Improved efficiency microwave BARITT oscillators by delayed injection

A new microwave BARITT diode structure which utilizes an intentionally added retarding-field region is presented. By taking advantage of the carrier diffusion at low velocities in this region, the injected current is delayed relative to the ac voltage and improved efficiency diodes are obtained. Microwave CW oscillation at C band with efficiency of ∼4 percent has been realized. Although the efficiency is improved over previous BARITT diodes, it is believed that considerable further improvements are possible by further optimization.     

Microwave BARITT diode with retarding field—An investigation

Studies have been made on a new n⁺ipvn⁺ reach through structure operated as a BARITT diode. Unlike earlier BARITT structures, the present one has an additional region (the i-layer) added to it so as to provide a longer retarding field region. By taking advantage of the carrier diffusion at low velocities against an opposing field in this region, the injected current may be delayed relative to the a.c. voltage and improved efficiency BARITT diodes can be expected. In this paper, a one dimensional model is used to illustrate the basic operating mode and advantages of the structure. A d.c. analysis is performed and expressions for the reach through voltage and, critical voltage at which the space charge effects of the injected carriers become apparent are obtained. Furthermore an a.c. analysis is made and a relation for the transit time delay in the retarding field region is developed.     

Epitaxial layer induced series resistance and microwave properties of NNP Si X band impatt diodes

Following Gummel-Blue approach [1] [H.K. Gummel, J. L. Blue, IEEE Trans. Electron. Devices 14(1967) pp. 569-572.], the effect of undepleted epitaxial layers on the series resistance (R_s) as well as on its microwave properties of single drift region (n⁺np⁺) Si IMPATT diodes [2] [M. Mitra, M. Das, S. Kar, S.K. Roy, IEEE Trans. Electron. Devices 40(1993) pp. 1890-1893.] with flat doping profile with capacity 0.2PF at X band under experimental bias current of 25 mA and temperature 373 K have been studied. The computation for series resistance fits well with the device data for flat doping profile [2] [M. Mitra, M. Das, S. Kar, S.K. Roy, IEEE Trans. Electron. Devices 40(1993) pp. 1890-1893.]. The same study has also been simulated on its low-high-low (lhl) doping profile counterpart. The value of R_s increases approximately linearly with the increase of undepleted epi-layer thickness determined by the doping density for both flat and lhl structure. The value of R_s decreases remarkably as the doping profile changes from flat to lhl type.     

Subnanosecond 4<Emphasis Type="Italic">H</Emphasis>-SiC diode current breakers

Mesa epitaxial 4H-SiC-based p ⁺-p-n ₀-n ⁺ diodes have been fabricated and their reverse recovery characteristics have been measured in modes typical of fast semiconductor current breakers, drift step recovery diodes, and SOS diodes. It has been found that, after the short (～10 ns) pulsed injection of nonequilibrium carriers by a forward current with a density of 200–400 A cm^?2 and the subsequent application of a reverse voltage pulse (with a rise time of 2 ns), diodes can break a reverse current with a density of 5–40 kA cm^?2 in a time of about (or less than) 0.3 ns. A possible mechanism for ultrafast current breaking is discussed.     

A discrete element small-signal equivalent circuit for forward-biased p+n diodes containing deep levels

A discrete element small-signal equivalent circuit model for p-n diodes containing deep defect levels is developed, by extending an existing model for undamaged devices. With the aid of a simple analytical expression which accurately describes the forward bias d.c. current, the enhanced small-signal conductance due to carrier recombination in the depletion region is included in the model. The influence of trapped charge on the space charge capacitance is incorporated using a simplified version of the analysis of Beguwala and Crowell. The predictions of the model are verified by experimental data from silicon p⁺n diodes, in which deep levels have been induced by electron irradiation. It is shown that the deep level activation energies may be estimated from the forward bias capacitance-voltage characteristics, yielding values which agree well with those obtained by established techniques.     

Small-signal analysis of punch-through injection microwave devices

A theoretical small-signal analysis of punch-through injection microwave devices is given. A numerical study of a silicon p⁺?n?p⁺ structure is performed, which shows good agreement with experimental measurements by Snapp and Weissglas for a diode with a doping density of 1·2 × 10¹⁵/cm³. Negative resistance is also calculated for diodes with doping densities of 0·6 × 10¹⁵/cm³ and 5 × 10¹⁵/cm³. A partially analytical mode, including the lowfield region, is developed and compared with the numerical calculation. Ohmic losses for devices with low impurity concentrations and diffusion for devices with high impurity concentrations are shown to be significant factors.The noise spectrum is calculated numerically from the assumption of two noise sources, injection noise and diffusion noise. The noise measure is determined and shown to be in good agreement with experiments by Björkman and Snapp.     

