Speed ​​saturation

The velocity saturation ( English velocity saturation ) designates one of the short-channel effects in field effect transistors (FETs).

With high drain voltages or short channel lengths (  <0.25 µm), the longitudinal electrical field in the channel becomes very large. According to the drift speed of the charge carriers would have to increase steadily. However, the charge carrier mobility decreases at high speeds. This hot carrier induced mobility degradation is based on a scattering of the charge carriers on the crystal lattice with the generation of optical phonons . Finally, the speed no longer increases with the field strength, it saturates. ${\displaystyle L}$ ${\displaystyle E}$ ${\displaystyle v=\mu \cdot E}$ ${\displaystyle v}$ ${\displaystyle \mu }$

As a consequence, the drain current in the transistor rises less strongly than one would expect, namely only because of the increasing charge carrier concentration. This is expressed e.g. B. in the transfer characteristic of the transistor, which instead of a square curve only has a linear current increase. The transconductance is thus independent of the current, which is usually very unfavorable. ${\displaystyle I_{\mathrm {D} }(V_{\mathrm {GS} })}$ ${\displaystyle g_{\mathrm {m} }}$

This effect is particularly relevant in DSM technologies (DSM, deep submicron ) due to the very short transistors.

literature

• Thomas Tille, Doris Schmitt-Landsiedel : Microelectronics: Semiconductor components and their application in electronic circuits . Springer, Berlin 2005, ISBN 3-540-20422-9 .