Low- and high-field electron-transport parameters for unstrainedand strained Si1-xGex

Ohmic minority and majority drift mobilities as well as saturation velocities are reported for unstrained and strained Si_1-xGe _x alloys up to z=0.31. The electron-transport model is verified by measurements of the in-plane majority drift mobility in strained Si_1-xGe_x samples for various dopant and Ge concentrations. Saturation velocities are determined by full-band Monte Carlo simulations. There is no substantial decrease in the mobility perpendicular to the Si/SiGe interface for doping concentrations above 10¹⁹ cm^-3 and growing x. In contrast, the saturation-drift velocity is strongly reduced with x     

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%     

Minority-carrier transport parameters in degenerate n-type silicon

Direct measurements of the minority-hole transport parameters in degenerate n-type silicon were done by analyzing transient photocurrent in the frequency domain. Minority-hole mobility is found to increase with doping for dopings larger than 4×10¹⁹ cm^-3 . The ratio of minority-hole to majority-hole mobility is found to be about 2.8 at N_D=7.2×10¹⁹ cm^-3. The measured lifetime shows a strongly Auger-dependent mechanism. The extracted Auger coefficient at 296 K is C_n =2.22×10^-31 cm⁶-s^-1, and is in agreement with that reported on other works. Self-consistent checking is used to validate the accuracy of the measured results     

Electron drift mobility model for devices based on unstrained andcoherently strained Si1-xGex grown on <001>silicon substrate

The electron drift mobility for unstrained and coherently strained Si_1-xGe_x grown on a <001> silicon substrate is analytically obtained for Ge fractions less than 30%. The method is based on the following two assumptions: the conduction bands of the unstrained alloy are Si-like for Ge fraction less than 30%, and in the case of the coherently strained alloy, strain-induced energy shifts occur in the conduction band valleys. The shifts in energy yield two different mobility values: one corresponding to the growth plane with a value larger than the unstrained mobility, and the other parallel to the growth direction and correspondingly smaller in value. In comparison to silicon, the results show a degradation of both the unstrained mobilities for doping levels up to 10¹⁷ cm^-3. Beyond this doping level, the strained mobility component parallel to the growth direction becomes slightly larger than the mobility of silicon     

The effect of channel boron implants on electron mobility inNMOSFET's

The Hall mobilities and Hall concentrations of channel electrons in boron-implanted NMOSFETs were measured at 77 and 300 K. At both temperatures, the mobilities were found to decrease with increasing implantation dose (10¹¹-10¹² cm^-2) only for electron concentrations <2×10¹² cm^-2, the effect being more pronounced at 77 K. It is suggested that the mobility degradation is mainly due to impurity scattering     

Electron mobility models for 4H, 6H, and 3C SiC [MESFETs]

Models for the electron mobility in the three most important silicon carbide (SiC) polytypes, namely, 4H, 6H, and 3C SiC are developed. A large number of experimental mobility data and Monte Carlo (MC) results reported in the literature have been evaluated and serve as the basis for the model development. The proposed models describe the dependence of the electron mobility on doping concentration, temperature, and electric field. The low-field mobility in 4H SiC is much higher than in 6H and 3C in the doping range interesting for RF power transistors (10¹⁶ cm^-3 ...10¹⁸ cm ^-3), whereas the saturation velocities in the three polytypes investigated are nearly the same (slightly above 2×10⁷ cm/s at 300 K). The models developed can be easily incorporated into numerical device simulators     

Au/Pt/Ti/Ni ohmic contacts to p-ZnTe

Ohmic contacts of Au/Pd/Ti/Ni to p-ZnTe show a minimum specific contact resistance of 10^-6 Ωcm² for a p-type doping level of 3×10¹⁹ cm^-3 and at an annealing temperature of 300°C. The Ni and Ti layers are very effective in improving the electrical properties of these contact     