Polycrystalline silicon p-n junctions

Silicon films deposited on recrystallized metallurgical silicon substrates have been used for the fabrication of low cost solar cells. The substrate is polycrystalline, and the active region of the solar cell is epitaxial with respect to the substrate. Since the dark current-voltage characteristics of a solar cell are important factors affecting its conversion efficiency, the characteristics of a number of epitaxial mesa diodes of the configuration n⁺-silicon/p-silicon/p⁺-metallurgical silicon/graphite have been measured over a wide temperature range to study the effects of grain boundaries. The results were analyzed on the basis of the two-exponential model.     

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.     

On the forward current-voltage characteristics of p+-n-n+ (n+-p-p+) epitaxial diodes

The forward-biased current-voltage characteristics of p⁺-n-n⁺ and n⁺-p-p⁺ epitaxial diodes are derived theoretically. Effects of the energy-gap shrinkage, the high-low junction built-in voltage, the high-level injection, and the minority-carrier life time on the forward-biased current-voltage characteristics are included. Good agreements between the theoretically derived results and the experimental data of Dutton et al. are obtained. The developed theory predicts that the leakage of the high-low junction is dominated by the recombination of minority carriers in the highly doped substrate, not by the recombination of minority carriers in the high-low space charge region, which is opposite to the previous prediction of Dutton et al.     

Double-drift-region ion-implanted millimeter-wave IMPATT diodes

A detailed experimental comparison between double-drift-region (DDR) and single-drift-region (SDR) millimeter-wave avalanche diodes is presented. For 50-GHz CW operation, DDR diodes have given a maximum of 1-W output power compared to 0.53 W for the SDR diodes, while maximum efficiencies of 14.2 percent for the DDR and 10.3 percent for the SDR diodes have been obtained. These results are in agreement with the theory of Scharfetter et al. [1] for DDR IMPATT diodes. Both the DDR and SDR diode measurements were made on room temperature, metal heat sinks. The DDR diodes were shown to operate at significantly lower junction temperatures for the same value of output power, indicating a potential reliability advantage. Ion implantation was used to make the p drift region of the p⁺p-n-n⁺50-GHz DDR devices. Otherwise the fabrication (which includes diffusion and epitaxial technologies) and the microwave measurement methods were identical for both types of diodes. Capacitance measurements were compared with calculations to determine the desired doping concentrations for frequencies from 43 to 110 GHz. Experimental results for the higher frequency millimeter-wave region have been obtained on DDR structures with both p and n drift regions implanted. At 92 GHz an output power of 0.18 W and an efficiency of 7.4 percent have been obtained.     

Numerical analysis of a double avalanche region IMPATT diode on the basis of nonlinear model

The analysis of the n⁺pvnp⁺ avalanche diode structure has been realized on the basis of the nonlinear model. This type of the diode that was named as double avalanche region (DAR) IMPATT diode includes two avalanche regions inside the diode. The phase delay which was produced by means of the two avalanche regions and the drift region v is sufficient to obtain the negative resistance for the wide frequency band. The numerical model that is used for the analysis of the various diode structures includes all principal features of the physical phenomena inside the semiconductor structure. The admittance characteristics of the DAR diode were analyzed in very wide frequency band. The obtained results contradict to the before performed analysis on basis of the approximate models and show that only diode with a sufficiently short drift region can produce active power in some frequency bands.     

CW oscillations in GaAs planar-type bulk diodes

CW Gunn oscillations at frequencies from 0.5 GHz to over 1 GHz have been obtained with GaAs bulk diodes constructed from an epitaxial n layer grown on semi-insulating substrates. Observations of current waveforms have been made for both transit-time and delayed-domain modes. CW microwave power of 12 mW has been obtained at 0.9 GHz.     

A simplified theory of the BARITT silicon microwave diode

A simplified analytical theory is developed for the BARITT microwave diode. This is based on the small-signal concept of effective source conductivity and the large-signal concept of space-charge-limited injection. The impedance parameters of equivalent series resistance and capacitance and the output power and efficiency are evaluated as functions of frequency, r.f. load and biasing conditions. Small-signal negative-resistance Q-factors of about ?20 are obtained at bias current densities in the range from about 50 Amps/cm² to about 200 Amps/cm². Large-signal power generation efficiencies of about 10% can be achieved with Q-factors of about ?100.     

