Large-Signal Equivalent Circuits of Avalanche Transit-Time Devices

A large-signal analysis for IMPATT diodes is derived, which allows carrier multiplication by impact ionization to occur at every point in the diode. Therefore, the operating characteristics of IMPATT diodes with a wide range of realistic doping profiles can be investigated. For a given operating frequency, RF voltage, dc bias current, and doping profile, the admittance, power output, efficiency, bias voltage of a diode can be obtained. An equivalent circuit the diode package, microwave circuit mount and diode, is obtained experimentally. Using this circuit, the admittance of the diode is measured by a reflection-type circuit and an oscillator circuit as a function of the RF voltage, dc bias current, and frequency.     

A large signal analysis of IMPATT diodes

This paper presents results on RF power output and efficiency of IMPATT oscillators obtained from a large-signal model of these devices. The results are obtained from a closed-form solution of the nonlinear equations describing a Read-type IMPATT diode. The closed-form solution is obtained by assuming a short transit time through the drift region compared to the RF period. The solution is used to obtain the large-signal diode impedance. The analysis shows that the power output of an IMPATT diode depends strongly on the series load resistance presented to the active part of the diode and that the change in diode reactance with increasing bias current also depends on the series resistance. Plots of power output as a function of frequency, bias current, and load resistance are presented. Frequency tuning of the oscillator through current variation is also discussed. Experimental results are presented and compared with the theoretical ones wherever possible. The results lead to an improved understanding of such oscillators and are extremely useful in optimizing their performance and determining their limitations.     

Effect of Temperature on Device Admittance of GaAs and Si IMPATT Diodes

The effect of temperature on the small-signal admittance of IMPATT diodes with uniformly doped and high-low doped (Read) structures is investigated experimentafly and theoretically. Small-signal admittance characteristics of X-band Si p+-n-n+, GaAs M-n-n+ (Schottky-uniform), and GaAs M-n+-n-n+ (Schottky-Read) IMPATT diodes are measured at various junction temperatures for different dc current levels. Small-signal analysis is performed on GaAs IMPATT diodes of uniformly doped and high-low doped structures, and the calculated results on temperature dependence of the device admittance are compared with the experimental results. Reasonable agreement is found between theory and experiment. It is shown that GaAs IMPATT diodes are superior to Si diodes in admittance temperature characteristics and that the uniformly doped structure has a small admittance temperature coefficient in magnitude, compared to the high-low doped structure. It is also shown by calculation that the admittance temperature coefficient of a punch-through diode is small in magnitude, compared to that of a non-punch-through diode.     

Dependency of the highest harmonic oscillation frequency on junction diameter of IMPATT diodes

The effect of series resistance and junction capacitance on the high-frequency limit of IMPATT diode operation is studied with a Read-type small-signal theory, and is confirmed experimentally. Oscillation frequencies from 30 to 400 GHz have been measured with Si p⁺-n-n⁺abrupt junction diodes with a depletion layer width of 0.2 µm. The highest oscillation frequency increases as the junction diameter is decreased, owing to reduced junction capacitance and increased bias-current density. The highest oscillation frequency observed is 423 GHz, which is obtained in the fifth harmonic mode with a diode of 16-µm junction diameter. Fundamental oscillation frequency is found to depend strongly on dc bias-current density, and to be close to the avalanche frequency of the small-signal theory.     

The Traveling-Wave IMPATT Mode

The small-signal analysis of a distributed IMPATT diode ifdicates the existence of a traveling-wave mode. The severe power-frequency limitation as well as the associated low impedance level of the discrete diode appear avoidable. No external resonant circuitry is needed. It is shown that the TEM parallel-plate waveguide mode of the junction is modified by the injection of electrons at the p+ -n junction (or Schottky contact). The transverse electric field takes on a traveling-wave nature in the transverse direction tracking the injected electrons, and a small longitudinal electric field will also be present. In previous papers on IMPATT traveling-wave structures, the IMPATT effect was lumped into an effective complex permittivity in a composite layer model or into an effective shunt admittance in a transmission line model. The current work attempts to incorporate the IMPATT mechanism into the wave model and considers the actual carrier field interaction. The srnall-signal analysis yields an analytic field solution and a characteristic equation for the complex propagation constant. Solutions are found and documented for various frequencies and bias current densities. For the particular structure considered, at 12 GHz with a bias current density of 1000 A/cm/sup 2/ a gain of 72 dB/cm was found.     

Pulsed measurement of BARITT-diode impedance against current and temperature

The small-signal impedances of M?n?p and p?n?p BARITT diodes have been measured as a function of bias current and diode temperature. A bridge method, developed by Van Iperen and Tjassens, has been adapted so that measurements can be made during short current pulses. In this way, the diode temperature can be kept close to the heatsink temperature. The method is equally suited for other microwave diodes, e.g. IMPATT and Gunn diodes.     

