Modeling silicon emitters for VLSI transistors

The accuracy and reliability of predictions from numerical simulations of advanced bipolar transistors for VLSI applications depend on model input parameters. These parameters include the variations with doping and carrier concentrations in both n-type and p-type silicon of (1) the valance and conduction band edges, (2) the effective intrinsic carrier concentrations, (3) the minority carrier mobilities, and (4) the minority carrier lifetimes. This paper reviews recent advances in device physics for modeling the emitters of bipolar transistors with submicrometer dimensions and high concentrations of dopant ions and carriers.     

Effects of dislocations in silicon transistors with implanted emitters

Silicon bipolar transistors have been made by substituting a shallow phosphorus implanation for the standard emitter deposition used in the manufacture of linear integrated circuits. The implantation was followed by a high temperature heat treatment (drive-in), which caused the implanted ions to diffuse deeper into the semiconductor to give emitter/base junction depths of typically 1.8 μm. When the high temperature heat treatment was performed in an oxidising atmosphere, the resulting transistors had lower gains and higher emitter/base leakages than the comparable standard diffused transistors. However, if an 1180°C drive-in, in an inert atmosphere, was performed prior to the oxidation drive-in, high gains and low emitter/base leakages were obtained. Alternatively, if the oxidation drive-in was omitted, and instead an inert drive-in performed at any temperature between 1000 and 1180°C, high gains and low emitter/base leakages were again obtained. Etching and TEM studies revealed that the low gains and high emitter/base leakages were again obtained. Etching and TEM studies revealed that the low gains and high emitter/base leakages were caused by emitter edge dislocations intersection the emitter/base junction around the perimeter of the emitter. A mechanism is suggested to describe the formation of the emitter edge dislocations.     

On heavy doping effects and the injection efficiency of silicon transistors

A study of heavy doping effects on the current gain of diffused bipolar silicon transistors is continued, with emphasis on the variation of the current gain with injection level. The sensitivity of the calculations to some assumptions and approximations made previously is appraised. Numerical solutions of the heavy doping model simultaneously with the carrier transport equations are included.     

High-performance transistors with arsenic-implanted polysil emitters

Integrated high-frequency transistors (f/SUB T/>3 GHz) with an arsenic implanted polysil emitter have been investigated. The results are compared with data of bipolar transistors made with the conventional planar technique. It is shown that better emitter efficiency higher current carrying capability, and improved emitter-base breakdown can be achieved for transistors with polysil emitters.     

Emitter injection efficiency in heterojunction bipolar transistors

Experimental approaches for measurement of the emitter injection efficiency in heterojunction bipolar transistors are discussed. The electron and hole currents crossing the base-emitter junction and the currents recombining within the quasi-neutral emitter and base region are also determined. The influence of the interface and space-charge region recombination is discussed qualitatively. New figures of merit for a bipolar transistor are introduced. Preliminary experimental results obtained on AlGaAs/GaAs transistors are presented.     

Analytical solutions for high-dose shallow arsenic profiles in silicon

Formulas for surface concentration, junction depth and impurity profile are given for high-dose shallow arsenic diffusions in silicon. These formulas are derived from the analytical solution of the diffusion equation with concentration-dependent diffusivity at constant dose. It was found that the Chebyshev polynomials provide a close approximation of the exact formulas. But although the functional dependence is the same, the coefficients are slightly different. The theoretical profile equation was verified with the SIMS analysis of annealed arsenic implants, which were done at 80 keV with 5 and 8e15 ions/cm².     

Self-aligned transistors with polysilicon emitters for bipolar VLSI

A new bipolar process technology for fabricating self-aligned transistors with polysilicon contacted emitters is described. The extrinsic base regions of the transistor are self-aligned to the emitter contact by exploiting the effects of concentration-dependent oxidation to selectively oxidize the polysilicon. The shallow emitter is fabricated with a thin oxide layer at the polysilicon-silicon interface, thereby enhancing the emitter efficiency and thus the current gain of the device. It is demonstrated that this gain enhancement can be traded for a considerable increase in active base doping, with a resulting decrease in base resistance and potential improvement in switching performance. Under certain circumstances, non-ideal electrical characteristics can be obtained from the self-aligned transistor which are caused by lateral spread of the extrinsic base region beneath the sidewall oxide of the polysilicon emitter contact. This leads to the formation of p⁺-n⁺junction at the periphery of the emitter and hence to tunneling of carriers across this region. It is shown that the same tunneling mechanism also limits the extent to which the active base doping can be increased. In order to avoid the formation of the peripheral p⁺-n⁺junction, a polysilicon base contact is employed which allows a self-aligned extrinsic base region to be fabricated with negligible lateral movement.     

Silicon heterojunction bipolar transistors with amorphous and microcrystalline emitters

The emitter Gummel number of bipolar transistors with an amorphous silicon emitter-base heterojunction is shown to be very large. The temperature dependence of the common-emitter current gain of such heterojunction bipolar transistors is much smaller than that of conventional homojunction transistors. With the actual technology the applicability of amorphous silicon emitters is limited by the emitter series resistance which is too high for VLSI applications. Microcrystalline silicon is a promising emitter material as it combines the high emitter efficiency of amorphous silicon emitters with a much lower resistivity, yielding lower emitter resistances.     

