A study of frequency response in silicon heterojunction bipolartransistors with amorphous silicon emitters

A detailed physical model of amorphous silicon (a-Si:H) is incorporated into a two-dimensional device simulator to examine the frequency response limits of silicon heterojunction bipolar transistors (HBT's) with a-Si:H emitters. The cutoff frequency is severely limited by the transit time in the emitter space charge region, due to the low electron drift mobility in a-Si:H, to 98 MHz which compares poorly with the 37 GHz obtained for a silicon homojunction bipolar transistor with the same device structure. The effects of the amorphous heteroemitter material parameters (doping, electron drift mobility, defect density and interface state density) on frequency response are then examined to find the requirements for an amorphous heteroemitter material such that the HBT has better frequency response than the equivalent homojunction bipolar transistor, We find that an electron drift mobility of at least 100 cm² V^-1 s^-1 is required in the amorphous heteroemitter and at a heteroemitter drift mobility of 350 cm ² V^-1 s^-1 and heteroemitter doping of 5×10¹⁷ cm^-3, a maximum cutoff frequency of 52 GHz can be expected     

Space-charge region recombination in heterojunction bipolartransistors

A new comprehensive model for space-charge region (SCR) recombination current in abrupt and graded energy gap heterojunction bipolar transistors (HBTs) is derived. It is shown that if a spike is present in one of the bands at the heterojunction interface, the SCR recombination current becomes interrelated with the collector current. A previously proposed charge control model for the HBT is modified to include the SCR recombination current. The model is used to study SCR recombination characteristics in HBTs     

Thermal design studies of high-power heterojunction bipolartransistors

A theoretical thermoelectro-feedback model has been developed for the thermal design of high-power GaAlAs/GaAs heterojunction bipolar transistors (HBTs). The power-handling capability, thermal instability, junction temperature, and current distributions of HBTs with multiple emitter fingers have been numerically studied. The calculated results indicate that power HBTs on Si substrates (or with Si as the collector) have excellent potential power performance and reliability. The power-handling capability on Si is 3.5 and 2.7 times as large as that on GaAs and InP substrates, respectively. The peak junction temperature and temperature difference on the chip decrease in comparison to the commonly used Si homostructure power transistor with the same geometry and power dissipation. Thereby HBTs are promising for high-speed microwave and millimeter-wave applications. It has been also found that the nonuniform distribution of junction temperature and current can be greatly improved by a ballasting technique that uses unequal-valued emitter resistors     

Material-based comparison for power heterojunction bipolartransistors

Various materials are studied to determine their potential in power heterojunction bipolar transistors (HBTs). The authors first start by generating an HBT figure of merit (FOM) which is defined as the product of operating frequency and output power of the HBT with 3-dB power gain. By using the FOM and available material parameters, a material-based comparison of HBT performance is done. The general tendency is for use of narrow-bandgap materials, such as Ge or InGaAs, as the base and wide-bandgap materials, such as AlGaAs, InP, SiC, or GaN, as the collector, technology permitting     

Effects of displaced p-n junction of heterojunction bipolartransistors

Two-dimensional simulations that demonstrate the effects of displacements of the p-n junctions from the heterojunctions of symmetrical Al_0.28Ga_0.72/GaAs double-heterojunction bipolar transistors (DHBTs) are reported. When the emitter and/or collector p-n junctions do not coincide with the AlGaAs/GaAs heterojunctions, the electrical characteristics are shown to be drastically altered due to changes in the potential profiles and to changes in recombination rates both in the neutral base and in the space-charge region of the emitter. The effects of a small displacement of the p-n junction from the emitter-base or the base-collector heterojunctions are examined and results for current gain β and cutoff frequency f_T are given that demonstrate enhanced performance for DHBTs with p-n junctions that are not coincident with the heterojunctions     

Low-frequency noise of InP/InGaAs heterojunction bipolartransistors

The equivalent base noise SI_b of InP/InGaAs heterojunction bipolar transistors (HBT's) with a circular pattern emitter is investigated experimentally at a low frequency ranging from 10-10⁵ Hz. The measured SI_b exhibits the 1/f dependence in an overall frequency range without any accompanying burst noise. Furthermore, SI_b varies as I_b^γ for the base current I_b and as d^-2 for the emitter diameter d, where the value of γ ranges from 1.62-1.72 depending on d of HBT's used. The 1/f noise model, which rigorously deals with the recombination current at the base surface I_bs as a function of I_b as well as of d is proposed. Applying our noise model to the dependence of SI_b on I_b, as well as on d, reveals that even though γ is less than two, the origin of SI_b is due to the recombination of electrons at the exposed base surface near the emitter edges. On the basis of theoretical considerations for the diffusion length of electrons and traps at the base surface, the Hooge parameter α_H for the noise due to the base surface recombination is deduced to be in the order of 10 ^-2 for the first time     