Observation of the current and voltage waveforms of the Si IMPATT diode

Current and voltage waveforms of the Si IMPATT diode were observed directly by means of the oscillating circuit using microstrip line. The results indicate that the conventional small-signal theory cannot be applied to the observed type of oscillation. The oscillation starts at the bias voltage just above the breakdon voltage of the diode; then along with its buildup, the bias voltage is lowered owing to the auto-bias effect, to reach a steady value considerably below the breakdown voltage. Large amplitude oscillation of high efficiency is expected over a wide frequency range.     

Computer-Aided Small-Signal Characterization of IMPATT Diodes

This paper is a discussion of IMPATT wafer small-signal characteristics in the frequency range of 2.0-8.0 GHz. These characteristics have been obtained by computer conversion of reflection phase-gain data. The data handling technique which allows establishment of the desired reference plane and the reduction of the admittance data into the desired equivalent circuit is presented. A calibration procedure using reference impedances consistent with the diode geometry is discussed. The validity of the microwave measurement technique and the data handling process is demonstrated by comparison of the values of junction capacitance determined at microwave frequencies with junction capacitance measurements at 30 MHz. Representative plots are given for wafer conductance and susceptance as a function of frequency with current density as a parameter. In addition, typical values obtained for the circuit elements are presented. These data illustrate the capability of determining package inductance, series resistance as a function of bias voltage, and, with the diode in avalanche, the parallel G, L, and C of the wafer admittance. The diode equivalent circuit was studied as a function of current density to compare results with the existing analytical small-signal theories. This procedure permits the separation of the wafer elements from the parasitic elements of the package. Data obtained from these measurements are extremely useful for ascertaining wafer design parameters and assisting in circuit design.     

A simplified field analysis of a distributed IMPATT diode usingmultiple uniform layer approximation

A small-signal field analysis of a distributed IMPATT diode is presented. The active region of the diode is assumed to consist of a uniform avalanche layer and avalanche-free drift layers. The propagation constant and field distributions are obtained without numerical solution of differential equations. The effects of losses caused by the presence of inactive layers are included in the analysis. Numerical examples of GaAs double-Read distributed IMPATT diodes are given which show the dependence of the amplification characteristics on the thicknesses of the avalanche and drift layers     

IMPATT diode quasi-static large-signal model

A quasi-static large-signal model of an IMPATT diode with general doping profile is derived. The numerical solution of this model has been implemented in a Fortran IV program which executes economically. This model has been used to analyze large-and small-signal admittances of GaAs double-drift and quasi-Read IMPATT diodes. The small-signal results are in good agreement with calculations done using a linearized small-signal model. The large-signal calculations exhibit power and efficiency saturation when reasonable values of parasitic resistance are included and are in good agreement with experimental GaAs diode performance. The generalized quasi-static formulation simplifies analysis of IMPATT structures with arbitrary doping profiles, specifically those with distributed avalanche zones, by providing the correspondence between these devices and the Read diode model.     

Effects of ionization rates on IMPATT device admittance

The significance of using different ionization rates on the operating characteristics of Si IMPATT devices is examined. The dc breakdown and small-signal results of IMPATT devices at room temperature are presented. Numerical results for p⁺nn⁺as well as the complementary n⁺pp⁺Si diodes in the millimeter-wave frequency range and at different current densities ranging from 2500 to 10 000 A/cm²are given. It is shown that large differences in some important device parameters are obtained, depending on the ionization rates employed.     

Broad-Band Small-Signal Impedance Characterization of Silicon (Si) P+-N-N+ Impatt Diodes

A method is described that permits a broad-band small-signal characterization of an IMPATT diode mounted in a package. From automatic-network-analyzer measurements on a package-shaped metal dummy, an empty package, and the diode under test biased below and above breakdown, the method allows first determination of a coupling-circuit parameter, bonding-wire inductance, and diode series resistance, and then evaluation of junction admittance above breakdown. Experimental results on silicon (Si) p+-n-n+ diodes over 2.5-15 GHz are shown. Nearly frequency-independent bonding-wire inductance is observed. Avalanche frequency squared (f/sub a//sup 2/) is found to be sublinear with respect to dc current density (I/sub d), possibly due to a variation of junction temperature (T/sub i/). An experimental formIda for (f/sub a//sup 2/) / (I/sub d) is obtained in terms of (T/sub i/).Detailed comparisons of the measured junction admittance with an existing analytical theory indicate a good agreement, if a suitable amount of saturation current is postulated, and also suggest that the estimated amount is in excess of the prebreakdown saturation current.     

Mixed tunneling and avalanche mechanisms in p-n junctions and their effects on microwave transit-time devices

Analytical models of dc and small-signal characteristics for Read-type diode structures are given which incorporate both tunneling and avalanche mechanisms. A "dead-space" analysis is shown to be fundamental to accurate models of thin generation regions. Pure tunneling and pure avalanche appear as the two limiting cases of the general models. In the pure tunneling limit, the diode oscillator will operate in the tunnel transit-time (TUNNETT) mode. The TUNNETT oscillator would be attractive for low-noise and medium power and efficiency applications. For diode structures which operate between the pure TUNNETT and IMPATT modes, there exists a noise performance-output power tradeoff. Computer solutions of the analytical models, for specific diode structures and operating conditions, are given, and the results are discussed and compared with experimental results whenever possible.     

