Low-level microwave limiting utilizing impact ionization in bulk germanium at 4.2°K

Abstract

Low-level garnet limiters operating in the coincidence region have previously been reported a t 4.2'K [l]. These devices, however, operate only within an octave frequency range which is a function of the saturation magnetization. (For a YIG sphere a t 4.2'K, for example, this frequency range is 2.3 to 4.6 Gc/s.) This correspondence describes a limiter, ha\-ing no such fundamental limitation, which utilizes the sharp change in conductivity due t o impact ionization in n-type germanium a t 4.2'K.. I high power T R device based on this principle was previously investigated by Seeger [2].Breakdown occurs in certain impuritydoped semiconductors a t !ow temperatures when free charge carriers acquire sufficient energy at a certain electric field intensity to ionize neutral impurities: The critical electric field (E,) is given by [3]where r=longitudinal velocity of sound, p =mobility, AE = ionization energy of the impurities, k = Boltmann's constant, and T =temperature. I t is clear from this equation that for low breakdown fields, AE should be small and p should be large. I n n-type germanium a t 4.2'K, E, is typically in the range 4 to 10 \'/cm. Below this critical field. germanium has a very high resistivity (as high as lo9 Q-cm) and appears essentially as a dielectric insulator (E= 16). Figure 1 shows a typical v-i characteristic at 4.2'K of a 5 a-cm arsenic doped germanium sample. The conductivity change is greater than five orders of magnitude.A cross-sectional view-of the limiter appears in Fig. 2(a). The resonator is in singleridge waveguide, and coupling is achieved by means of irises. The faces of the germanium slab are nickel plated (by means of an electroless process) to provide ohmic contacts. The ridge is insulated from the rest of the structure by 0.001-inch mylar in order to permit the application of a dc bias voltage. A photograph of the structure is shown in Fig. 2(b). The cavity is well under a quarter-wavelength long due to the large capacitive loading of the thin (0.012 inch) germanium element.A typical limiting characteristic obtained from this device at 3875 Mc/s appears in Fig. 3. The limiting level is -6 dBm and the low-level insertion loss is 1.5 d B with a 3-dB bandwidth of 200 Mc/s. Lower insertion loss and wider bandwidth can be obtained merely by increasing the circuit loading. The dynamic range should be in excess of 50 dB. Unfortunately, the exact value could not be measured because a high power source was not available.Efforts were made to obtain a lower limiting level bl-applying a dc bias field across the germanium. The maximum imsupported by the Research and Technology Diy.. Manuscript received July 20. 1965. This work was Rome Air Development Center, under Contract ho. AF 30(602)-2989.Fig. 1. V-I characteristic of bulk germanium semiconductor. ,/ ,rI.Q/S / IF--(b) Fig. 2. (a) Cross section of ridge-waveguide structure. (b) Structure with top removed. 5.0 OW-CU SAMPLE (0.IM) INCH X 0.100 I N C M X 0.012 INCHI f . s 7 5 I O 81'2WYc -16 -12 -8 -4 0 4 8 I2 16 20 INWT POWER IN dl.Fig. 3. L...