Low turn-on voltage InGaP/GaAsSb/GaAs double HBTs grown by MOCVD

A novel InGaP/GaAs_0.92Sb_0.08/GaAs double heterojunction bipolar transistor (DHBT) with low turn-on voltage has been fabricated. The turn-on voltage of the DHBT is typically 150 mV lower than that of the conventional InGaP/GaAs HBT, indicating that GaAsSb is a suitable base material for reducing the turn-on voltage of GaAs HBTs. A current gain of 50 has been obtained for the InGaP/GaAs_0.92Sb_0.08/GaAs DHBT. The results show that InGaP/GaAsSb/GaAs DHBTs have a great potential for reducing operating voltage and power dissipation     

Electrical properties of high permittivity ZrO2 gate dielectrics on strained-Si

Deposition and electrical properties of high dielectric constant (high-k) ultrathin ZrO₂ films on tensilely strained silicon (strained-Si) substrate are reported. ZrO₂ thin films have been deposited using a microwave plasma enhanced chemical vapor deposition technique at a low temperature (150 °C). Metal insulator semiconductor (MIS) structures are used for high frequency capacitance–voltage (C–V), current–voltage (I–V), and conductance–voltage (G–V) characterization. Using MIS capacitor structures, the reliability and the leakage current characteristics have been studied both at room and high temperature. Schottky conduction mechanism is found to dominate the current conduction at a high temperature. Observed good electrical and reliability properties suggest the suitability of deposited ZrO₂ thin films as an alternative as gate dielectrics. Compatibility of ZrO₂ as a gate dielectric on strained-Si is shown.     

Breakdown behavior of GaAs/AlGaAs HBTs

Avalanche breakdown behavior at the collector junction of the GaAs/AlGaAs HBT (heterojunction bipolar transistor) has been studied. Junction breakdown characteristics displaying hard breakdown, soft breakdown, and negative resistance breakdown behavior were observed and are interpreted by analysis of localized microplasma effects, uniform microplasma-free behavior, and associated current gain measurements. Light emission from the collector-base junction of the GaAs/AlGaAs HBT was observed and used to investigate breakdown uniformity. Using a simple punchthrough breakdown model, the theoretical breakdown curves at different collector doping concentrations and thicknesses were computed and found to be in agreement with maximum breakdown voltages measured from devices displaying the most uniform junction breakdown. The serious current gain degradation of GaAs/AlGaAs HBTs at low current densities was analyzed in connection with the measurement of a large collector-emitter breakdown voltage. The unexpected functional relationship between the collector-emitter breakdown voltage and collector-base breakdown voltage is explained by the absence of a hole-feedback effect for devices not exhibiting transistor action     

Submicrometer self-aligned AlGaAs/GaAs heterojunction bipolartransistor process suitable for digital applications

A self-aligned process is developed to obtain submicrometer high-performance AlGaAs/GaAs heterojunction bipolar transistors (HBTs) which can maintain a high current gain for emitter sizes on the order of 1 μm². The major features of the process are incorporation of an AlGaAs surface passivation structure around the entire emitter-base junction periphery to reduce surface recombination and reliable removal of base metal (Ti/W) deposits from the sidewall by electron cyclotron resonance (ECR) plasma deposition of oxide and ECR plasma etching by NF₃. A DC current gain of more than 30 can be obtained for HBTs with an emitter-base junction area of 0.5×2 μm² at submilliampere collector currents. The maximum f_T and f_max obtained from a 0.5×2 μm² emitter HBT are 46 and 42 GHz, respectively at I_C=1.5 and more than 20 GHz even at I_C=0.1 mA     

Density of States-Based DC $I$– $V$ Model of Amorphous Gallium–Indium–Zinc-Oxide Thin-Film Transistors

The density of states (DOS)-based DC I-V model of an amorphous gallium-indium-zinc oxide (a-GIZO) thin-film transistor (TFT) is proposed and demonstrated with self-consistent methodologies for extracting parameters. By combining the optical charge-pumping technique and the nonlinear relation between the surface potential (phiS) and gate voltage (V _GS), it is verified that the proposed DC model reproduces well both the measured V _GS-dependent mobility and the I _DS-V _GS characteristics. Finally, the extracted DOS parameters are N _TA = 4.4 times 10¹⁷ cm^-3 middot eV^-1, N _DA = 3 times 10¹⁵ cm^-3 middot eV^-1, kT _TA = 0.023 eV, kT _DGA = 1.5 eV, and EO = 1.8 eV, with the formulas of exponential tail states and Gaussian deep states.     