Small-signal equivalent circuit for IMPATT transistors

A lumped-distributed small-signal equivalent circuit of an IMPATT transistor, a 3-terminal transistor-like device having a base-collector junction operated as an IMPATT diode, is presented. The equivalent circuit is composed of RLC elements, two controlled sources and one transmission line. Characterisation of the device based on this model gives clear insight into the small-signal and noise behaviour of the IMPATT transistors.     

A Small-Signal and Noise Equivalent Circuit for IMPATT Diodes (Short Papers)

A frequency-independent small-signal equivalent circuit for an IMPATT diode is proposed. It incorporates five circuit elements, including a negative resistance, and is valid over an octave range of frequency. With the addition of two white noise sources it also serves as a noise equivalent circuit.     

Premature collection mode in IMPATT diodes

Experimental efficiencies of up to 35.5 percent have been reported for Read-type GaAs diodes, whereas theoretical calculations have predicted an upper limit of approximately 30 percent for the conversion efficiency of IMPATT diodes. The concept of a premature collection mode is shown to resolve this discrepancy by predicting maximum efficiencies close to 40 percent. Premature collection refers to large-signal conditions where the modulation of the drift width is sufficiently large to result in collection of the avalanche current pulse at drift angles smaller than the small-signal angle. It is shown that a discontinuous transition between the IMPATT and the premature collection modes takes place when the drift angle in the small-signal limit is greater than π. Designing the diode for close to punchthrough conditions in small-signal operation extends the practical frequency range for inducing premature collection by avoiding long drift angles and corresponding rapid conductance saturation in the IMPATT mode. The onset of premature collection is accompanied by a substantial increase in power output because of a more favorable drift angle, and in high noise because of the high RF levels involved. The jump in transit angle causes a discontinuous increase in negative conductance. The hysteresis in the tuning characteristic resulting from this discontinuity has been observed experimentally. Noise measures in the range 60-70 dB have been measured and calculated for the premature collection mode compared to 40-50 dB under large-signal conditions for the IMPATT mode. Therefore, the high efficiencies available with the premature collection mode are expected to be usable only in applications where high noise levels can be tolerated.     

An HBT noise model valid up to transit frequency

A comprehensive HBT noise model for circuit simulation is presented that describes the microwave noise behavior up to the transit frequency. It is based on diode noise theory, and requires only the small-signal equivalent circuit, including the thermal resistance, and the dc bias point. A main feature is correlation of the shot-noise sources at the pn junctions. The model is verified by measurements of the four noise parameters of an InGaP/GaAs HBT, varying frequency and bias conditions     

Carrier transport in the drift region of read-type diodes

The transport properties of the drift region significantly impact the conversion efficiency of Read-type IMPATT diodes. A detailed numerical study has been undertaken to gain insight into the carrier transport under large-signal conditions and the roles of depletion-width modulation and the carrier-induced electric field. A phase delay in the peak of the avalanche current results from assuming a sinusoidal voltage rather than a sinusoidal field and is incorporated in the model. induced current waveforms, transport factors, diode admittances, and conversion efficieneies are presented as functions of RF level, bias current density, and the small-signal transit angle. A region of partial collection of the drifting carriers exists between regular IMPATT operation and the onset of the premature collection mode, which provides a smooth transition between these two modes. The effects of premature collection are most pronounced at long transit angles where the corresponding large increase in the transport factor at a high RF level will give high-efficiency operation, but also a large hysteresis in the tuning characteristic. The displacement current has an appreciable effect on the induced external current under these conditions. The presented results aid the understanding of tuning behavior and tradeoffs in design for Read diodes.     

A rate equation analysis for the frequency chirp to modulated power ratio of a semiconductor diode laser

An expression for the frequency chirp to modulated power ratio (CPR) is derived from a rate equation analysis of the small-signal, injection current modulation in a semiconductor diode laser. The model includes the effect of lateral carrier diffusion across the active region of the laser diode. The modulation frequency dependence of the CPR is flat from dc to a few hundred megahertz, beyond which it is proportional to the modulation frequency.     

Oscillation Characteristics of Millimeter-Wave IMPATT Diodes Mounted in Low-Impedance Waveguide Mounts (Short Papers)

Experiments on dc-bias-current-tuned IMPATT diodes mounted in low-impedance waveguide mounts are described. Broad-band bias-current-tuned IMPATT oscillators were obtained which cover almost the full waveguide band; 20-, 24-, and 18-GHz tuning bandwidths were obtained with the R-500, R-620, and R-740 waveguide, respectively. From experiments it became evident that there are some suitable relations for broad-band bias tuning among the diode breakdown voltage, the oscillation frequency, and the waveguide dimension. The results are very useful for the design of the circuit and diode parameter for broad-band millimeterwave IMPATT sweep oscillators. The feasibility of applying bias-current-tuned IMPATT oscillators to a broad-band measuring instrument is expected.