Ion-sensitive field-effect transistors with inorganic gate oxide for pH sensing

Ion-sensitive field-effect transistors (ISFET's) have been fabricated by using silicon films on sapphire substrates (SOS). Using this structure SiO2, ZrO2, and Ta2O5films are examined as hydrogenion-sensitive materials, and Ta2O5film has been found to have the highest pH sensitivity (56 mV/pH) among them. The measured pH sensitivity of this SOS-ISFET's is compared with the theoretical sensitivity based on the site-binding model of proton dissociation reaction on the metal oxide film and good agreement between them is obtained.     

p-Channel TFT's using magnetron-sputtered Ta2O5films as gate insulators

Metal-gate thin-film transistors (TFT's) have been fabricated in layers of laser-recrystallized polycrystalline silicon on fused quartz substrates at processing temperatures below 625°C. Tantalum pentoxide (Ta2O5) was used as a gate insulator instead of a conventional thermally grown silicon dioxide (SiO2). Ta2O5gate insulator was deposited onto the recrystallized silicon layer at room temperature, using an RF-magnetron sputtering system. The reactive ion etching method, using CF4as a reactive gas, was employed in patterning deposited Ta2O5. These TFT's have exhibited p-channel depletion-mode characteristics with a threshold voltage of 2.5 V and a transconductance of 70 µS at Vg= - 2 V. An on-off current ratio exceeding 10⁵has been obtained.     

Characteristics of MOSFET prepared on Si/MgO.Al2O3/SiO2/Si structure

A new silicon on insulator (SOI) wafer with epitaxial-Si/ epitaxial-MgO.Al2O3(0.1 µm)/SiO2(0.5 µm)/     

The equivalent circuit model in solid-state electronics—Part II: The multiple energy level impurity centers

In this paper, the equivalent circuit of a defect center with multiple energy levels in a semiconductor is formulated and applied to the calculation of the characteristic time constants of the multiple energy level system. Detailed numerical example of gold-doped silicon is given, including the constant characteristic time constant contour maps. The steady-state charge distribution ratio RN= Ns+1/Ns(s = charge state of the center) and recombination rate ratio RR(s) = RSS(s = ½)/(RSS(s-½) diagrams are developed and applied to the numerical calculations of the steady-state recombination and generation rate of electrons and holes in gold-doped silicon.     

The SIS tunnel emitter: A theory for emitters with thin interface layers

Silicon n-p-n transistors are made with emitters consisting of a polycrystalline and monocrystalline region with a thin (20-60-Å) "insulating" interfacial layer in between (SIS structure). These transistors show a remarkable increase in emitter efficiency with emitter Gummel numbers up to 7 × 10¹⁴s.cm^-4, and a low positive or even negative temperature coefficient of the current gain. A model is proposed to explain the mechanism in terms of tunneling through the interfacial layer. The electrical characteristics are measured in the temperature range 290-415 K. From the measurements it is deduced that the tunnel probability for holes (Ph) is 10^-2to 10^-3and that for electrons (Pe) is >10^-5. There also exists a band bending of 30-90 mV at the interfaces with the interfacial layer.     

A review of recent experiments pertaining to hole transport in Si3N4

In spite of the recent accumulation of experimental evidence for hole conduction in Si3N4it has been largely ignored in modeling of MNOS devices, especially, when hole injection from the metal electrode should have been considered. A review of recent experiments related to hole conduction in Si3N4films deposited on silicon is given. The experiments include: photo-induced and darkI-V; C-Vand flat-band tracking; charge-centroid; and shallow junction "carrier-type" experiments. The case for hole conduction is established firmly for both polarities of applied voltage; however, while agreement exists that holes dominate the conduction for negative polarity (injection of holes from the silicon substrate), differences of opinion remain about the role of electron conduction under positive polarity. A comparison of the qualitative features of the valence band structure of Si3N4and SiO2is included to show that the same reason for low hole conduction in SiO2is not expected in Si3N4.     

