Elemental and compound semiconductors, including wide band gap semiconductor SiC, are critically examined for
high-power performance of IMPATT diodes at 94 GHz, in terms of efficiency, high-temperature operation and noise
measure. Based on a new analysis applicable to a wide range of semiconducting materials and by using the available
measured physical parameters, it is shown that wide band gap semiconductor, SiC, offer significant advantages
compared to conventional Si and Ge for these applications. The new analysis uses peak electric field strength at
avalanche breakdown, avalanche response time and noise-measure as critical material parameters for evaluating the
quality of a semiconducting material for high-frequency, high-power operation of an IMPATT oscillator. The study
show improvement by order of magnitude in the maximum breakdown electric field, four hundred fold improvement in
the RF power output, and over all potential for successful operation beyond 600K for SiC W-band IMPATTs. The
present study depicts the improvement in the efficiency by three orders of magnitude and avalanche response time by
an order of amplitude of the IMPATT device, by replacing Si with SiC. The noise analysis shows that under similar
operating condition, Si0.5Ge0.5 IMPATT is less noisy device than its counterparts. Though SiC IMPATTs outclass Si
IMPATTs in terms of RF power, efficiency, noise level of these two types of IMPATTs are comparable (~ 17 dB).
Prospects of Si/Si0.8Ge0.2 heterojunction IMPATT devices are studied through a modified simulation technique and
their performances are further compared with those of homojunction devices. This high-frequency study includes the
effects of mobile space charge, parasitic resistance and also considered the detrimental role of elevated junction
temperature on the maximum exploitable power level from the devices. The study indicates that under similar operating
condition, Si/SiGe heterojunction IMPATT is capable of delivering a RF-power output of nearly 6W (efficiency 21%),
two times higher than that from Si0.8Ge0.2 homojunction diode, which is only 13% efficient. The overall study indicates
the superiority of the heterojunction diodes over their homojunction counterparts as far as negative resistance, device
impedance and quality factor are concerned. To the best of author’s knowledge, this is the first report on highfrequency
analysis of Si/Si0.8Ge0.2 heterojunction IMPATT.
Full-scale, non-linear large-signal model of IV-IV SiC Double-Drift IMPATT diode with general doping profile is derived. This
model, for the first time, has been used to analyze large-signal characteristics of SiC-IMPATTs at 94 GHz of MM-wave window
frequency. Under small-voltage modulation (~ 2%, i.e. small-signal condition) results are in good agreement with calculations done
using a linearised small-signal model. The large-signal values of the diode’s negative conductance (5x106 Sm-2), susceptance (104.0
×106 Sm-2), breakdown voltage (207.6 V), and power generating efficiency (15%, RF power: 25.0 W at 94.0 GHz) are obtained at
modulation amplitude ~ 50% of DC breakdown voltage, for a fixed average current density. The large-signal calculations exhibit
power and efficiency saturation for large-signal (< 50%) voltage modulation and thereafter decrease gradually with further
increasing voltage-modulation. This generalized large-signal formulation is applicable for all type of IMPATT structures with
distributed and narrow avalanche zones. The simulation is made more realistic by incorporating the diffusion effects, space-chare
effects and the realistic field and temperature dependent material parameters in SiC. The electric field snap-shots and the large-signal
impedance and admittance of the diode with current excitation are expressed in closed loop form.
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