Multi-layer epitaxially grown silicon impatt diodes at millimeter-wave frequencies

Complementary (N⁺PP⁺) and double-drift (N⁺NPP⁺) silicon IMPATT diodes were prepared and investigated as oscillators in the millimeter-wave frequency region of 50 to 70 GHz. All the diodes were fabricated from multi-layer epitaxially grown silicon structures. A maximum CW output power level of 198 mW at 62.9 GHz and a maximum conversion efficiency of 7.3% have been measured for the complementary diodes. The double-drift IMPATT diodes have a maximum CW output power of 400 mW at 56.3 GHz and a maximum conversion efficiency of 8.5%.     

Voltage dependence of the differential capacitance of a p +-n junction

The dependences of the differential capacitance and current of a p ⁺-n junction with a uniformly doped n region on the voltage in the junction region are calculated. The p ⁺-n junction capacitance controls the charge change in the junction region taking into account a change in the electric field of the quasi-neutral n region and a change in its bipolar drift mobility with increasing excess charge-carrier concentration. It is shown that the change in the sign of the p ⁺-n junction capacitance with increasing injection level is caused by a decrease in the bipolar drift mobility as the electron-hole pair concentration in the n region increases. It is shown that the p ⁺-n junction capacitance decreases with increasing reverse voltage and tends to a constant positive value.     

Heat flow resistance measurements of silicon p+-v-n+ diodes under forward and reverse biased conditions

A detailed study of the heat flow resistance measurements in a p⁺-v-n⁺ diode is studied in both forward and reverse biased conditions. Measurements are made by continuously switching the diode from the power dissipation state into the temperature measuring state. Safe operating power limits are identified for the diodes depending upon their mode of operation either as a microwave switch or as an IMPATT oscillator.     

Analytic modeling of transit-time device drift regions with field-dependent transport coefficients

An iterative procedure for obtaining two-carrier d.c. solutions in regions of rapidly varying carrier concentration is presented. The procedure uses an analytic solution for the carrier concentrations in a region of linear spatial electric field variation. Field-dependent diffusion and field-dependent velocities are assumed. A single-carrier small-signal model for a drift region with a spatially varying field and field-dependent transport properties is presented. When applied to a BARITT device, results consistent with published experimental data are obtained. The importance of momentum relaxation effects in Si BARITT drift regions is discussed.     

Effect of irradiation with fast neutrons on electrical characteristics of devices based on CVD 4<Emphasis Type="Italic">H</Emphasis>-SiC epitaxial layers

The effect of irradiation with 1-MeV neutrons on electrical properties of Al-based Schottky barriers and p⁺-n-n⁺ diodes doped by ion-implantation with Al was studied; the devices were formed on the basis of high-resistivity, pure 4H-SiC epitaxial layers possessing n-type conductivity and grown by vapor-transport epitaxy. The use of such structures made it possible to study the radiation defects in the epitaxial layer at temperatures as high as 700 K. Rectifying properties of the diode structures were no longer observed after irradiation of the samples with neutrons with a dose of 6×10¹⁴ cm^?2; this effect is caused by high (up to 50 GΩ) resistance of the layer damaged by neutron radiation. However, the diode characteristics of irradiated p⁺-n-n⁺ structures were partially recovered after an annealing at 650 K.     

Small-signal noise measure of avalanche diodes

A study of small-signal noise measure of avalanche diodes is useful for establishing a reference for large-signal noise theories as well as to determine conditions for low noise operation in low power applications. Such a study depending largely on numerical calculations was presented by Haus et al. Similar results are derived by analytic techniques through the use of a second-order approximation for the avalanche current accounting for transport delay of the carriers in the avalanche region.A uniformly avalanching PIN diode is studied to derive the properties of the avalanche region. In the lossless case the noise measure has a minimum for a carrier transit angle of approximately 2π, while it is inversely proportional to frequency squared for smaller transit angles. Operation close to the avalanche frequency is favorable in a practical case because of parasitic series resistance.The effects of a drift region is included to obtain reasonable modeling of practical avalanche diodes. In the lossless case the impedance transformation and space-charge smoothing in the drift region cause a strong modulation of the noise measure with minima for drift transit angles π < θ_d < 2π and 3π < θ_d < 4π. The noise measure decreases for increasing avalanche widths. Series resistance limits the practical upper drift angle to approximately 1.2π due to the decrease of the negative resistance with frequency.Similar calculations have been carried out for double-drift-region diodes, and show that these in general are noisier than the corresponding single-drift structures in the small-signal limit.