Field Effect Transistors

ἀ ere are several different types of field effect transistors (FETs), each of which has a different operational principle. For example, there are metal oxide semiconductor (MOS) transistors, junction field effect transistors (JFETs), static induction transistors (SITs), the punch-through transistors (PHTs), and others. All of these devices employ the flow of majority carriers. ἀ e most popular one among this group is the MOS transistor, which is primarily used in integrated circuits [T99, N06, S05]. In contrast, the JFET is not suitable for integration and so it is primarily fabricated as an individual device [E97, R99]. All FETs have very large input resistance on the order of 1012 Ω. ἀ e MOS transistor typically operates with very small currents [N02] and thus for power electronics applications thousands of MOS transistors are connected in parallel. A JFET usually operates with larger currents. Both JFET and MOS transistors have relatively small transconductances, and this means that they cannot control current flow as effectively as bipolar junction transistors (BJTs). Since the parasitic capacitors are of the same order of magnitude, BJTs can charge and discharge these capacitors much faster and so BJTs are more suitable for high-frequency operations. Because current flow in MOS transistors is very close to the silicon surface where surface states can fluctuate with time, MOS devices have a relatively higher noise level, especially at low frequencies.