Hole current in dual dielectric under positive gate voltage

n-channel dual-dielectric transistors with SiO2:Si3N4gate insulators were fabricated with and without boron implant in the channel. Under positive gate voltage stress, electrons can enter the insulator from the silicon, and holes can enter the silicon from the insulator. The electrons and holes were measured by the technique described by Ginovker et al.[1]. For oxides thicker than 30 Å, it is always observed that the silicon hole current is 3-4 orders of magnitude below the silicon electron current. The physical origin of this hole current is shown to be field-enhanced excitation of electrons from the valence band to the conduction band in the silicon prior to entering the insulator, and is not due to the holes from the insulator entering the silicon.     

High-voltage planar structure using SiO2-SIPOS-SiO2film

The BVCB0leakage current hFEwere studied for high-voltage planar transistors which had three kinds of passivation films; SiO2-semi-insulating polycrystalline silicon (SIPOS)-SiO2; SIPOS-SiO2; and SiO2-phosphosilicate glass (PSG)-SiO2. The SiO2-SIPOS-SiO2type had a lower leakage current (surface generation current) and higher hFEthan the conventional SIPOS-SiO2type. The SiO2-SIPOS-SiO2type also had the highest BVCB0due to the field-plate effect.     

Controlling void formation in WSi2polycides

Polycides, composite wiring/electrode films formed by depositing a refractory metal silicide such as WSi2, MoSi2, or TaSi2atop a polysilicon film [1]-[4], are finding their way into IC technologies as low-resistivity electrodes/interconnects. One of the desirable features of polycide composite films is their ability for self-passivation through thermal oxidation. In some cases, however, oxidation of the two-layer materials results in the formation of large "voids" in the polysilicon film (bottom layer in the polycide) [5], [6]. A method for preventing this void formation has been found. The solution involves deposition of a thin silicon layer onto the existing two-layer material. The additional layer is designed to provide some of the silicon required for oxidation during the initial stages of self-passivation. In cases where over 200 nm of SiO2were grown atop a WSi2polycide, a silicon layer as thin as 15-nm prevented void formation.     

In-situ low energy BF2+ion doping for silicon molecular beam epitaxy

An experimental study of the p-type ion dopant BF2+ in silicon molecular beam epitaxy (MBE) is described. BF2+ was used to dope MBE layers during growth to levels ranging from 1 × 10¹⁶/cm³to 4 × 10¹⁸/cm³over a growth temperature range of 650°C to 1000°C. The layers were evaluated using spreading resistance, chemical etching, and secondary ion mass spectroscopy. Complete dopant activation was observed for all growth temperatures. Remnant fluorine in the epitaxial layer was less than 2 × 10¹⁶/cm³in all cases. Diffused p-n junction diodes fabricated in BF2+-doped epitaxial material showed hard reverse breakdown characteristics.     

Design considerations of low-noise high-efficiency silicon IMPATT diodes

The noise and efficiency of p⁺-n1-n2-n⁺and n⁺-p1- p2-p⁺high-low silicon IMPATT diodes have been studied as a function of doping ratio n1/n2or p1/p2. In contrast to GaAs IMPATT diodes whose efficiency can be improved with some degradation of noise performance, both the efficiency and noise of Si IMPATT diodes can be improved. As an example, for a 6-GHz silicon n⁺-p1-p2-p⁺IMPATT structure with a doping ratio of 10, the efficiency is 21 percent and the incremental noise as compared to a uniformly doped structure is about -6 dB. These results indicate that silicon high-low structures can compete favorably with GaAs structures in both efficiency and noise performances.     

Reliability of MINP compared to MIS, SIS, and N/P silicon solar cells under 1.0-MeV electron and environmental effects

The effects of 1.0-MeV electron radiation are compared for MIS, SIS, N/P, and MINP silicon solar cells. MIS, SIS, and N/P silicon solar cells are comparable in performance except that SIS cells degraded faster due to use of n-type Si substrates. MINP cells exhibited superior performance in that efficiency degraded 9 percent at a fluence of 1 × 10¹⁵e^-/cm²and 32 percent at a fluence of 1 × 10¹⁶e^-/cm²compared to 29 percent and 49 percent, respectively, for N/P cells. MINP cells utilize an SiO2insulator layer over a thin N-region, and a low work function metal contact. This design gives a high ultraviolet response and low surface recombination velocity which maintains high efficiency since most of the radiation loss occurs in the infrared region due to bulk damage effects.     