Direct extraction and numerical simulation of the base andcollector delay times in double heterojunction bipolar transistors

A new method is presented to evaluate the base and collector transit times, τ_B and τ_C in heterojunction bipolar transistors (BBT's) from the phase and magnitude of the common-base current gain, α(ω), which itself was directly extracted from measured S-parameter data. The method is applied to InGaP/GaAs single and double HBT's. A smaller cutoff frequency in the latter device is attributed to τ_B and τ_C due to two effects: trapping of electrons in the conduction band triangular barrier existing at the base-collector (B-C) heterojunction and smaller saturation velocity of electrons in InGaP as compared to GaAs. Finally, a new B-C design of InGaP/GaAs DNBT's is proposed to partially compensate the transit time effects. Numerical simulation of the cutoff frequency demonstrates the superiority of the proposed structure for high-frequency applications     

Stress current behavior of InAlAs/InGaAs and AlGaAs/GaAs HBT's withpolyimide passivation

InAlAs/InGaAs and AlGaAs/GaAs HBTs, with heavily Be-doped base layers, have been fabricated and their reliability under excessive forward current tested. To understand the HBT material difference, a common process based on a polyimide planarization method is applied to the fabrication. While short-term degradation induced by stress current is observed for AlGaAs/GaAs HBTs, InAlAs/InGaAs HBTs are stable up to a current density of 1.5×10⁵ A/cm², indicating the absence of substantial Be diffusion. An analysis of base current has shown a striking contrast between the HBTs in terms of the stressing effect on the surface recombination along emitter junction periphery     

Design study of AlGaAs/GaAs HBTs

The frequency performance of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) having different layouts, doping profiles, and layer thicknesses was assessed using the BIPOLE computer program. The optimized design of HBTs was studied, and the high current performances of HBTs and polysilicon emitter transistors were compared. It is shown that no current crowding effect occurs at current densities less than 1×10⁵ A/cm² for the HBT with emitter stripe width S_E<3 μm, and the HBT current-handling capability determined by the peak current-gain cutoff frequency is more than twice as large as that of the polysilicon emitter transistor. An optimized maximum oscillation frequency formula has been obtained for a typical process n-p-n AlGaAs/GaAs HBT having base doping of 1×10 ¹⁹ cm^-3     

High-performance InGaP/GaAs pnp δ-doped heterojunction bipolar transistor

In this article, a novel InGaP/GaAs pnp δ-doped heterojunction bipolar transistor is first demonstrated. Though the valence band discontinuity at InGaP/GaAs heterojunction is relatively large, the addition of a δ-doped sheet between two spacer layers at the emitter-base (E-B) junction effectively eliminates the potential spike and increases the confined barrier for electrons, simultaneously. Experimentally, a high current gain of 25 and a relatively low E-B offset voltage of 60 mV are achieved. The offset voltage is much smaller than the conventional InGaP/GaAs pnp HBT. The proposed device could be used for linear amplifiers and low-power complementary integrated circuit applications. The article is published in the original.     

Significant time constants defined by high-current charge dynamicsin advanced silicon-based bipolar transistors

Analytical expressions for the time constants in advanced silicon-based bipolar transistors defined by the charge dynamics in the base-collector junction space-charge region (τ_C) and by the charge modulation in the quasi-neutral base (τ_BM) are derived based on an accounting for the high-current-induced perturbation of the space-charge region. The derivations show that voltage drops in the intrinsic and extrinsic collector regions and in the extrinsic emitter region are important in defining τ_C and τ_BM, and that τ_BM is approximately proportional to collector-current density. Application of the results to an aggressive SiGe-base HBT technology shows that τ_C and τ_BM are comparable to the base transit time, and hence that they are significant in defining high-current speed of the HBT     

Modeling of current transport and 1/f noise in heterojunction bipolar transistors

An analytic model is proposed to determine the effect of band offsets at heteroemitter interface on the current transport and 1/f noise in heterojunction bipolar transistors (HBTs). The proposed model uses the modified form of drift-diffusion formalism, which requires that the net recombination rates be proportional to the densities of other type carriers across the heterointerface. The numerical analysis of the current–voltage and 1/f noise characteristics of Npn AlGaAs/GaAs HBT and npn GaAs BJT demonstrates that the role of band offsets at heteroemitter interface in the overall current transport and 1/f noise is very important in HBTs at low forward biases. The junction resistance due to diffusing minority electrons is much stronger (weaker) at small (high) forward biases than that due to recombined electrons and holes across the heteroemitted space charge region in both Npn AlGaAs/GaAs HBTs and npn GaAs BJTs.     

