Noise of field-effect transistors at very high frequencies

The minimum noise factor of a field-effect transistor has been computed at high frequencies on the basis of the thermal noise of the real parts of the equivalent circuit. A treatment of the intrinsic FET is followed by a consideration of the influence of feedback, parasitic output impedance and parasitic impedance in series with the source on the noise factor. Moreover, the difference between common-gate and common-source configuration has been considered. For frequencies smaller than the gain-bandwidth product fgbthe factorF_{min} - 1varies linearly with the frequency, whereas at higher frequencies this factor varies with f². The computed results are compared with measurements on both JFETs and MOSFETs in the frequency range 100-1500 MHz at different conditions of operation. The agreement is rather good. For the JFET the value ofF_{min}(f_{gb}) approx 2.5; for the MOSFET somewhat higher values are found due to the presence of substrate depletion effects.     

A GaAs 1-kbit static RAM with a shallow recessed-gate structure FET

A novel GaAs FET structure, the shallow recessed-gate structure, has been proposed and applied to a 1-kbit static RAM. In order to decrease the source resistance Rsand gate capacitance Cg, the shallow n⁺implanted layer was formed between the gate and source/drain region; then the gate region was slightly recessed. This FET has a high transconductance gm, low source resistance Rs, small gate capacitance Cg, and small deviation of threshold voltagepart V_{th}, and thus is suitable for high-speed GaAs LSI's. A 1-kbit static RAM has been designed and fabricated applying this FET structure and an access time of 3.8 ns with 38- mW power dissipation has been obtained.     

Low-frequency noise in permeable base transistors

The predominant noise is1/fnoise and consists of two parts: a) Noise varying asImin{C}max{2}, generated mostly with conducting channel and predominating for normal values of the collector voltage VCE. b) Noise at low VCEand practically independent of VCE; it is generated chiefly in the space charge region around the base grating and gives collector1/fnoise atV_{CE} = 0. The turnover frequency of the first noise source lies at about 20 MHz forV_{CE} = 0.30V,V_{BE} = 0.20V. At sufficiently high frequencies the PBT shows thermal noise of the output conductanceg_{c0}at zero bias. Generation-recombination noise is observed at large VBEand low VCEand comes mostly from the space charge region around the base grating.     

Noise figure of UHF transistors as a function of frequency and bias conditions

It is shown that the observed values of the minimum noise figureF_{min}of UHF transistors in common base connection can be explained in terms of the device parameter(1-alpha_{dc}) r_{b'b}/R_{e0}and fαfor frequencies up to 1000 MHz. An interesting collector saturation effect is observed that gives a strong increase in UHF noise figure at high currents. Many features of the dependance ofF_{min}on operating conditions can be explained by this effect. The current dependence ofF_{min}for large values of |VCB| and high currents suggests a distribution in diffusion times through the base region. At intermediate frequencies, the noise figure increases with increasing collector bias |VCB| due to an increase inr_{b'b}, which in turn is caused by the dependence of the base width on |VCB|.     

Compound semiconductors for low-noise microwave MESFET applications

In order to determine the low-noise potential of microwave MESFET's fabricated from materials other than GaAs, a one-dimensional FET model is employed. From material parameters and device geometry the model enables the calculation of a small-signal equivalent circuit from which performance information is acquired. Material parameters, as predicted from Monte Carlo calculations, are used to simulate 1-µm devices fabricated from GaAs as well as InP, Ga0.47In0.53As, InP0.8As0.2, Ga0.27In0.73P0.4As0.6, and Ga0.5In0.5As0.96Sb0.04. Results obtained from simulations comparing a Ga0.5In0.5As0.96- Sb0.04device to an equivalent GaAs device indicate that a decrease in minimum noise figure of almost a factor of two is possible. Considerable improvement in noise performance over a GaAs device is also predicted for devices fabricated from Ga0.47In0.53As and Ga0.27In0.73P0.4As0.6. In addition, the quaternary and ternary devices as well as the InP device should exhibit superior gain and high-frequency performance compared to GaAs devices.     

