Low-frequency noise phenomena in InP-based HFETs related to stress induced degeneration and interface properties

The normalized noise power density is shown for HFETs with InAlAs as barrier layer and those where InP is used. All transistors were manufactured in our Institute using the same geometrical parameters and exhibit almost the same sheet carrier concentration in the channel. The depicted spectrum of the HFET containing InAlAs is a typical result for these devices grown by MBE as long as no degeneration effects occur. On the other hand, HFETs without aluminium which were grown by LP-MOVPE and in which the In mole fraction has been varied from 53% up to 81%, show excellent device properties. The comparison between both material systems reveals that noise of the HFET with InAlAs is slightly lower. This can be understood since during the growth of the interface in this device only one component must be exchanged. Also, due to the larger conduction band discontinuity the penetration depth of the 2DEG into the barrier is reduced and with it the activation of traps located there. The latter also explains the decreasing noise power with increasing indium portion in the channel of the aluminium-free HFETs. However, at 67% indium, the minimum is reached and further increasing the strain does not improve the noise properties. Concluding, it may be stated that there exists a trade-off dependent on the special application between the slightly better performance of the InAlAs/InGaAs/InP HFETs and those devices with a InP barrier, which are less sensitive to stress