For the first time silicon double-drift IMPATT structures have been grown completely by Si molecular-beam epitaxy. The n-type layers are grown at 750 °C on low-resistivity n+-type substrates followed by p-type layers at 650 °C. The highly doped p+-layers are grown by solid-phase epitaxy in the MBE system. Device design is made for CW operation in W-band. The material is investigated by inspection of beveled samples, defect etching, TEM, SIMS, and spreading resistance measurements. Double-drift flat-profile diodes are housed and mounted employing a technological procedure approved for single-drift diodes. For initial device characterization, dc measurements are performed. Information about doping profile, series, and thermal resistances is obtained. Preliminary RF measurements delivered a maximum output power of 600 mW at 94 GHz with 6.7-percent efficiency from an unoptimized structure.

A 90-GHz double-drift IMPATT diode made with Si MBE

Reliability of terahertz-frequency (~1.0 THz) characteristics of wide-bandgap (WBG) wurtzite (Wz)-GaN- and 4H-SiC-based p++nn++-type single-drift-region (SDR) impact avalanche transit time (IMPATT) devices (normal and photoilluminated) is compared through a simulation scheme. The simulation experiment reveals that an RF power density of 3.37 times 1011 W middotm-2 (efficiency of 18.2%) at around 1.126 THz may be realized from the optimized unilluminated GaN IMPATT device, whereas the unilluminated 4H-SiC IMPATT device is expected to generate an RF power density of 1.35 times 1011 W middotm-2 (efficiency of 9%) at 1.05 THz. However, the parasitic series resistance reduces the maximum exploitable power density from the terahertz devices. Under optical illumination, additional photogenerated carriers are created in the devices, and these carriers change the admittance and negative resistance properties of the terahertz IMPATT diodes. The performance modulation of the terahertz devices is simulated, and the results are compared in this paper. Under external radiation, the operating frequencies of the GaN- and SiC-based diodes are found to shift upward by 6.0 and 40.0 GHz, respectively, with degradation of maximum output-power density level and device negative resistance. The extensive simulation experiments establish that, although the photosensitivity of the 4H-SiC-based IMPATT device is better than its GaN counterpart, the overall terahertz performance of the unilluminated GaN IMPATT device is far better than the 4H-SiC-based device, particularly in terms of output power and efficiency. The simulation results and the proposed experimental methodology presented here can be used for realizing optically tuned WBG IMPATT oscillators for terahertz communication.

Photosensitivity Analysis of Gallium Nitride and Silicon Carbide Terahertz IMPATT Oscillators: Comparison of Theoretical Reliability and Study on Experimental Feasibility

An attempt is made in this paper to study the influence of skin depth on the parasitic series resistance of millimeter-wave IMPATT devices based on Silicon. The method is based on the concept of depletion width modulation of the device under large-signal condition. A large-signal simulation model based on non-sinusoidal voltage excitation is used for this study. The electric field snap-shots of 35 GHz Single-Drift Region (SDR) and 94 GHz Double-Drift Region (DDR) IMPATT devices are first obtained from which the series resistances are estimated by incorporating the effect of skin depth in the modeling and simulation. The series resistances of these devices are also obtained by neglecting the effect of skin depth. The values of series resistances obtained from the simulation are compared with the corresponding experimentally reported values. It is observed that the series resistance estimated by including the skin effect is in closer agreement with the experimental values as compared to that without including the same. Thus the skin effect plays an important role for determining the series resistance of IMPATT devices at millimeter-wave frequency bands.

Influence of skin effect on the series resistance of millimeter-wave IMPATT devices

High-performance GaAs double-drift Read IMPATT diodes have been demonstrated at 60 GHz. 1.24-W CW output power at 11.4-percent dc to RF conversion efficiency was obtained with a junction temperature rise of 225°C. The doping profiles and test circuits have not yet been optimized and we expect that still higher power and efficiency should be achievable.

GaAs IMPATT diodes for 60 GHz

The dynamic performance of wide-bandgap 4H-SiC based double drift region () IMPATT diode is simulated for the first time at terahertz frequency (0.7 Terahertz) region. The simulation experiment establishes the potential of SiC based IMPATT diode as a high power ( ) terahertz source. The parasitic series resistance in the device is found to reduce the RF power output by 10.7%. The effects of external radiation on the simulated diode are also studied. It is found that (i) the negative conductance and (ii) the negative resistance of the diode decrease, while, the frequency of operation and the quality factor shift upward under photoillumination. Holes in 4H-SiC based IMPATT are found to dominate the modulation activities. The inequality in the magnitude of electron and hole ionization rates in the semiconductors may be correlated with these findings.

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Prospects of 4H-SiC Double Drift Region IMPATT Device as a Photo-Sensitive High-Power Source at 0.7 Terahertz Frequency Regime

A new method is given for determining the electrical series resistance of an IMPATT diode. The measurement is based on observation of the oscillation threshold bias current for a diode in a standard circuit. The method is applied to GaAs diodes near 40 GHz. The values obtained are used to quantitatively explain other performance characteristics of the diodes.

L.H. Holway

Papers

Measurement of series resistance in IMPATT diodes