Static electric field profiles of (a) 35 GHz, (b) 94 GHz, (c) 140 GHz, and (d) 220 GHz DDR GaAs IMPATTs for different crystal orientation of GaAs.
Bar graphs in Figures 5(a) and 5(b) show the values of V D/V B and x A/W (in percentage) for 35, 94, 140, and 220 GHz DDR IMPATTs based on 〈111〉, 〈100〉, and 〈110〉 oriented GaAs. Highest ratio of drift region voltage drop to breakdown voltage (V D/V B) and lowest ratio of avalanche zone width to total depletion layer width (x A/W) are observed in 〈111〉 oriented GaAs based DDR diodes up to 94 GHz, while the ratio of drift region voltage drop to breakdown voltage (V D/V B) and the ratio of avalanche zone width to total depletion layer width (x A/W) are found to be highest and lowest, respectively, in DDR diode based on 〈110〉 oriented GaAs at 140 and 220 GHz. Higher value of the ratio V D/V B indicates greater DC to RF conversion efficiency (since η L ∞V D/V B) [28]. Therefore, η L of DDR diodes based on 〈111〉 oriented GaAs are expected to be highest at 35 and 94 GHz, whereas at 140 GHz and 220 GHz the DDR diodes based on 〈110〉 oriented GaAs are expected to excel others at both 140 GHz and 220 GHz as regards the DC to RF conversion efficiency.
Bar graphs showing the (a) ration of drift zone voltage drop to breakdown voltage and (b) ration of avalanche zone width to total epitaxial layer width of 35, 94, 140, and 220 GHz DDR IMPATTs based on 〈111〉, 〈100〉, and 〈110〉 oriented GaAs.
The important L-S parameters such as optimum frequency (f p), avalanche resonance frequency (f a), peak negative conductance (G p), corresponding susceptance (B p), quality factor or Q-factor (Q p), negative resistance (Z R), RF power output (P RF), and L-S DC to RF conversion efficiency (η L) of DDR IMPATTs based on 〈111〉, 〈100〉, and 〈110〉 oriented GaAs designed to operate at 35, 94, 140, and 220 GHz for bias current densities of 0.85 × 108, 5.60 × 108, 10.20 × 108, and 22.45 × 108 for 50% voltage modulation are given in Table 4. The L-S admittance characteristics of the above-mentioned devices are shown in Figures 6(a)–6(d). It is interesting to observe from Table 4 and Figures 6(a)–6(d) that the magnitude of G p is highest in DDR diodes based on 〈111〉 oriented GaAs up to 94 GHz, but beyond that frequency the magnitude of the same parameter is highest in the DDR diodes based on 〈110〉 oriented GaAs. Similar nature is also observed for Z R. Q-factor (Q p = −B p/G p) of the device determines the growth rate and stability of IMPATT oscillation. Lower Q-factor closer to one (i.e., Q p ≈ 1) suggests higher oscillation growth rate and better stability. It is interesting to note that, at higher mm-wave frequencies (i.e., at 140 and 220 GHz), the Q-factors of DDR diodes based on 〈110〉 oriented GaAs are smallest among all the devices under consideration, while, at 35 and 94 GHz frequencies, DDR diodes based on 〈111〉 oriented GaAs possess better oscillation growth rate and stability due to their smaller Q-factors as compared to DDR diode based on 〈100〉 and 〈110〉 oriented GaAs.
Admittance characteristics of (a) 35 GHz, (b) 94 GHz, (c) 140 GHz, and (d) 220 GHz DDR IMPATTs based on 〈111〉, 〈100〉, and 〈110〉 oriented GaAs.