Low-frequency noise in Cr—SiO2—N-Si tunnel diodes

Low-frequency noise of Cr-SiO2-n-Si tunnel diodes with about 30-Å-thick oxides is investigated as function of bias, frequency, and temperature. Measurements of1/fnoise are explained by a theory employing the two step tunneling model of Sah. Electrons from the Si conduction band are trapped by states at the Si-SiO2interface and then tunnel into bound states of the oxide located close to the interface. The oxide states of density N00can be represented by a frequency dependent parallel admittance exhibiting frequency-dependent thermal noise that modulates the dc currentItunneling through the oxide barrier. This generates flicker noise at the device terminals proportional toI^{2}N_{00}and inversely proportional to frequencyfand tunneling areaA. The valueA = 5. 10-3A0, determined by fitting theoretical and experimental curves at low frequency, is only a small fraction of the gate area A0, since tunneling preferentially occurs through the thinnest parts of the oxide. The currentIalso exhibits full shot noise at high frequency and low current. Qualitative agreement between theoretical and measured noise is found over 9 decades. Measurements at low temperature show additional noise of generation-recombination centers at larger frequencies and currents.     

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.     

1/f noise in n-channel silicon-gate MOS transistors

It is found that equivalent gate noise power for l/f noise in n-channel silicon-gate MOS transistors at near zero drain voltage at room temperature is empirically described by two noise terms, which vary asK_{1}(q/C_{ox}) (V_{G} -V_{T})/f and K_{2}(q/C_{ox})^{2}/f, where V_{G}is gate voltage, VTis threshold Voltage, and Coxis gate-oxide capacitance per unit area. Unification of carrier-density fluctuation (McWhorter's model)and mobility fluctuation (Hooge's model) can account for the experimental data. The comparison between the theory and experiment shows that the carrier fluctuation term K2is proportional to oxide-trap density at Fermi-level. The mobility fluctuation term K1is correlated to K2, being proportional toradic K_{2}. The origin of this correlation is yet to be clarified.     

Magnetic focusing of electron beams in the presence of transverse velocity components

The behavior of nonlaminar electron beams having an originally Gaussian current density distribution in a uniform magnetic field is studied by analytical and numerical methods. The effects of transverse velocity components vTin the beam are described by a fictitious, equivalent perveance which is proportional to the square ofbar{v}_{T}, the rms value of vT. Simple expressions are given for the magnetic field required to achieve a certain beam transmission through a tunnel of given diameter, and for the optimum beam filling factor. In the Appendix a simple experimental method for the determination ofbar{v}_{T}is descried.     

Multichannel coherent optical communications systems

Balanced coherent receivers perform substantially better than single-detector receivers in multichannel optical fiber FDM communications systems since the balanced approach eliminates the direct-detection and signal-cross-signal interference. The permissible channel spacingDdepends on the intermediate frequency fIF, on the bit rate Rb, and on the modulation/demodulation format. In particular,Dincreases by 2 Hz for every 1-Hz increase of the fIF. The signal-to-interference ratio SIR, as defined in the text, provides a simple measure of the amount of the interference generated by undesired channels. The criterion SIR = 30 dB is selected in this paper and leads to the following minimum channel spacings: for heterodyne systems,3.8R_{b}for FSK,9.5R_{b}for ASK, and12.4R_{b}for PSK; for homodyne systems,7.5R_{b}for ASK and10.5R_{b}for PSK. Simultaneous transmission of several channels generates an excess shot noise studied here for the first time. If the local oscillator power is 40 dB above the received signal power and 2000 channels are transmitted without optical prefiltering, the excess shot noise power penalty is less than 1 dB.     