Reliability investigation of InGaP/GaAs heterojunction bipolartransistors

During elevated-temperature bias stress, InGaP/GaAs HBT's grown by MOCVD show a medium-term degradation in current gain of about 20%, with an activation energy of 0.64 eV. They also show a corresponding decrease in base resistance and an increase in turn-on voltage. InGaP/GaAs HBTs grown by GSMBE, however, do not show this degradation. SIMS measurements show a five times greater than GSMBE-epi hydrogen concentration of about 10¹⁹ cm^-3 in the base layer of the MOCVD-grown epi. The degradation can be explained by acceptor depassivation due to hydrogen out-diffusion from the epi during stress     

High-frequency characterization of heterojunction bipolartransistors using numerical simulation

The high-frequency performance of semiconductor devices is estimated using a small-signal numerical calculation based on drift-diffusion equations. In particular, unity current gain frequency in the common-emitter configuration (f_T) and maximum frequency of oscillation (f_max) are calculated for a heterojunction bipolar transistor. f_max is calculated from numerically obtained y parameters using formulas for maximum available gain, Mason's invariant (U), and a passivity criterion. They all give the same value for f_max. The influence of extrinsic and intrinsic base resistance on f_max is investigated for one device design. It is also found that a frequency used approximation formula for f_max is inaccurate, especially at higher current levels     

Current gain rolloff in graded-base SiGe heterojunction bipolartransistors

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     

Orientation effect on AlGaAs/GaAs heterojunction bipolartransistors

Orientation effects on N-p-n AlGaAs/GaAs heterojunction bipolar transistors (HBT's) have been demonstrated for the first time. We have observed that the current gains of HBT's fabricated on the same wafer are strongly dependent on the emitter direction. The HBT's with emitter direction of [010] show the highest current gain and the smallest emitter-size effect. This orientation effect could be attributed to the piezoelectric effect, which superposes the piezoelectric charges to the original emitter doping and generates the weak lateral electric field that drifts the injected carriers at the emitter periphery. The difference of the saturation voltage between collector-emitter of those HBT's corresponds to the superposed piezoelectric charges     

Failure mechanisms in AlGaAs/GaAs power heterojunction bipolartransistors

We report the failure mechanisms resulting in the second breakdown characteristics found in AlGaAs/GaAs power heterojunction bipolar transistors (HBTs). The dominant failure mechanism is identified to be the increasingly larger base-collector leakage current at elevated junction temperatures. This failure mechanism is compared with those found in silicon bipolar transistors     

Excess noise reduction in GaInP/GaAs heterojunction bipolartransistors

In this paper an annealing procedure which gives an excess noise reduction both of heavily C-doped resistive structures and GaInP/GaAs Heterojunction Bipolar Transistors (HBTs) of 5 dB is proposed. The investigation of the correlation between the noise generators indicate that the annealing leads to a decrease of noise voltage attributed to a strain reduction both in the intrinsic and in the extrinsic base related to a site switching effect of carbon atoms. The reduction of noise current with annealing is attributed to the surface improvement related passivation process by hydrogen atoms     

A broad-band amplifier using GaAs/GaAlAs heterojunction bipolartransistors

A compact wideband amplifier (or gain block) designed around a Darlington pair of GaAs/GaAlAs heterojunction bipolar transistors (HBTs) is discussed. This circuit has been fabricated by an ion-implanted process with a transistor f_t of 40 GHz. Two variants of the circuit gave either a 8.5-dB gain with a DC-to-5-GHz 3-dB bandwidth or a 13-dB gain with a DC-to-3-GHz bandwidth. These amplifiers gave 11.8- and 18.3-dBm output, respectively, at 1-dB gain compression     

Current transport mechanism in GaInP/GaAs heterojunction bipolartransistors

GaInP/GaAs heterojunction bipolar transistors (HBTs) and both graded and abrupt AlGaAs/GaAs HBTs were fabricated. A total of 20 wafers were analyzed. Comparisons of the experimental results establish that the dominant carrier transport mechanism in GaInP/GaAs HBTs is the carrier diffusion through the base layer. This suggests that the conduction-band barrier across the GaInP/GaAs emitter-base junction is so small that the barrier spike does not affect the carrier transport. This result differs from other published results which, by studying device structures other than HBTs, determined the conduction band barrier to be as large as ~50% of the bandgap difference. The findings of the present investigation, however, agree well with another published work which also examined an HBT structure. The difference between these works is discussed     