L-S parameters of 35, 94, 140, and 220 GHz DDR IMPATTs based on 〈111〉, 〈100〉, and 〈110〉 oriented GaAs.
| f d (GHz) | Crystal orientation | f a (GHz) | f p (GHz) | G p (×107 S m−2) | B p (×107 S m−2) | Q p (−B p/Gp) | Z R (×10−9 Ω m2) | P RF (mW) | η L (%) |
|---|---|---|---|---|---|---|---|---|---|
| 35 | 〈111〉 | 22.91 | 35.29 | −0.2417 | 0.7160 | 2.96 | 42.32 | 1498.50 | 16.24 |
| 〈100〉 | 20.43 | 34.94 | −0.2038 | 0.7475 | 3.67 | 33.95 | 1309.20 | 13.94 | |
| 〈110〉 | 19.92 | 34.80 | −0.1930 | 0.7514 | 3.89 | 32.07 | 1286.10 | 13.44 | |
| 94 | 〈111〉 | 70.08 | 94.00 | −2.0650 | 3.8847 | 1.88 | 10.67 | 1230.60 | 10.26 |
| 〈100〉 | 61.78 | 94.24 | −1.6119 | 4.2898 | 2.66 | 7.68 | 1066.00 | 8.44 | |
| 〈110〉 | 64.88 | 95.13 | −1.7686 | 4.2225 | 2.38 | 8.44 | 1107.60 | 9.01 | |
| 140 | 〈111〉 | 107.50 | 139.99 | −5.3110 | 8.7317 | 1.64 | 5.09 | 526.40 | 8.27 |
| 〈100〉 | 94.00 | 139.25 | −3.9431 | 9.6815 | 2.46 | 3.61 | 490.60 | 6.88 | |
| 〈110〉 | 107.91 | 139.13 | −6.0521 | 8.5827 | 1.42 | 5.49 | 578.84 | 9.25 | |
| 220 | 〈111〉 | 169.77 | 221.00 | −12.9818 | 21.3811 | 1.65 | 2.08 | 358.00 | 1.11 |
| 〈100〉 | 147.79 | 217.57 | −9.2000 | 23.3634 | 2.54 | 1.46 | 335.00 | 1.02 | |
| 〈110〉 | 180.17 | 218.14 | −15.2177 | 19.2157 | 1.26 | 2.53 | 397.90 | 1.19 |
Bar graphs in Figures 7(a) and 7(b) show the RF power output and DC to RF conversion efficiency of DDR diodes based on 〈111〉, 〈100〉, and 〈110〉 oriented GaAs designed to operate at 35, 94, 140, and 220 GHz frequencies. It is noteworthy from Table 4 and Figures 7(a) and 7(b) that DDR diodes based on 〈111〉 oriented GaAs are capable of delivering maximum peak RF power with maximum DC to RF conversion efficiency at 35 and 94 GHz. But at 140 and 220 GHz, both the P RF and η L are maximum in DDR diodes based on 〈110〉 oriented GaAs.
Bar graphs showing the (a) RF power output and (b) DC to RF conversion efficiency of 35, 94, 140, and 220 GHz DDR IMPATTs based on 〈111〉, 〈100〉, and 〈110〉 oriented GaAs.
The L-S characteristics of DDR IMPATTs based on 〈111〉, 〈100〉, and 〈110〉 oriented GaAs designed to operate at mm-wave window frequencies such as 35, 94, 140, and 220 GHz are presented in this paper. Both the DC and L-S performance of the above-mentioned devices are investigated by using a NSVE L-S simulation technique developed by the authors. Results show that the DDR IMPATTs based on 〈111〉 oriented GaAs are most suitable for generation of RF power with maximum conversion efficiency up to 94 GHz. However, at higher mm-wave frequencies, the DDR IMPATTs based on 〈110〉 oriented GaAs exceed its other counterparts as regards both the RF power output and DC to RF conversion efficiency. Thus, for higher mm-wave frequencies greater than 94 GHz, 〈110〉 oriented GaAs substrate is best choice for the fabrication of DDR IMPATT device, while, up to 94 GHz, 〈111〉 oriented GaAs substrate must be preferred over other two orientations.
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