Measurement of high-frequency equivalent circuit parameters of junction and surface barrier transistors

Methods are described for the determination of the high-frequency parameters of junction and surface barrier transistors and involve, in addition to the knowledge of the usual low-frequency parameters, measurement of the product of Ccand the extrinsic base resistancer_{b0},r_{b0}itself, and the alpha cutoff frequency fα. A previously described method is used to determine the productr_{b0}C_{c}, but new methods are described for the measurement ofr_{b0}and fα. The measurements are of "bridge" type, involving simple circuit adjustments for a response null at a single frequency. Typical experimental results are given for transistors having fαvalues as high as 85 mc, Ccvalues down to 2.3 pF andr_{b0}ranging from 45 to 400 ohms. The limits quoted for fαand Ccrefer to surface barrier transistors. Comparison with results derived by alternative methods of measurement show good agreement.     

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.     

Linear and nonlinear optical properties of some ternary selenides

The refractive indices of the ternaryA^{I}B^{III}C_{2}^{VI}semiconductors CuAlSe2, AgGaSe2, CuGaSe2, and AgInSe2have been measured over most of the transparency range of these crystals. The optical nonlinear coefficients for second-harmonic generation of AgGaSe2CuGaSe2, and AgInSe2have also been measured. Three-frequency colinear phase matching is analyzed in detail for AgGaSe2. The birefringences of the other three crystals are not sufficient to permit three-frequency colinear phase matching within the range of the measured index. The merits of AgGaSe2for nonlinear optical applications are evaluated in comparison with other promising infrared nonlinear materials.     

The effect of collector capacity on the transient response of junction transistors

The effect of collector depletion layer capacity on the transient response of junction transistors to a current input is calculated for the case of a resistive load. Expressions are given for the small-signal rise-time of the common-base, emitter, and collector configurations and for the large-signal turn-on and decay times of the common-emitter and collector configurations. The analysis shows that the transient durations under most conditions of operation are approximately (1 + omega_{alpha}R_{L}C_{c}) times those which would be predicted for the short-circuit output approximation reported by Moll [1]. Experimental results are reported which exhibit an excellent agreement with the analysis over a wide range ofomega_{alpha}R_{L}C_{c}. An empirical examination has been made of the dependence of large-signal switching time on the range of operating point excursion. A satisfactory approximate representation of this dependence is provided by a first-order correction factor, which takes into account the functional dependence of ωαand Ccon collector voltage.     

GaAs integrated optical circuits by wet chemical etching

An integrated optical circuit containing a laser, passive waveguide, and extra-cavity detector is described. These devices are fabricated from AlxGa1-xAs layers grown by liquid-phase epitaxy. Reflectors are formed by a two-step preferential etch procedure. For proper laser orientation on {100} wafers, the mirrors are found to be oblique, tapered so as to enhance coupling into the underlying passive waveguide; this increases the efficiency of radiation transfer to the detector. Device operation with high differential transfer efficiency and low threshold has been achieved; e.g., for devices with one etched mirror and one cleaved mirror, ηt= 16 percent andJ_{t} = 2.4kA/cm², whereas these values areeta_{t} = 6.5percent and Jt= 3.0 kA/cm²for devices with two etched mirrors. Other orientations on {100} and {110} wafers have also been investigated. The reflectivity of the etched mirrors is small,R_{e} simeq 1-2percent, but transfer efficiencies into the external passive waveguide as large asT = 50percent have been observed. The effect of small Reon device performance is examined both experimentally and theoretically. The fabrication of ribbed interconnections between active circuit components is also described.     

Thermal noise measurements in GaAs MESFET's

The absolute magnitude of the thermal drain current fluctuations and the associated effective thermal noise coefficient of 1-µm gate-length MESFET's have been measured under various bias conditions. At low drain-source voltages the magnitude of current fluctuations are in good agreement with the thermal noise theory which is based on the gradual channel approximation. However, under normal operating conditions (V_{ds} geq 1.5V,V_{gs} approx 0), we find for the thermal drain noise currentimin{d}max{2} approx 1 - 2 times 10^{-22}A²/Hz with a noise coefficientP approx 0.1in disagreement with the commonly used, theoretically predicted value P = 1.1. Our results are qualitatively consistent with a more comprehensive FET noise theory which properly takes into account high-field effects.     

