW with a supply voltage of -8 V     

Improved current gain and fT through dopingprofile selection in linearly graded heterojunction bipolar transistors

Analytical and experimental results are used to show that extension of a thin p-doped layer of base doping into the graded-gap region, close to the base, of an n-p-n AlGaAs/GaAs heterojunction bipolar transistor and removing n-type dopant from the rest of the linearly graded AlGaAs region improves current gain β and unity gain cutoff frequency f_T. Current gain is significantly improved by reducing recombination near the metallurgical interface and using the effective electric field from the grading to accelerate electrons as they are injected into the p-base. The doping profile also inhibits the formation of a potential minimum in which electrons can be stored in close proximity to the base. This greatly improves f_T, and does not hamper the current injection or increase the turn-on voltage. Space-charge recombination current is also reduced, due to the carrier density reduction associated with the effective electric field due to the graded gap     

Extremely high peak specific transconductance AlGaAs/GaAsheterojunction bipolar transistors

Self-aligned AlGaAs/GAs heterojunction bipolar transistors with peak specific transconductances as high as 25 mS/μm² of emitter area are discussed. These are the highest specific transconductances ever reported for a bipolar transistor. These devices, which contain no indium in the emitter, display specific parasitic emitter resistances of less than 1×10^-7 Ω-cm². This low parasitic resistance is attributed to an improved n-type contact technology, in which a molybdenum diffusion barrier and a plasma-enhanced chemical vapor deposition SiO₂ overlayer are used to achieve low specific contact resistivities     

Thermal effects on the characteristics of AlGaAs/GaAsheterojunction bipolar transistors using two-dimensional numericalsimulation

Two-dimensional numerical simulations incorporating the spatial distributions of the energy band and temperature were used to study AlGaAs/GaAs heterojunction bipolar transistor characteristics. It was found that the negative differential resistance and the reduction of the base-emitter voltage for a constant base current in the active region are due to thermal effects. The differential current gain and cutoff frequency decrease when the transistor is operated at high power levels. The temperature distribution of the transistor operated in the active region shows a maximum at the collector region right beneath the emitter mesa. When the transistor is operated in the saturation region, the emitter contact region may be at a slightly lower temperature than the heat sink temperature. This thermoelectric cooling effect results from the utilization of the thermodynamically compatible current and energy flow formulations in which the energy band discontinuities are part of the thermoelectric power     

Tradeoff between 1/f noise and microwave performance in AlGaAs/GaAsheterojunction bipolar transistors

The 1/f noise characteristics and microwave gain of AlGaAs/GaAs heterojunction bipolar transistors (HBT's) have been investigated as a function of surface passivation ledge length. These measurements clearly demonstrate that the ledge length imposes a tradeoff between the 1/f noise and microwave power gain performance. Compared to a conventional unpassivated self-aligned HBT, HBT's with 0.4 and 1.1 μm ledge lengths improve the equivalent input base noise current spectral density at 100 Hz by as much as 2 dB and 6.5 dB, respectively; while degrading the maximum available gain at 18 GHz by 0.3 dB and 2.4 dB, respectively     

Bias dependence of fT and fmax in anIn0.52Al0.48As/n+-In0.53Ga0.47As MISFET

Detailed microwave characterization of a recently fabricated In _0.52Al_0.48As/n⁺-In_0.53Ga _0.47As MISFET reveals that high values of current-gain cutoff frequency (f_T) and unilateral-gain cutoff frequency (f_max) are obtained for a broad range of gate bias voltage values. A significant peak in f_T and f _max has also been observed at high gate-source bias values. The peak coincides with the onset of electron accumulation at the heterointerface and is attributed to reduced ionized impurity scattering coupled with reduced drain conductance. This result suggests an improved device structure that optimizes operation in the accumulation regime     

Hot-electron InGaAs/InP heterostructure bipolar transistors withfT of 110 GHz

A hot-electron InGaAs/InP heterostructure bipolar transistor (HBT) is discussed. A unity-current-gain cutoff frequency of 110 GHz and a maximum frequency of oscillation of 58 GHz are realized in transistors with 3.2×3.2-μm² emitter size. Nonequilibrium electron transport, with an average electron velocity approaching 4×10⁷ cm/s through the thin (650 Å) heavily doped (p=5×10¹⁹ cm^-3) InGaAs base and 3000-Å-wide collector space-charge region, results in a transit delay of 0.5 ps corresponding to an intrinsic cutoff frequency of 318 GHz     

On the leverage of high-fT transistors foradvanced high-speed bipolar circuits

A detailed study on the leverage of high-f_T transistors for advanced high-speed bipolar circuit applications is presented. It is shown that for the standard ECL (emitter-coupled logic) circuit, the leverage of high f_T is limited by the passive resistors (emitter-follower resistor and collector load resistor) and wire delay, especially in the low-power regime. For the standard NTL (nonthreshold logic) circuit, the leverage is higher due to its front-end configuration and lower power supply value. As the passive resistors are decoupled from the delay path in various advanced circuits utilizing active-pull-down schemes, the leverage of high F_T becomes more significant     

89-GHz fT room-temperature silicon MOSFETs

The authors report the implementation of deep-submicrometer Si MOSFETs that at room temperature have a unity-current-gain cutoff frequency (f_T) of 89 GHz, for a drain-to-source bias of 1.5 V, a gate-to-source bias of 1 V, a gate oxide thickness of 40 Å, and a channel length of 0.15 μm. The fabrication procedure is mostly conventional, except for the e-beam defined gates. The speed performance is achieved through an intrinsic transit time of only 1.8 ps across the active device region     

75-GHz fT SiGe-base heterojunction bipolartransistors

The fabrication of silicon heterojunction bipolar transistors which have a record unity-current-gain cutoff frequency (f_T) of 75 GHz for a collector-base bias of 1 V, an intrinsic base sheet resistance (R_bi) of 17 kΩ/□, and an emitter width of 0.9 μm 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     

Reduction of ft by nonuniform base bandgapnarrowing

Analytical and simulation results are presented to illustrate qualitatively the effect of doping on base transit time. Nonuniform base bandgap narrowing (BGN) in silicon bipolar transistors can give rise to an electric field that is comparable to and against the built-in field. The base transit time τ is subsequently increased, leading to a deterioration of the cutoff frequency f₁. It is shown that the BGN effectively reduces the impurity profile grading factor K and subsequently the transit-time coefficient η. Physically, the minority carriers can be thought of as moving in a new profile characterized by a reduced η but in the absence of BGN. Unlike earlier investigations which also consider effective BGN dopings but ignore the field effects, this treatment includes their impact on the minority-carrier base transit time. For a steep exponential profile with strong BGN, an increase of η by a factor 3.57 at 300 K is calculated. Device simulations predict a smaller f_t reduction factor of 1.5 for more general profiles