Frequency response of AlInAs/GaInAs/InP modulation-doped field-effect transistors at cryogenic temperatures

The microwave S-parameters of MODFETs have been measured at 300 and 90 K up to a frequency of 26.5 GHz. Measurements were made using a microwave network analyzer and a probe station that was modified for liquid-nitrogen-cooled operation. The MODFET layers included an Si-doped Al/sub 0.48/In/sub 0.52/As barrier and an undoped Ga/sub 0.47/In/sub 0.53/As channel grown by molecular-beam epitaxy on a semi-insulating InP substrate. Results indicate a striking difference between the high-frequency responses at the two temperatures. At low frequencies the gain is higher at 90 K, where the maximum unilateral gain G/sub u/ at 1 GHz is 28 dB at 90 K vs. 20 dB at 300 K. At 300 K, G/sub u/ rolls off at 6 dB/octave and the maximum frequency of oscillations f/sub max/, is 51 GHz. However, at 90 K, the gain roll-off steepens to 12 dB/octave and f/sub max/ is only 21 GHz. The roll-off in current gain is 6 dB/octave at both temperatures, and the current gain cutoff frequency, f/sub T/, is 33 GHz at 300 K and 41 GHz at 90 K. It was found that GaInAs MODFETs can have a lower frequency response at cryogenic temperatures than at room temperature. The measured 12 dB/octave roll-off rate of G/sub u/ indicates a second pole in the device response function.<
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