Noise and transit time in ungated FET structures

Using a classical Monte-Carlo simulation, tested by means of the comparison with experimental measurements of I-V characteristics and noise temperature, we compare the frequency and noise behavior of GaAs/Al_0.2Ga_0.8As ungated HEMT (called u-HEMT) with respect to GaAs ungated MESFET (u-MESFET). The small fraction of Al in the AlGaAs makes the real-space transfer between the GaAs and AlGaAs layers become important; its effects will be analyzed. As main tool for the comparison we employ the transit time of the electrons through the devices. This magnitude can give exact information not only about the speed but also about the noise behavior of the devices. We have observed an important reduction of the switch-on time which passes from 14 ps in the u-MESFET to 10 ps in the u-HEMT (at a biasing of 2.0 V). The transit-time distribution shows that the current in the u-HEMT comes from electrons whose velocities do not spread as much as in the u-MESFET, where the distribution Is wider. This leads to a lower value of the current variance, which is related to the total noise spectrum of the devices