The GaAs traveling-wave amplifier as a new kind of microwave transistor

A versatile solid-state microwave amplifier has evolved from a combination of transistor mechanisms and the transferred-electron effect. The prototype device, called a "traveling-wave transistor," employs a 2-µm layer of n-type GaAs grown epitaxially on a semi-insulating substrate. A transistor-like input launches a traveling space-charge wave that grows exponentially as it moves along the surface to a relatively distant output. There it is converted back into an electromagnetic wave. The long distance between input and output minimizes the feedback capacitance that often limits gain and bandwidth in high-frequency devices. Twelve experimental units show broad-band net gain in X band, with 10-30-dB built-in isolation. One unit exhibits instantaneous net gain from 6.7 to 15.3 GHz; another yields 28 dB at 9.2 GHz. All devices are good for linear microwave signal processing: voltage-controlled phase modulation at constant net gain, or voltage-controlled gain modulation at constant phase. Different bias conditions make possible threshold-sensitive saturated-amplitude amplification of pulses or sinusoids for logic or digital functions.