3.7 Summary

There are two types of MOSFETs: the depletion-enhancement or DE-MOSFET and the enhancement-only or E-MOSFET. Both devices are constructed using an insulated gate instead of a PN junction and both devices exhibit a square-law characteristic curve. Like the JFET, MOSFETs are modeled as voltage-controlled current sources. Both devices show very, very small gate currents due to the insulated gate. They are static sensitive and precautions must be taken when handling them to avoid damage from ESD.

The DE-MOSFET exhibits the same characteristic curve as the JFET, however, the curve extends into the first quadrant (enhancement mode). Consequently, 𝐼_𝐷𝑆𝑆 is no longer the largest drain current possible, but rather, represents a middle ground. The DE-MOSFET can utilize all of the bias prototypes that are used with JFETs, including self bias, constant current bias and combination bias. Due to its dual quadrant capability, other biasing types are also possible including zero bias and voltage divider, both of which are variations on constant voltage bias.

The E-MOSFET operates in the first quadrant only (enhancement mode). Compared to the DE-MOSFET, its characteristic curve is shifted positive such that 𝑉_{𝐺𝑆(𝑜𝑓𝑓)} is now 𝑉_{𝐺𝑆(𝑡ℎ)}, and 𝐼_𝐷𝑆𝑆 signifies the off-state leakage current. E-MOSFETs are available in both low power and high power variants. The high power versions utilize an alternate internal structure that allows drain current to flow vertically rather than horizontally. This results in very high current carrying ability and very low values for 𝑟_{𝐷𝑆(𝑜𝑛)}.