High-speed low-power ring oscillator using inverted-structure modulation-doped GaAs/n-AlGaAs field-effect transistors

Modulation-doped GaAs/n-AlGaAs field-effect transistors with inverted structures, i.e. undoped GaAs on top of n-AlGas, have been successfully used in E/D-type DCFL ring oscillators. An inverted heterojunction structure grown by MBE showed a high electron mobility of about 20,000–92,000 cm 2 /(Vs) at 77 K. In the enhancement mode (E-mode) FETs with 1.2 µm gate length, a maximum transconductance of 250 mS/mm and a K-value of 7.3 mA/V 2 were achieved at 77 K. Low ohmic contact resistances of 0.1–0.4 Ωmm with a mean value of 0.2 Ωmm were obtained in a 2 inch-diameter wafer at 300 K and 77 K. A 21-stage ring oscillator showed a minimum gate delay of 26.3 ps with a power dissipation of 234 µW/gate at 77 K.