Emitter injection and collector current ideality in abrupt heterojunction AlInAs/GaInAs HBTs

We investigated the electron injection process for high-speed N-p-n AlInAs/GaInAs HBTs by measuring collector and base currents as a function of base-emitter voltage with collector-base voltage equal to zero (Gummel plots) at temperatures from 77 to 300 K. We compared the measured collector current with calculations based on electron injection from emitter to base by tunneling through the conduction band spike and thermionic emission over it, using a modified version of the thermionic-field emission theory developed by Crowell and Rideout. Good agreement was obtained between the experimental collector current ideality factor and tunneling-thermionic emission theory for all temperatures and currents. This is an improvement over drift-diffusion and thermionic emission models, which have been used for HBTs but which do not correctly describe the experimentally observed temperature and current dependence of the ideality of the collector current. The tunneling-thermionic emission model explains the increase in collector current ideality factor that occurs as the transistor is biased at high collector current density (J_C 10⁵ A cm⁻²), which is the regime of operation in which f_T is maximized and a low ideality factor is most important. The model also explains the experimentally observed variation of h_FE with ln I_C. Thus the tunneling-thermionic emission model is a useful aid in the design of the epitaxial structure for high-frequency HBTs.     

Monolithically integrated SQW laser and HBT laser driver viaselective OMVPE regrowth

An AlGaAs/GaAs nan heterojunction bipolar transistor (HBT) laser driver circuit and a pseudomorphic InGaAs/GaAs/AlGaAs graded index single-quantum-well (SQW) laser have been laterally integrated to maintain surface planarity using selective organometallic vapor-phase epitaxy (OMVPE) regrowth of the HBT. The self-aligned HBTs exhibit a DC current gain of 30 and an f_t (f_max ) of 45(60) GHz. The 980-nm lasers exhibit room-temperature threshold current densities as low as 420(320) A/cm² for CW (pulsed) operation. The cavities are 40(7) μm×500 μm and have less than 1(2) Ω of series resistance. SPICE simulations of the integrated driver indicate that operating speeds of over 10 Gb/s are possible     

Small-scaled InGaP/GaAs HBTs with WSi/Ti base electrode and buriedSiO2

This paper describes the fabrication and characteristics of small-scaled InGaP/GaAs HBTs with high-speed as well as low-current operation. To reduce both the emitter size S_E and the base-collector capacitance C_BC simultaneously, the HBTs are fabricated by using WSi/Ti as the base electrode and by burying SiO₂ in the extrinsic base-collector region under the base electrode. WSi/Ti simplifies and facilitates processing to fabricate a small base electrode, and makes it possible to reduce the width of the base contact to less than 0.4 μm without the large increase in the base resistance. The DC current gain of 20 is obtained for an HBT with S _E of 0.3×1.6 μm² due to the suppression of emitter size effect by using InGaP as the emitter material. An HBT with SE of 0.6×4.6 μm² exhibited f_T of 138 GHz and f_max of 275 GHz at I_C of 4 mA; and an HBT with SE of 0.3×1.6 μm² exhibited f_T of 96 GHz and f_max of 197 GHz at I_C of 1 mA. These results indicate the great potential of these HBTs for high-speed and low-power circuit applications     

High-speed small-scale InGaP/GaAs HBT technology and itsapplication to integrated circuits