On the time-bandwidth concentration of signal functions forming given geometric vector configurations

Landau, Pollak, and Slepian, [4]-[6] have shown that the prelate spheroidal wave functions play an important role in determining the approximate dimensionality of a space of functions whose energies are concentrated in a given time bandwidthWT. They have also shown the extent to which this space may be assumed2 WTdimensional. The function space which they consider is actually infinite dimensional and a subset of{cal L}_{infty}^{2}, but it is not a {em linear subspace} of{cal L}_{infty}^{2}, nor in general does it necessarily contain any linear subspace of dimensionality2 WT. However, in the problem of the discreteM-nary channel with additive Gaussian noise and perhaps other types of noise, one is mainly concerned with givenn-dimensional linear subspaces of{cal L}_{infty}^{2}and given geometric configurations of vectors in those subspaces. Thus to be conveniently applied to this problem, the results of Landau, Pollak and Slepian should be reformulated in terms of arbitrary given finite dimensional linear subspaces of{cal L}_{infty}^{2}, with given geometric configurations therein. This paper undertakes such a reformulation for some important special cases. In particular, for the cases of orthogonal, biorthogonal and simplex configurations, it is shown that one can orient the configuration such that the time-bandwidth concentration of the least concentrated vector in the configuration is maximized. The maxi-min criterion is chosen because, as is also shown, the average concentration for these three configurations is always independent of orientation.     

Comparison of experimental and theoretical excited-state spectra for rhodamine 6G

The stimulated emission cross section (σe) and the excited-state absorption cross section (sigma*) for this dye are resolved across the fluorescence and the lowest energy ground-state absorption bands. The absorption is weak (sim0.4 times 10^{16}cm²), particularly at the longer wavelengths [Fig. 3(c)]. Lasing properties are predicted from Fig. 4(b), which shows a plot of the (σe) curve displaced with respect to the S1state as the zero-energy level of reference, on top of the long-axis polarized ground-state absorption band. Spectral assignments are proposed in terms of symmetry arguments based on this observation.     

Microwave phototube design considerations

The general behavior of the traveling-wave microwave phototube (TWP) as a broadband light demodulator has been described in previous publications. This paper presents specific theoretical and experimental analyses. It is shown that at low average currents the microwave power output of a TWP is given byP = frac{1}{2}i^{2} R_{eq}whereiis the peak value of the microwave current modulation at the cathode (i.e., the ac current) and Reqis an "equivalent interaction resistance" given byR_{eq} = pi^{2}N^{2}Z_{c}whereNis the helix length in electronic wavelengths and Zcis the longitudinal beam-circuit interaction impedance. Typical values of Reqare from 10⁵to 10⁷ohms. The bandwidth is determined by the productf^{2}Z_{c}, which can vary less than 3 db over an octave. The major noise contributions are found to be shot noise and thermal noise. The power output and signal-to-noise calculations are verified by experiments on an S-band TWP at low average current levels. Preliminary analysis of the TWP at high average currents shows that even higher values of Reqshould be obtainable. However, in that case the frequency-dependent microwave interactions in the gun region may provide a serious bandwidth limitation.     

Electromigration and low-frequency resistance fluctuations in aluminum thin-film interconnections

Low-frequency noise spectra originating from resistance fluctuations in Al films during electromigration were measured in the absolute temperature and current density intervals327 leq T leq 396K and1.34 times 10^{6} leq j leq 2.22 times 10^{6}A/cm². The values of SR, the resistance power spectral density, at 20 × 10^-3Hz allowed the construction of an Arrhenius plot from which a grain-boundary activation energy value of about 0.6 eV was deduced. This value lies in the range of values found by other authors using different techniques. A first attempt to model the observed dependence of SRonjandTis also described.