Electronic processes at grain boundaries in polycrystalline semiconductors under optical illumination

The dependence of minority carrier lifetime (τ) on the doping concentration Nd, grain sizedand interface state density Nisat the grain boundaries in (n-type) polycrystalline semiconductors has been calculated analytically. The recombination velocity at grain boundaries is enhanced by the diffusion potential Vdadjacent to the boundaries, and ranges fromsimeq 10^{2}to 10⁶cm . s^-1depending on Nisand Nd. Under illumination, the population of the interface states is altered considerably from its dark level and as a result, Vddecreases to that value which maximizes recombination (equal concentrations of electrons and holes at the boundary). This causes τ to decrease with increasing Nd. Sample calculations for polycrystalline silicon show that for low angle boundaries with interface state densities ofsimeq 10^{11}cm^-2eV^-1, τ decreases from 10^-6to 10^-10s as the grain size is reduced from 1000 to 0.1 µm (forN_{d} = 10^{16}cm^-3). For a constant grain size, τ decreases with increasing Nd. The open-circuit voltage of p-n junction solar cells decreases fortau leq 10^{-7}s, whereas that for Schottky barrier cells remains at its maximum value untiltau lsim 10^{-8}s.     

Metallization by ionized cluster beam deposition

Preferentially oriented and epitaxial Al films have been deposited by ionized cluster beams (ICB). The thermal stability of these films has been examined by SEM, AES, ion backscattering, and electrical characterization. In preferentially oriented films, long electromigration lifetime and low-temperature contact formation are expected. In epitaxial films on silicon, alloy penetration at the interface, a shift in barrier height, and degradation of crystalline quality were not observed after heat treatment up to 550 °C. Extremely long electromigration lifetime was also confirmed. Epitaxial growth Of Al film on CaF2, GaAs, and sapphire was attempted and preliminary results are given.     

Application of laserQswitching for infrared spectroscopy

PassiveQswitching of IR lasers (N2O as well as CO2) has been applied for the detection of coincidences within a few 100 MHz between laser lines (νL) and molecular transitions (νM). Polar (¹²CH3F,¹³CH3F, C2H5OH, and C6H5CH3) as well as nonpolar molecules (SiH4, GeH4, CCl4, C2H4, C3H4, and C6H6) have been studied. The phenomenon depends strongly on the frequency mismatchDeltanu = nu_{M} - nu_{L}. 81combinations between laser lines and molecular absorption lines were found to produce theQ-switch effect.     

P-N junction and Schottky barrier diode fabrication in laser recrystallized polysilicon on SiO2

P-N junction and Schottky barrier diodes have been fabricated in laser recrystallized polycrystalline silicon on SiO2. A repetitively Q-switched Nd³⁺:YAG laser was used to anneal polysilicon deposited by LPCVD on oxide thermally grown on     

A laser augmented reaction: SF6+ SiH4→S*2+ SiF4+ HF + H2· retention of isotopic selectivity during detonation