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     

Numerical simulation and comparison of Si BJTs andSi1-xGex HBTs

Using the Monte Carlo method for the solution of the Boltzmann transport equation, the authors analyze the low-field carrier mobilities of strained layer and bulk Si and Si_1-xGe_x alloys. Strained alloy layers exhibit higher low-field mobility compared with bulk Si at doping levels >10¹⁸ cm^-3 and for a Ge mole fraction x⩽0.2, while the unstrained alloy bulk low-field mobility is always lower than that of Si for any doping level or mole fraction. These mobilities are then used in a two-dimensional drift-diffusion equation solver to simulate the performance of Si BJTs (bipolar junction transistors) and Si_1-xGe_x HBTs (heterojunction bipolar transistors). The substitution of a Si_0.8 Ge_0.2 layer for the base region leads to a significant improvement in current gain, turn-on voltage, and high-frequency performance. Maximum unity current gain frequency f_T increases two times over that of an Si BJT if the bulk alloy mobility is used for the alloy base layer; it increases three times if strained-layer mobility is used. Maximum frequency of oscillation also improves, but not as dramatically as f_T     

Carbon-doped base GaAs-AlGaAs HBT's grown by MOMBE and MOCVDregrowth

High-quality GaAs-AlGaAs heterojunction bipolar transistors (HBTs) in which the carbon-doped base layers (p=10¹⁰-10²⁰ cm^-3, 400-800 Å thick) and Sn-doped collector and subcollector layers are grown by metalorganic molecular-beam epitaxy (MOMBE) and a subsequent regrowth using metalorganic chemical vapor deposition (MOCVD) is used to provide the n⁺ AlGaAs emitter and GaAs/InGaAs contact layers are discussed. A current gain of 20 was obtained for a base doping of 10¹⁹ cm^-3 (800 Å thick) in a 90-μm-diameter device, with ideality factors of 1.0 and 1.4 for the base-collector and emitter-base junctions, respectively, demonstrating the excellent regrowth-interface quality. For a base doping of 10²⁰ cm^-3 (400 Å thick), the current gain decreased to 8     

Modeling of currents in a vertical p-n-p transistor with extremelyshallow emitter

Detailed simulations of the collector current in a vertical poly-emitter p-n-p transistor have been carried out to verify the minority-hole mobility model of S.E. Swirhun et al. (ibid., vol.7, no.3, p.168-71, 1986). The simulations were based on the SIMS profile, incorporating all published physical parameters, and the results showed good agreement with the measurements for base doping ranging from 10¹⁸ to 10¹⁹ cm^-3. In addition, the effective surface recombination velocity of electrons at the p⁺ poly/Si interface was found by fitting the measured base current to be ~1.4×10⁵ cm/s, which is comparable to its n-p-n counterpart     

Minority-carrier transport parameters in heavily doped p-typesilicon at 296 and 77 K

Minority-carrier electron lifetime, mobility and diffusion length in heavily doped p-type Si were measured at 296 and 77 K. It was found that a 296 K μ_n (pSi)≈μ_n (nSi) for N _AA≲5×10¹⁸ cm^-3, while μ_n (pSi)/μ_n (nSi)≈1 to 2.7 for higher dopings. The results also show that for N_AA≲3×10¹⁹ cm^-3, D (pSi) at 77 K is smaller than that at 296 K, while for higher dopings D_n (pSi) is larger at 77 K than at 296 K. μ_n (pSi) at 77 K increases with the increasing doping above N_AA>3×10¹⁸ cm^-3, in contrast to the opposite dependence for μ_n (nSi) in n⁺ Si     

KrF excimer laser annealed TFT with very high field-effect mobilityof 329 cm2/V-s

A thin-film transistor (TFT) with a maximum field-effect mobility of 320 cm²/V-s, an on/off current ratio of 7.6×10⁷ , a threshold voltage of 6.7 V and a subthreshold slope of 0.37 V/decade was fabricated by using pulse laser annealing processes. Amorphous silicon films (a-Si:H) with a very low impurity concentration of 4×10¹⁸ cm^-3 for oxygen, 1.5×10¹⁸ cm^-3 for carbon, and 2×10¹⁷ cm^-3 for nitrogen were deposited by a plasma chemical vapor deposition (CVD) method and annealed by KrF excimer laser (wavelength of 248 nm). The Raman spectroscopy technique was a useful tool for optimizing laser annealing conditions. Experimental results show that two factors are very important for fabricating very-high mobility TFTs: (1) utilizing high-purity as-deposited a-Si:H film; and (2) performing whole laser annealing processes sequentially in a vacuum container and optimizing illumination conditions     

