Improvements In Modfet Performance Realized Through ION Implantation In The Gate Region

Abstract

The performance of MODFET's as microwave power devices6v7 is limited by the relatively low sheet carrier density in the 2-dimensional electron gas and the low drain breakdown voltage due to the highly doped Alo.3 Gao7 As. In conventional modulation-doped structures, there is always a tradeoff between the gate breakdown voltage and the doping level in the AlGaAs. A higher doping level implies higher sheet carrier concentration but reduced gate breakdown voltage. To circumvent this problem, several research groups have fabricated multiplechannel MODFET'S.~.~ Such devices not only have a higher sheet carrier concentration in the 2-dimensional electron gas, they also have a lower output conductance and a higher drain breakdown voltage. This is due to the confinement by the Si-AIGaAs layers underneath each channel. In this work, a new approach was used to improve the performance of MODFETs. A very shallow, low-dose p-type implantation was performed under the gate region, directly above the 2dimensional electron gas of the channel. Significant improvements were observed in the source-drain breakdown voltage and the gate-channel forward turn-on and reverse breakdown volatges. In addition, extremely low output conductance and very high &1 to gd ratios were obtained. To clearly understand the effect of implantation and high temperature annealings on device performance, the dependence of VDS,max, gd, gm and V, on annealing temperatures ranging from 78OOC to 93OOC was studied. EXPERIMENTAL METHODS