We have developed the advanced performance, small-scale InGaP/GaAs heterojunction bipolar transistors (HBTs) by using WSi/Ti base electrode and buried SiO₂ in the extrinsic collector. The base-collector capacitance C_BC was further reduced to improve high-frequency performance. Improving the uniformity of the buried SiO ₂, reducing the area of the base electrode, and optimizing the width of the base-contact enabled us to reduce the parasitic capacitance in the buried SiO₂ region by 50% compared to our previous devices. The cutoff frequency f_T of 156 GHz and the maximum oscillation frequency f_max of 255 GHz were obtained at a collector current I_C of 3.5 mA for the HBT with an emitter size S_E of 0.5×4.5 μm², and f_T of 114 GHz and f_max of 230 GHz were obtained at I_C of 0.9 mA for the HBT with S_E of 0.25×1.5 μm². We have also fabricated digital and analog circuits using these HBTs. A 1/8 static frequency divider operated at a maximum toggle frequency of 39.5 GHz with a power consumption per flip-flop of 190 mW. A transimpedance amplifier provides a gain of 46.5 dB·Ω with a bandwidth of 41.6 GHz at a power consumption of 150 mW. These results indicate the great potential of our HBTs for high-speed, low-power circuit applications     

First demonstration of monolithic InP-based HBT amplifier with PNPactive load

An InP-based integrated HBT amplifier with PNP active load was demonstrated for the first time using complementary HBT technology (CRBT). Selective molecular beam epitaxy (MBE) regrowth was employed and a merged processing technology was developed for the monolithic integration of InP-based NPN and PNP HBTs on the same chip. The availability of PNP devices allowed design of high gain amplifiers with low power supply voltage. The measured amplifier with PNP HBT active load achieved a voltage gain of 100 with a power supply (V_CC) of 1.5 V. The corresponding voltage swing was 0.9 V to 0.2 V. The amplifier also demonstrated S₂₁ of 7.8 dB with an associated S₁₁ and S₂₂ of -9.5 dB and -8.1 dB, respectively, at 10 GHz     

Analysis of I–V measurements on PtSi-Si Schottky structures in a wide temperature range

I–V Measurements on PtSi-Si Schottky structures in a wide temperature range from 90 to 350 K were carried out. The contributions of thermionic-emission current and various other current-transport mechanisms were assumed when evaluating the Schottky barrier height Φ₀. Thus the generation-recombination, tunneling and leak currents caused by inhomogeneities and defects at the metal-semiconductor interface were taken into account.

Taking the above-mentioned mechanisms and their temperature dependence into consideration in the Schottky diode model, an outstanding agreement between theory and experiment was achieved in a wide temperature range.

Excluding the secondary current-transport mechanisms from the total current, a more exact value of the thermionic-emission saturation current I_te and thus a more accurate value ofΦ_b was reached.

The barrier height Φ_b and the modified Richardson constant A^** were calculated from the plot of thermionic-emission saturation current I_te as a function of temperature too. The proposed method of finding Φ_b is independent of the exact values of the metal-semiconductor contact area A and of the modified Richardson constant A^**. This fact can be used for determination of Φ_b in new Schottky structures based on multicomponent semiconductor materials.

Using the experimentally evaluated value A^** = 1.796 × 10⁶ Am⁻²K⁻² for the barrier height determination from I–V characteristics the value of Φ_b = 0.881 ± 0.002 eV was reached independent of temperature.

The more exact value of barrier height Φ_b is a relevant input parameter for Schottky diode computer-aided modeling and simulation, which provided a closer correlation between the experimental and theoretical characteristics.     

High-performance InP/In0.53Ga0.47As/InPdouble HBTs on GaAs substrates

InP/In_0.53Ga_0.47As/InP double heterojunction bipolar transistors (HBTs) were grown on GaAs substrates. A 140 GHz power-gain cutoff frequency f_max and a 207 GHz current-gain cutoff frequency f_τ were obtained, presently the highest reported values for metamorphic HBTs. The breakdown voltage BV_CEO was 5.5 V, while the dc current gain β was 76. High-thermal-conductivity InP metamorphic buffer layers were employed in order to minimize the device-thermal resistance     

The effects of base dopant diffusion on DC and RF characteristicsof InGaAs/InAlAs heterojunction bipolar transistors

The effects of base p-dopant diffusion at junction interfaces of InGaAs/InAlAs HBTs with thin base thicknesses and high base dopings are reported. It is shown that HBTs with compositionally graded emitter-based (E-B) junctions are very tolerant to base dopant outdiffusion into the E-B graded region. The RF performance is nearly unaffected by the diffusion, and the DC current gain and E-B junction breakdown voltages are improved with finite Be diffusion into the E-B graded region