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     

Hole and electron mobilities in heavily doped silicon: comparison of theory and experiment

Most device models for npn or pnp transistors assume that hole (electron) mobilities in n-type and p-type silicon are equal. Partial-wave phase shift calculations for the contributions of carrier-dopant ion scattering to the carrier mobilities lead to unequal minority hole (electron) and majority hole (electron) mobilities at the same doping density. These calculations are valid over the doping range of 2 x 10¹⁹ to 8 x 10¹⁹ cm⁻³ in n-type and p-type silicon and contain the assumptions that the holes and electrons move in isotropic parabolic energy bands and are scattered by the screened Coulomb potentials of the dopant ions. When the effects of carrier-acoustic phonon and carrier-carrier scatterings are included, these calculations agree to within the spread of experimental value for the majority mobilities reported in the literature. This agreement is a substantial improvement by factors of 2–4 over the results of earlier theories such as first order Born and nondegenerate theories. The results of this work, particularly the inequality of minority and majority carrier mobilities, have implications for the modeling of both bipolar and field effect transistors.     

On the universality of inversion layer mobility in Si MOSFET's:Part I-effects of substrate impurity concentration

This paper reports the studies of the inversion layer mobility in n- and p-channel Si MOSFET's with a wide range of substrate impurity concentrations (10¹⁵ to 10¹⁸ cm^-3). The validity and limitations of the universal relationship between the inversion layer mobility and the effective normal field (E_eff) are examined. It is found that the universality of both the electron and hole mobilities does hold up to 10¹⁸ cm ^-3. The E_eff dependences of the universal curves are observed to differ between electrons and holes, particularly at lower temperatures. This result means a different influence of surface roughness scattering on the electron and hole transports. On substrates with higher impurity concentrations, the electron and hole mobilities significantly deviate from the universal curves at lower surface carrier concentrations because of Coulomb scattering by the substrate impurity. Also, the deviation caused by the charged centers at the Si/SiO₂ interface is observed in the mobility of MOSFET's degraded by Fowler-Nordheim electron injection     

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     

The characteristics of CMOS devices in oxygen-implantedsilicon-on-insulator structures

The characteristics of CMOS devices fabricated in oxygen-implanted silicon-on-insulator (SOI) substrates with different oxygen doses are studied. The results show that transistor junction leakage currents are improved by orders of magnitude when the oxygen dose is decreased from 2.25×10¹⁸ cm^-2 to 1.4×10¹⁸ cm^-2 . The floating-body effect, i.e. transistor turn-on at lower gate voltage with dramatic improvement in subthreshold slope when the drain voltage is increased, is enhanced by the reduction in leakage current and hence the oxygen dose. In SOI substrates implanted with 1.4×10¹⁷ cm^-2 oxygen dose and annealed at 1150°C, back-channel mobilities are decreased by several orders of magnitude compared to the mobilities in the precipitate-free silicon film. These device characteristics are correlated with the microstructure at the silicon-buried-oxide interface, which is controlled by oxygen implantation and post-oxygen-implantation anneal     

Low-threshold room-temperature operation of injectorless quantum-cascade lasers: influence of doping density

The influence of the doping density in the active sections of InP-based injectorless quantum cascade lasers, emitting at 6.8 mum, is investigated. The doping sheet density is varied in the range 2.5-8.6times10¹⁰ cm^-2. Lasing is observed in the whole range, with a threshold current density as low as 1.2 kA/cm² at 300 K for the smallest doping sheet density of 2.5times10 ¹⁰ cm^-2. Further improvement has been made by additionally increasing the number of periods in the active region from 40 to 60. With the same doping level of 2.5times10¹⁰ cm^-2 record low threshold current densities of 0.73 kA/cm² at 300 K were achieved     

A physically-based model of the effective mobility in heavily-dopedn-MOSFETs

We present a new analytical mobility model for channel electrons in heavily-doped MOSFETs biased from weak to strong inversion suitable for implementation in device simulation codes. The model accounts for the two-dimensionality of the electron gas and for the effect of charge trapping on the measurements and has been validated by comparing the theoretical curves with an extensive set of mobility measurements performed on devices with channel doping ranging from 3.8×10¹⁷ to 1.25×10¹⁸ cm^-3 over a wide bias and temperature range (141-400 K)