A series connection of IMPATT diodes

A microwave oscillator using a series combination of three packaged IMPATT diodes has been successfully operated. Since the series combination increases the power output of IMPATT oscillators without decreasing the impedance level, the 1/f²limitation of the power-impedance product for IMPATT diodes can be avoided. This type of series combination is suited for use at millimeter-wave frequencies.     

Measurement technique for largesignal admittance of IMPATT diodes

A method is described for determining the large-signal admittance of an IMPATT diode from reflection-coefficient measurements at the output of a well-characterised IMPATT oscillator/amplifier circuit. Measurements at 10 GHz are presented and compared with the computed performance for the diode.     

Lateral IMPATT diodes

Conventional IMPATT diodes are the highest-power microwave semiconductor devices, but they are difficult to couple light into, challenging to integrate into monolithic circuits, to incorporate a third terminal, or to series combine. The lateral IMPATT diode is proposed as a solution to these problems. This device is planar and features contact and drift regions that are all adjacent to the wafer surface. Two types of fabrication schemes are discussed and pulsed RF power results, up to 17.4 GHz, are demonstrated. This device structure promises to be well suited for microwave, millimeter-wave, and electrooptic integrated circuits in which maximum power is required     

Heterojunction IMPATT diodes

Heterojunction IMPATT diodes, which incorporate an abrupt GaAs/Al _0.3Ga_0.7As p/N heterojunction in place of the standard p/n junction, have shown a number of significant properties that represent a considerable technological advance. Ku-band experimental devices exhibit up to 2.0 dB higher power, superior DC characteristics, and 3-6 dB less phase noise content. These and other properties are examined in detail, and a first-order theory of operation is proposed     

Double-velocity IMPATT diodes

A new IMPATT diode structure is proposed. The device incorporates a heterojunction between materials having different electric field saturated carrier velocities. Analysis shows that such a diode can have significantly higher dc-to-microwave-power conversion efficiencies than conventional Read IMPATT diodes.     

High-power parallel-array IMPATT diodes

Fabrication of parallel arrays of silicon IMPATT diodes in which the arrays are formed in a single diode chip is described. The technique includes formation of an integral heatsink for the diode arrays during wafer processing. For a given total active device area, the use of a parallel array of smaller diodes, rather than one large diode, allows a significant reduction in thermal impedance and consequently larger power-dissipation capability. The contribution shown in the letter is the ease and economy with which parallel arrays on an integral heatsink can be fabricated and handled as a single entity. In a diode operated at 6.4 GHz, 3.5 W of c.w. output power has been achieved with a room-temperature copper heatsink and a junction temperature of about 280°C.     

Frequency conversion in IMPATT diodes

A large-signal model of the Read-type IMPATT diode has been used to analyze the frequency-mixing properties of the oscillating diode. The self-oscillating, two-port frequency converter is described in terms of its short-circuit admittance parameters. It is shown that in the proper circuit, parametric frequency conversion may result in a negative conductance at the input and output ports of the converter. Therefore, high-gain frequency conversion and parametric amplification are possible. Under some conditions, spurious oscillations may occur due to this negative conductance. Experimental circuits have been built which demonstrate conversion gain and parametric amplification and confirm qualitatively the theoretical results. It is also shown experimentally that some of the sideband noise of the IMPATT oscillator is due to low-frequency noise which is up converted from the bias circuit. Some of this noise can be eliminated by proper circuit design.     

Ring-geometry IMPATT oscillator diodes

Ring-geometry IMPATT diodes have been developed which give 1.15 watts output power at 8.4 to 10 GHz, compared with 0.87 watt obtained from solid-circular-geometry diodes of the same basic design. The improved performance is due to better thermal dissipation in the ring structure.     

Junction-temperature measurement of IMPATT diodes

The breakdown voltage VB of an IMPATT diode is a function of the junction temperature. Pulse techniques are applied to measure VB directly during actual operation, thus giving the temperature within an accuracy of a few per cent. The method also provides a display of 'space-charge resistance at the operating temperature.     

High-speed modulation of IMPATT diodes

A high-speed bias circuit modulation process which occurs in Gunn oscillators by virtue of their equivalent circuit is shown to also occur in IMPATT oscillators.     

