Pulse-doped AlGaAs/InGaAs pseudomorphic MODFETs

MODFETs have been fabricated using heterojunctions consisting of AlGaAs and pseudomorphic InGaAs, grown on GaAs substrates. The large conduction band discontinuity (about 0.46 eV for 25% In and Al concentration) leads to a 2-D electron density as high as 2.3×10 ¹² cm^-2, with electron mobilities of 7000 and 16000 cm²/V-s at 300 and 77 K, respectively. Such a high electron density in combination with reasonable transport properties leads to MODFETs with exceptional characteristics. Devices with 0.15-0.25-μm gate length have room-temperature drain currents as high as 600 mA/mm and room-temperature transconductance as high as 500 mS/mm. The f_T is as high as 98 GHz, as determined by 20-dB/decade extrapolation of microwave data taken to 25 GHz. A comparison of the effect of bias on the total delay through standard and pseudomorphic MODFETs suggests that the excellent microwave performance exhibited by the pseudomorphic device arises from a reduction in parasitic and drain delays and not from a higher electron velocity under the gate