A program to demonstrate the feasibility of GaAs-based Kₐ -band power amplifiers has generated a number of technological advances. The goals of the program included (1) capability of amplifier operation at center frequencies of 23, 29, and 32.5 GHz; (2) bandwidth of 5 percent at each center frequency; and (3) gains and output power levels as specified in the table. Each amplifier was to contain three metal/semiconductor field-effect transistor (MESFET) stages, the MESFET gate width in each stage being larger than that of the preceding stage (see figure). During the program, one- and two-stage amplifier submodules with various input and output network configurations were also constructed and tested to characterize the input, output, and interstage-matching electrical characteristics of the networks.
At the beginning of the program, odd-shaped vapor-phase epitaxial (VPE) MESFET wafers were used. A breakthrough in power and efficiency was achieved by use of highly doped (doping density 8 × 1017cm¯3) MESFET material grown by molecular-beam epitaxy. At an operating frequency of 34 GHz, a monolithic amplifier that had gate widths of 50, 100, and 250 µm exhibited a gain of 16 dB, yielding an output power of 112 mW with 21.6-percent efficiency.
The next breakthrough came with the use of heterostructures grown by MBE (AlGaAs/InGaAs wherein the InGaAs was highly doped). These heterostructures made it possible to achieve high power density with high efficiency. For example, a single-stage monolithic microwave integrated circuit (MMIC) amplifier containing a MESFET with gate width of 100 ?m exhibited an efficiency of 40 percent at 32.5 GHz. The corresponding three-stage amplifier (with gate widths of 50, 100, and 250 µm) put out 180 mW at a gain of 23 dB and an efficiency of 30.3 percent.
The next breakthrough was achieved with 3-in. (7.6-cm) pseudomorphic high-electron-mobility transistor (PHEMT) wafers, each incorporating an etch-stop layer for the gate recess (made by reactive-ion etching). Again, state-of-the-art performances were achieved: efficiency of 40 percent with output power of 235 mW and gain of 20.7 dB. A single-stage 2 × 600-µm chip generated an output power of 794 mW with a gain of 5 dB and a power-added efficiency of 38.2 percent.
This work was done by Edward J. Haugland of Lewis Research Center and Paul Saunier and Hua Quen Tserng of Texas Instruments, Inc.
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