J. Dickmann

InGaP/InGaAs HFET with high current density and high cut-off frequencies

Doped channel pseudomorphic In/sub 0.49/Ga/sub 0.51/P/In/sub 0.20/Ga/sub 0.80/As/GaAs heterostructure field effect transistors have been fabricated on GaAs substrate with 0.25 /spl mu/m T-gates and self-aligned ohmic contact enhancement. By introducing the channel doping and reducing the series resistances, a high current density of 500 mA/mm is obtained in combination with cut off frequencies of f/sub T/=68 GHz and f/sub max/=160 GHz. The channel doping did not affect the RF-performance of the device essentially, which is additionally reflected in noise figures below 1.0 dB with an associated gain of 14.5 dB at 12 GHz.

Influence of surface layers on the RF-performance of AlInAs-GaInAs HFETs

The influence of thickness and doping level of the GaInAs cap layer in AlInAs-GaInAs-InP HFET structures on the DC and RF performance is systematically investigated. The authors compare three different approaches, the undoped cap layer, the highly doped thick cap layer, and, as a new approach, the thin doped and therefore surface depleted cap layer. HFET devices with 0.3 mu m gates have been processed. While all devices demonstrate f/sub T/-values around 80 GHz, distinct differences are observed for the f/sub max//f/sub T/ ratios from 1 (highly doped cap) over 1.3 (undoped cap) to 2.7 (surface depleted cap). The best f/sub max/ of 240 GHz is achieved for the new cap layer approach. A systematic investigation of the influence of the g/sub m//g/sub d/ and C/sub gs//C/sub ds/ ratios demonstrates the strong influence of a proper layout of the cap layer at the drain side of the gate region.<<ETX>>

Recess dependent breakdown behavior of GaAs-HFETs

GaAs based HEMT devices were fabricated with a constant recess towards the source, whereas the recess width towards the drain was varied. While the off-state breakdown voltage has been improved by the use of a wide recess towards the drain, no dependence of the on-state breakdown on the recess configuration was observed. The constant breakdown voltage in the on-state is analysed by the feedback parameters obtained from an extraction of the small signal equivalent circuit. Although the extrinsic gate drain capacitance could be reduced by the use of a wider recess configuration, it is assumed that the intrinsic drift region is independent of the recess configuration.<<ETX>>

Double-side planar-doped AlGaAs/InGaAs/AlGaAs MODFET with current density of 1 A/mm

AlGaAs/InGaAs/AlGaAs double-side planar-doped (DSPD) pseudomorphic MODFETs of 0.3- mu m gate length with both excellent DC and RF performances are reported. A maximum unilateral gain cutoff frequency of 170 GHz and a maximum current gain cutoff frequency of 60 GHz are achieved. The devices exhibit a maximum transconductance of 500 mS/mm and an extremely high current density of 1 A/mm. These are the highest frequencies reported so far for MODFET devices capable of driving 1-A/mm current density. This current density is the highest ever reported with this type of layer structure.<<ETX>>

Determination of the optimum condition to introduce the doping in the channel of high speed doped channel AlGaAs/InGaAs HFET's

An investigation of the introduction of doping into the channel of Al/sub 0.25/Ga/sub 0.75/As/In/sub 0.2/Ga/sub 0.8/As HFETs to obtain high saturation currents and reasonable low noise performance is discussed. The device performance is experimentally compared to that of undoped channel HFETs having the same layer structure. For a 0.35- mu m gate length backside doped channel HFET a maximum transconductance of 625 mS/mm, a maximum saturation current at V/sub Gs/=+0.8 V of 720 mA/mm, a power gain cutoff frequency of 195 GHz, and at 18 GHz a noise figure of 1.1 dB with 17 dB associated gain were measured.<<ETX>>

Depleted surface layer AlInAs/GaInAs/InP HFETs

An investigation of InP HFETs that combine a highly doped, thick cap layer with an undoped surface layer by using a highly doped thin surface layer that is depleted by the surface potential is discussed. Very good DC characteristics without kinks, good gate characteristics, and excellent microwave performance are observed. For 0.3- mu m gate length devices f/sub maxGU/ of more than 250 GHz is obtained. From measurements of the bias dependent S-parameters, the corresponding variation of equivalent circuit elements is deduced. The measurements show the influence of the depleted surface layer.<<ETX>>

RF-current de-confinement in III-V HFETs

A criterion for the carrier confinement in III-V HFETs is derived from the linear correlation between the two ratios Cgs/Cgd and gm/gds, where Cgs is the gate/source capacitance, Cgd is the gate/drain capacitance, gm is the transconductance, and gds is the output conductance. FETs of a number of distinctly different layer structures have been compared on this basis. The model finds carrier flow confinement dominated by the structural aspect ratio and the extension of the drain region. In the RF analysis no evidence for additional confinement of hot carriers by a deep-quantum-well channel configuration is found either in GaAs-based or in InP-based material systems.<<ETX>>