Cubic versus hexagonal SiC vertical pin SPST/SPDT/SPMT switches for MMW communication systems: a modified quantum drift-diffusion model for switching characteristics analysis

A modified quantum drift-diffusion (QDD) model is developed for non-linear analysis of SiC (4H, 6H and 3C polytypes) pin semiconductor diodes at W-band frequency regime. Effects of incorporation of a buffer layer (n-type), in between substrate and low doped active region of the hexagonal (4H and 6H) pin (p⁺⁺-p⁺-n⁻-n-n⁺⁺) vertical mesa structure is thoroughly studied in this paper for the improvement of MM-wave performance of the single device. Also, a thin layer of Ge has been introduced in between Si substrate and n⁺ cubic-SiC layer in 3C-SiC pin-device for minimising the lattice mismatch issue in between Si/3C-SiC interface. The comprehensive analysis establish that the forward characteristics, reverse recovery time (1 ns) and breakdown voltage (171 Volt) in case of 4H-SiC device are quite good in comparison to its cubic (3C) and other hexagonal (6H) counterparts, however, the switching characteristics of 3C-SiC pin diode array is comparatively better than its hexagonal counterparts. This observation could be explained in terms of lowest series resistance in 3C-SiC based single pin device that has been achieved by incorporating Ge layer in mesa structure. The authors have made a comparative analysis among SPST, SPDT and SPMT pin switches made up with 4H, 6H and 3C-SiC poly-types. At 94 GHz, W-band central frequency, series resistance in 4H-SiC single device is 0.59 Ω, whereas, the same is much lower (0.27 Ω) in case of 3C-SiC. Insertion loss and isolation in 3C-SiC pin array of switches are found to be 0.18 dB and 38 dB (SPST switch), 0.19 dB and 67 dB (SPDT switch), 0.20 dB and 90 dB (SPMT shunt type switching array) and 0.23 dB and 74 dB (SPMT series-shunt type switching array). This newly proposed QDD model validation is done through comparative studies between experiment and analytical results for 4H-SiC SPST switches in low-frequency Microwave region. The validated QDD model, coupled with PSpice and Comsol Multi-physics simulator, then used for designing of the W-band devices and corresponding switches. However, as far as author’s knowledge is concerned, no experiment is yet done with SiC pin diodes at W-band frequencies, in literature for comparison. From simulation point of view also such an extensive study on hexagonal and cubic SiC pin diode switches at W-band region has not yet been done by any other researcher group. This paper, for the first time, establishes the feasibility and potentiality of IV-IV group semiconductor based pin (p⁺⁺-p⁺-n⁻-n-n⁺⁺) switches for W-band applications. Comparative analysis also reveals that 3C-SiC based shunt type pin (p⁺⁺-p⁺-n⁻-n-n⁺⁺) SPMT switches are the best for MMW-communication systems. Thermal modelling of the designed devices are also compared and reported in this paper. The quasi-3D thermal analysis is done to optimize the mesa and heat-sink diameter/dimensions so as to minimize the thermal runaway issues. The results may further be used for developing low-cost and fast semiconductor switches for potential application in THz communication systems.