Microwave Devices

Silicon Carbide Microwave Devices

Jason Henning, Andreas Przadka, Luo Yuan, Ivan Milos, J. A. Cooper, Jr., M. R. Melloch, and K. J. Webb

Supported by the Office of Naval Research through a subcontract from UCSB

Because of its high breakdown field, silicon carbide is an ideal semiconductor for the fabrication of high-power microwave devices operating in the 1 - 10 GHz range [1,2]. Short-channel MESFETs in SiC have demonstrated f_t of 22 GHz and f_MAX of 50 GHz [1], and static induction transistors (SITs) have reached power levels of 470 W (1.36 W/mm) at 600 MHz and 38 W (1.2 W/mm) at 3 GHz [2]. An added advantage of SiC is its high thermal conductivity (10x higher than GaAs, 3x higher than GaN), which simplifies heat removal through the substrate during microwave operation.

The goal of this project is to develop high power SiC microwave amplifier devices for use in a broadband linear microwave amplifier being developed by the Office of Naval Research. The primary device under development is the static induction transistor (SIT), shown schematically in Fig. 1. The SIT is a majority carrier (unipolar) device in which the flow of electrons from the source to drain is controlled by a saddle-shaped two-dimensional potential barrier in the semiconductor between the metallic gates. If the doping and lateral dimensions are chosen correctly, the height of the potential barrier will be modulated both by the gate and the drain. Since the current increases exponentially as the barrier is lowered, the output characteristics of the SIT are usually non-saturating or "triode-like", i.e. resembling a "triode" vacuum tube. The SIT is important as a microwave device at low GHz frequencies because it delivers extremely high power per unit area.

Figure 1. Cross section of a SiC static induction transistor (SIT). Electrons flow from source to drain through a "saddle point" of electrostatic potential between the gate electrodes.

Fabrication of the SiC SIT requires straight-wall anisotropic etching of trenches 2 - 3 痠 deep using reactive ion etching (RIE), followed by deposition of Schottky metallization on the trench bottom without covering the trench sidewall. The lateral dimensions of the mesa between gate trenches is on the order of 0.5 - 1.5 痠. Low-resistance ohmic contacts are established to the source regions on the top of the mesas. Figure 2 (below) shows an SEM photo of a completed SIT having a mesa width of 1 痠 and a total mesa length of 1 cm (100 fingers). For clarity, this photo is taken before deposition of source air-bridge connections. Experimental static I-V characteristics of a smaller SIT are shown in Fig. 3. The maximum drain voltage is 250 V, the on-current at the knee is about 80 mA/mm, and the blocking gain is approximately 10. These values are comparable to the best reported in the literature for a SiC SIT.

Figure 2. SEM photo of a large-area experimental SIT device fabricated in our laboratory. The mesa fingers are 1 痠 wide and 100 痠 long. The total mesa length is 1 cm (100 fingers).

Figure 3. Measured static I-V characteristics of an experimental SIT. The gate voltage steps from zero (top curve) to -18 V (bottom curve) in -2 V steps. The horizontal scale is 20 V/div. and the vertical scale is 2 mA/div. The maximum drain voltage shown in this photo is 200 V.

To achieve high frequency operation, it is necessary to aggressively scale the mesa and trench widths, increase the doping of the channel region, and minimize parasitic capacitances. Figure 4 shows an SEM photograph of a C-band SIT recently fabricated in our laboratory. The mesa width and trench widths are each 0.5 痠. Source contacts are formed by an airbridge interconnection to minimize parasitic capacitances. This device exhibited an f_T of 7 GHz, the highest value yet reported for a SiC SIT [3]. A plot of small-signal current gain versus frequency is shown in Fig. 5.

Figure 4. SEM photograph of a C-band SIT having mesa and trench widths of 0.5 痠. A series of airbridges formed by e-beam lithography connects the source contacts. Gate metal lies at the bottom of each trench, but does not extend up the side of the source fingers.

Figure 5. Small signal short-circuit current gain h₂₁as a function of frequency for the C-band SIT of Fig. 4. The unity gain cut-off frequency is 7 GHz, a new record for SiC SITs.

In another thrust under this project, we have fabricated sub-micron T-gate MESFETs in SiC for use as high-power microwave amplifiers. These devices utilize direct E-beam lithography for patterning the gate and N+ implant levels, with optical lithography used for all non-critical levels. The I-V characteristics of a completed MESFET on semi-insulating 4H-SiC are shown in Fig. 6. This device has ion implanted source and drain regions and a 0.5 痠 T-gate. The saturated drain current is 350 mA/mm, the transconductance is 20 mS/mm, the drain breakdown voltage is 120 V, and the maximum available RF power is 3.2 W/mm.

Figure 6. Measured static I-V characteristics of a 4H-SiC microwave MESFET fabricated in our laboratory. This device has a gate width of 50 痠, gate length of 0.5 痠, gate-source spacing of 1 痠, and gate-drain spacing of 2 痠.

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[1] J. W. Palmour, et al., WOCSEMMAD 97, San Antonio, TX, February 1997.

[2] R. C. Clarke, A. K. Agarwal, R. R. Siergiej, C. D. Brandt, and A. W. Morse, "The Mixed Mode 4H-SiC SIT as an S-Band Microwave Power Transistor," IEEE Device Research Conf., Santa Barbara, CA, June 24-26, 1996.

[3] J. P. Henning, A. Przadka, M. R. Melloch, and J. A. Cooper, Jr., "Design and Demonstration of C-Band Static Induction Transistors in 4H Silicon Carbide," IEEE Device Research Conf., Santa Barbara, CA, June 28 - 30, 1999.

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