Temperature effects in bulk GaAs amplifiers

The microwave-gain characteristics of a bulk GaAs amplifier have been investigated experimentally as a function of temperature. The resistivity of the samples showed a strong temperature dependence, hence the results have been interpreted in terms of changing carrier concentration assuming a constant mobility. It has been found that stable amplification only occurs within a narrow range of temperature (carrier concentration). The highest
n . L
product (carrier density × sample length) for which gain has been observed was about 5 . 10
¹¹
cm
^-2
, and this is in agreement with earlier results and theory. Amplification did not occur below
n . L = 8 . 10^{10}
cm
^-2
. The frequency band where negative conductance appears was found to be strongly dependent on temperature (carrier density) and bias field. Noise figures, measured at temperatures where gain occurred, lay between 20 and 30 dB. Noise figure appears to be nearly independent of temperature. Gain versus field measurements on a stable amplifier indicate that the peak field of the velocity-field curve is about 4000 V/cm. Gain versus frequency and noise figure have also been measured on a bulk semiconductor amplifier which was operated with a coaxial transformer to increase the gain over a broad microwave frequency range.