Properties of millimeter-wave IMPATT diodes

A study of the effect of the doping profile on the properties of IMPATT devices has been carried out and the results of a small-signal analysis for different ram-wave Si IMPATT-diode structures are presented. Properties of single-drift abrupt-junction diodes of both Si complementary structures with different punch-through factors (PTF) are described and compared to those of symmetrical and asymmetrical double-drift diode structures. Since the initiation of the TRAPATT mode may be related to the IMPATT performance, these results should also be useful in assessing the effects of the doping profile on TRAPATT initiation.     

Accelerated ageing of IMPATT diodes

An investigation conducted to determine suitable reliability test methods for IMPATT diodes has shown that elevated temperature accelerated their normal degradation in a predictable way only when the diodes were stressed with normal d.c. power applied and controlled in a constant-voltage mode. Normal degradation was manifested initially by an increase in thermal resistance and ultimately by short-circuits, both of which were caused by the progressive interaction of the constituents of the contact metallizations. Initial instabilities and infant mortalities had to be eliminated by burning-in the diodes before overstress results could be related to long-term wearout. Thereafter, simple d.c. was adequate to assess life expectancy because the changes in thermal resistance could be used to estimate consequent changes in r.f. performance.     

Thermal runaway of IMPATT diodes

A simple one-dimensional computer model of the dc-thermal behavior of a Schottky-barrier GaAs IMPATT diode has been formulated to compute the conditions for thermal runaway in IMPATT diodes of various designs. The model has been used to determine the thermal stability conditions for three designs of GaAs IMPATT's. The computations lead to several conclusions, the most important of which are the following. a) Junction thermionic emission (leakage) current is thermally unstable, whereas avalanche multiplication is thermally stabilizing. Diode thermal stability at high junction temperature requires that the thermionic emission current be low and the avalanche multiplication be large. b) Lowering of the barrier height caused by contaminants or defects at the junction increases the likelihood of thermal runaway. c) For a given barrier height, the higher the doping of the IMPATT diode, the more resistant it will be to thermal runaway.     

Avalanche induced dispersion in IMPATT diodes

Numerical analysis has been performed on the effect of the carrier dispersion caused by avalanching on the small signal admittance and the transient step response in the avalanche region of an idealized IMPATT diode having a uniform electric field profile. The degree of dispersion, referred to hereafter as Avalanche Induced Dispersion (AID), depends on the relative magnitudes of ionization rates of the two carriers. AID becomes largest when the two ionization rates are equal and decreases with increasing discrepancy between them.It is found that the build-up of an avalanche can be faster if either electrons or holes are strongly ionizing than when both of them ionize equally. Also, the small signal negative conductance is minimum when the dispersion is most pronounced. Since the time delay in the avalanche build-up depends strongly on AID, the upper limit of the high-frequency performance of IMPATT diodes can be estimated from the theoretical value of AID.     

Millimeter-wave GaAs Schottky-barrier IMPATT diodes

Recent experimental observations on a Schottky-barrier GaAs IMPATT diode for F-band operation are presented. The diode slices were thinned to 10 to 20 µm by removing the substrate by precision polishing. Output power of 304 mW at 50 GHz with 4.58 percent efficiency was observed. The highest efficiency was 4.72 percent at 55 GHz.     

Filamentary thermal instabilities in IMPATT diodes

The unstable growth of thermal filaments in a diode with a fixed bias current is calculated on the basis of a model which includes thermal conduction in the plane of the junction, as well as perpendicular to it. The growth time of the instability is shown to be longer than the thermal relaxation time by the ratio of the space-charge resistance to the differential negative resistance. Analytic results are obtained for small temperature disturbances which are initially Guassian functions of the transverse coordinate. If A is the area of the junction, the negative differential resistance must be of the order of the space-charge resistance multiplied by 16W²/π²A (whereWis the thickness of the active region) or the filament can dissipate itself by diffusing outward and spreading over the entire junction area before the temperature rise becomes very large. The voltage fluctuation relaxes to its equilibrium value in the thermal relaxation time, which is independent of the differential resistance. Pulsing a diode will tend to prevent an instability from becoming destructive, provided the off-time is long enough to cool the heated filaments which develop during the pulse transmission.     

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.     

High-efficiency X-band GaAs IMPATT diodes

Epitaxial GaAs diodes have been fabricated giving 65¹mW CW output power at 10 GHz under room temperature operation. A dc to RF efficiency of 10.1 percent was obtained from a diode operating CW while the measured output of another diode was over 20 mW for 3 mA dc with 70-volt biasing.