The spectral response and efficiency of heavily doped emitters in silicon photovoltaic devices

An analytical model of the spectral response and efficiency of an emitter is developed. The model is valid for an emitter of any thickness or any doping concentration and profiles. Explicit expression for the spectral response and efficiency of the emitter are derived. They take into account the space dependent mobility μ_p, band gap narrowing ΔE_g, lifetime and electric field. The expression reduces to simpler forms for quasi-transparent and transparent emitters. It is found that unlike a dark emitter, the effect of electric field on the behavior of the illuminated emitter is not negligible. The effect of change in ΔE_g and μ_p models on the spectral response and efficiency of the emitter is discussed. An increase in ΔE_g degrades the performance of the emitter. An increase in μ_p at high doping also degrades the behavior but only if the surface recombination velocity S_p is large. If S_p is very small, an increase of μ_p improves the spectral response and the efficiency of the emitter. The results calculated theoretically are in good agreement with the experimental results of Ref. [25] both for thin and thick emitters except for λ ≤ 0.4 μm and λ > 1.0μm. This discrepancy is attributed to the uncertainties in the experimental values of α at these wavelengths. It is shown that the value of μ_p at high dopings can be determined from the spectral response measurements provided that the value of S_p can be determined accurately from other independent measurements.     

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     

Biasing of silicon transistors

A modification of the voltage-feedback bias circuit applicable to modern silicon transistors is described, and formulas are given for the calculation of performance. For moderate power-supply potentials, variations in VCE can be held within 1.0 V for a temperature rise of 75 degC.     

Characteristics of silicon transistors

The d.c. and low-frequency a.c. characteristics of a number of silicon transistors have been measured over a collector-current range of 10 ?A?1 mA. The variations of d.c. and a.c. current gain, input resistance and mutual conductance with collector current are described by simple equations.     

Improving hydrogenation efficiency of polycrystalline silicon thin film transistors by a new approach

In this paper, a new hydrogenation process of poly-Si thin film for the fabrication of poly-Si thin film transistors (TFTs) is proposed. In the new approach, the hydrogenation of TFTs is performed before deposition of contact metal. N-channel and p-channel poly-Si TFTs with various channel lengths and widths were fabricated with the new and conventional processes for comparison. The results verified that the efficiency of hydrogenation has been improved remarkably by the new process. The field-effect mobility of carriers, the on state current, threshold voltage and the on/off states current ratio have been greatly improved, and the trap state density has been reduced significantly.     

Microcavities with field emitters sealed by silicon wafer bonding

A method of fabricating micrometre-sized field emission diodes is described. The devices use silicon field emitters that have been grown by selective epitaxy into 0.9 mu m sized contact holes in a 1 mu m thick field oxide. The anode of the device structure is a silicon wafer directly bonded to the field oxide.<>     

Polycrystalline silicon thin-film transistors with two-stepannealing process

Thin-film transistors (TFTs) were fabricated from poly-Si crystallized by a two-step annealing process on glass substrates. The combination of low-temperature furnace annealing and high-temperature rapid thermal annealing leads to a significant improvement in the material quality. The TFTs obtained with this two-step annealing material exhibit better measured characteristics than those obtained by using conventional furnace annealing     

Stable,high quantum efficiency,UV-enhanced silicon photodiodes by arsenic diffusion

Very high quantum efficiency, UV-enhanced silicon photodiodes have been developed by arsenic diffusion into p-type silicon as an alternative to the inversion layer photodiodes commonly used in precise radiometric and spectroscopic measurements. The fabricated diodes had an unbiased internal quantum efficiency that was 100% from 350 to 550 nm, and that exceeded 100% at shorter wavelengths. A typical responsivity at 200 nm was 0.1 A/W. No degradation in responsivity was detected anywhere in the 200–1100 nm range when these devices were exposed to 20 mW/cm² of 254 nm radiation for 60 days. Thus the theoretical maximum value of internal quantum efficiency for a diffused photodiode appears to have been achieved in the UV and short wavelength visible, without compromising the diode's long term stability. This is in marked contrast to older types of diffused photodiodes, which either were “dead” in the UV, or exhibited a spectral response vs flux characteristic that changed considerably with UV exposure.     

Influence of the dopant density profile on minority-carrier current in shallow,heavily doped emitters of silicon bipolar devices

Experimental determination of the dependence of recombination current in p⁺ and n⁺ regions on the dopant profile for shallow emitters of ion-implanted silicon solar cells is described. The results are analyzed by extending a previous analytical model for the transport of minority carriers in heavily doped regions. The extension accounts for an effective electric field, defined by heavy-doping effects at the surface, and suggests that the energy-gap narrowing for p⁺ silicon is slightly smaller than that for n⁺ silicon and/or that minority-carrier diffusivities are substantially lower than the majority-carrier ones at comparable dopant densities. The very high dopant densities achieved with the ion implantation/laser annealing technique provide an in situ surface passivation that supresses surface recombination and minimizes the emitter recombination current.