Influence of Real-Space Transfer on Transit Time and Noise in HEMTs

Using a classical Monte-Carlo simulation, tested by means of the comparison with experimental measurements of I-V characteristics and noise temperature, we explain the fast, low noisy behaviour of GaAs/Al0.2Ga0.8As high electron mobility transistors (HEMTs) with respect to GaAs MESFETs. As main tool for the comparison we employ the transit time of the electrons through the devices. The transit time distribution shows that the current in the HEMT comes from electrons whose velocities do not spread as much as in the MESFET, where the distribution is wider. This leads to a lower value of the current variance, which explains the better noise performance of the heterojunction devices.