INTRODUCTION0.13pm T-gate metamorphic Ino,32Alo,~8As~no,3~G~,6gAs HEMTs grown on GaAs substrates were fabricated with composite InGaAs channels, combining the superior transport properties of In0,52Ga,,48As with low impact ionization in In0,32G%.68A~. The use of composite InGaAs channels leads to excellent DC-characteristics, high drain currents of 750mA/mm, extrinsinc transconductances of 600mS/mm combined with still very low output conductance values of 20mS/mm, thus giving g,/go ratios of 30. A maximum frequency of oscillation f, , , of 350GHz with current gain cut-off frequency fT of 15OGHz has been obtained at VDs=1.5V. These are the best microwave frequency results ever reported for any FET on GaAs substrates. Using theoretical as well as experimental support, we show that excess output conductance in 0.13pm lattice matched and metamorphic Ino~52A1,,48Asflno,S2G~,48A~ HEMTs is based on weak impact ionization in high gate-drain field region.Ino,52A~o,4gAslIno,5~G~,48A~ HEMTs lattice matched to InP are well suited for high speed and low-noise applications.Unfortunately, the high output conductance and low breakdown voltage remain as obstacles for optimum power performances. In order to overcome such undesirable characteristics, a double-recess gate process or undoped surface cap-layer was introduced to reduce the peak electrical field in the InGaAs channel near the drain. This in turn increases the breakdown voltage and reduces the output conductance [ 1,2]. 0.15ym InAlAsLnGaAs HEMTs have shown excellent high-frequency performance: fmax of 405GHz with fT of 130GHz have been obtained [I]. The fmax was improved to record values of 455GHz using undoped InGaAs cap-layers [l]. On the other hand a record fT of 340GHz has been demonstrated with a gate length of 50nm and 80% indium in the channel [3]. However, the associated fmax was only 280GHz. Such results demonstrate the tradeoff between fma/fT ratios and fT values for sub-0.2pm lattice matched InAIAslInGaAs HEMTs. In this paper, we report on novel 0.13pm metamorphic InAIAsLnGaAs HEMTs on GaAs substrate with composite InGaAs channels. This leads to a high power gain at higher saturation velocity regime, an fmax of 350GHz and fT of 15OGHz. We demonstrate, with theoretical as well as experimental support, that this performance is due to the wider band gap of the sub-channel, which reduces the impact ionization in high gate-drain field, hence, the output conductance.
I. DEVICE STRUCTURESMetamorphic InAIAslInGaAs HEMTs layers ( MM ) were grown by MBE on GaAs substrates, using low-temperature grown linearly graded InAlAs buffer with an Incomposition from 0.03 to either 0.32, or 0.52, i.e. the composition corresponding to lattice matched material on InP. The MM-HEMT structure consists of a 250nm thick undoped InAlAs buffer embedded between two ten period superlattices, composite channels consisting of 12nm InGaAs channel layer with an In content of 0.52 on top of 20nm InGaAs with In-content of 0.32, a 2nm InAlAs spacer layer, a 12.5nm InAlAs supply layer doped to 10'9c...