InGaAs/InAlAs/InP collector-up microwave heterojunction bipolartransistors

Collector-up InGaAs/InAlAs/InP heterojunction bipolar transistors (HBTs) were successfully fabricated, and their DC and microwave characteristics measured. High collector current density operation (J_c>30 kA/cm²) and high base-emitter junction saturation current density (J₀>10^-7 A/cm²) were achieved. A cutoff frequency of f _t=24 GHz and a maximum frequency of oscillation f _max=20 GHz at a collector current density of J₀ =23 kA/cm² were achieved on a nominal 5-μm×10-μm device     

Simulation and design of InAlAs/InGaAs pnp heterojunction bipolartransistors

The performance capabilities of InP-based pnp heterojunction bipolar transistors (HBT's) have been investigated using a drift-diffusion transport model based on a commercial numerical simulator. The low hole mobility in the base is found to limit the current gain and the base transit time, which limits the device's cutoff frequency. The high electron majority carrier mobility in the n⁺ InGaAs base allows a reduction in the base doping and width while maintaining an adequately low base resistance. As a result, high current gain (>300) and power gain (>40 dB) are found to be possible at microwave frequencies. A cutoff frequency as high as 23 GHz and a maximum frequency of oscillation as high as 34 GHz are found to be possible without base grading. Comparison is made with the available, reported experimental results and good agreement is found. The analysis indicates that high-performance pnp InP-based HBT's are feasible, but that optimization of the transistor's multilayer structure is different than for the npn device     

Large-signal characteristics of InP-based heterojunction bipolartransistors and optoelectronic cascode transimpedance amplifiers

A large-signal model for InP/InGaAs-based single HBTs incorporating soft-breakdown effects to the LIBRA Gummel-Poon (GP) model is developed and its validity is established from DC to microwave frequencies and over a wide range of input excitation levels. The large-signal characteristics of a cascode InP-based transimpedance optoelectronic preamplifier employing such devices are studied. Gain compression for the preamplifier was found to take place at an input power level of -20 dBm. Input power excitation varying from -65 to -5 dBm results in a degradation of the amplifier transimpedance gain of the order of 3 dBn. Experimental and theoretical characteristics are presented for the InP-based HBTs and transimpedance amplifier. Self-biasing effects are suggested as possible origin of the transimpedance variations with input power     

A proposed collector design of double heterojunction bipolartransistors for power applications

A new collector design for the AlGaAs-GaAs double heterostructure bipolar transistor (DHBT) is proposed, analyzed, and simulated. The base-collector junction is linearly graded and terminated with a highly doped thin layer to offset the adverse alloy grading electric field. Simple analytical formulas are derived to facilitate the implementation of the design. A proof-of-principle simulation has been carried out for an X-band AlGaAs-GaAs power DHBT to confirm the design and the derived formula. The simulation shows the breakdown voltage can be increased from 30 V to about 45 V while the critical current density is about the same. It is also shown that, unlike other refined DHBT structures, the proposed structure does not require critical control in the fabrication of the base-collector junction     

Comparison of GaInP/GaAs heterostructure-emitter bipolartransistors and heterojunction bipolar transistors

Carbon-doped GaInP/GaAs heterojunction bipolar transistors (HBT's) and heterostructure-emitter bipolar transistors (HEBT's) grown by MOCVD were fabricated. Experimental comparison of HBT's and HEBT's has been made based on the dc and the RF performance. HBT's have higher current gains than those of HEBT's in the high current regime, while HEBT's offer a smaller offset voltage and better uniformity in dc characteristics across the wafer. The current gain and cutoff frequency of the DEBT with a 150 Å emitter set-back layer are comparable to those of HBT's. DC (differential) current gains of 600 (900) and 560 (900) were obtained at a collector current density of 2.5×10⁴ A/cm² for the HBT and HEBT, respectively. The cutoff frequencies are 37 and 31 GHz for the HBT and HEBT, respectively. It is shown that there is negligible contribution of the diffusion capacitance to the emitter capacitance in HEBT's with a thin emitter set-back layer but not with a thick emitter set-back layer. The behavior of HEBT's both in dc and RF characteristics is similar to that of HBT's