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_0.48In_0.52As barrier and an undoped Ga _0.47In_0.53As 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_u at 1 GHz is 28 dB at 90 K vs. 20 dB at 300 K. At 300 K, G_u rolls off at 6 dB/octave and the maximum frequency of oscillations f_max, is 51 GHz. However, at 90 K, the gain roll-off steepens to 12 dB/octave and f_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_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_u indicates a second pole in the device response function