A single unfocused pulse of a free running CO2laser, area ∼ 8 cm², initiates an explosive reaction between SF6and SiH4. This occurs at a minimum energy of 4 J [full width at half maximum (FWHM)sim 1.5 /mus] of which about one half is absorbed in an 8 cm long cell; total pressure 12 torr; 0.65 <p(SiH4)/p(SF6) < 1.8. The spectral and temporal distributions of the emitted chemiluminescence depend sensitively on the fuel to oxidizer ratio, and on the pulse energy; we investigated the range 4 → 20 J. The principal emission is due to S2(B^{3}Sigma-_{u} rightarrow X^{3}Sigma-_{g}). Transitionsupsilon' (0-4) rightarrow upsilon" (2-15)were recorded. In the³Sigma-_{u}state, vibrational temperatures range from 3000-13000 K. The luminosity peaks sharply at (SiH4)/(SF6) = 1.0 ± 0.05. On each side of the maximum of the emission versus composition curve [at (SiH4)/(SF6) ≈ 0.95 and 1.22, for a 12 J pulse] the residual SF6(0.2-0.5 percent of initial amount) is enriched in³⁴SF6; the observed fractionation factors at these two compositions are 8 ± 2. The separation between the two sharply peaked optimum compositions appears to increase with increasing pulse energy. Preliminary results with other fuels suggest that the concurrent absorption of CO2laser radiation by the fuel, as well as a highly exothermic reaction, are pre-requisite for fine tuning of composition, injected power, and total pressure for optimum isotope fractionation.     

Effects of gas flow on gain of 10.6 micron CO2laser amplifiers

Small-signal gain of flowing gas CO2laser amplifiers at 10.6 microns has been optimized for media including pure CO2CO2: N2, CO2: He, CO2: CO, CO2: O2, CO2: N2: He, CO2: CO : He, and CO2: CO : N2. Optimum gain of all flowing gas systems studied increases monotonically with increasing gas flow rate. In the low CO2flow rate region, 10 < RCO2: < 50 cm³/min, gas flow enhances the gain most for systems containing N2. Results provide strong evidence that the rapid increase in gain with flow rate in CO2: N2mixtures is due to removal by convection of the dissociated product CO. For 50 < RCO2< 200 cm³/min, a slow linear increase in gain of all gas mixtures with increasing flow rate occurs and is attributed to the cooling of gas temprature by convection. A stronger dependence of gainGon amplifier boreD, viz.,G propto I/D, was obtained for flowing gas media relative to that previously observed for nonflowing gas mixtures which is consistent with the proposed mechanism of gas cooling by convection. Highest gain values obtained were 7.8 and 6.2 dB/m with the flowing gas mixtures CO2: N2: He and CO2: CO : He, respectively, in a 12 mm bore water-cooled amplifier tube. Similarities between CO2: N2and CO2: CO systems suggest that pumping of the CO2laser by resonant transfer from CO* (upsilon = 1) can be significant.     

On the origin of leakage currents in silicon-on-sapphire MOS transistors

n-channel n-p-n metal-oxide-semiconductor transistors (MOST's), fabricated in thin films of silicon-on-sapphire, exhibit values of source-to-drain leakage currents (IL)which vary from wafer to wafer, typicaily from 10^-11to 10^-7A/mil of channel width. Conversely, p-channel (p-n-p) devices exhibit low leakage current values in the range of 10^-11∼ 10^-10A/mil of channel width, consistent from wafer to Wafer. A model of a high concentration of donorlike states in the silicon in the vicinity of the Al2O3-Si interface creating a back-surface Conductive channel is proposed to account for both the inconsistently high n-channel and consistently low p-channel leakage current values. Experimental measurements of IL, which support the general conclusions of the model, are presented. ILis shown to be a strong function of a) the annealing temperature of the sapphire substrate prior to film growth, b) the silicon-film growth rate, c) the impurity concentration profile in the channel region, and d) the device geometry. These measurements show that the dominant factor controlling the overall magnitude of ILis the state of the Al2O3-Si interface immediately prior to silicon-film growth. A set of silicon-film growth conditions and device processing steps is outlined which achieve consistent n- and p-channel leakage current values of less than 10^-9A/mil of gate width.     

Noise temperature in silicon in the hot electron region

The noise temperature as a function of the applied field has been measured on an epitaxial silicon layer at the frequencies 2 GHz and 4 GHz. It has been found that the experimental results are in good agreement with the theory given by Moll. It is shown that for noise calculations in silicon field-effect transistors with pronounced carrier velocity saturation the noise temperature Tnversus field E may be approximated byT_{n}/T_{0}= 1 + γ(E/E_{c})^{2}with T0= lattice temperature, Ec= saturation field, γ = const.