JFET vs MOSFET: Differences Explained
Both are field-effect transistors in which a gate voltage controls the current between source and drain. In a MOSFET the gate is insulated by an oxide layer. In a JFET the gate is a reverse-biased PN junction that squeezes the channel. This makes a JFET normally on and gives it very low noise, while the insulated gate of the MOSFET draws almost no current and makes it far easier to integrate by the billions.
| Aspect | JFET | MOSFET |
|---|---|---|
| Gate structure | A PN junction, which must stay reverse-biased | A metal gate separated from the channel by a thin oxide |
| Default state | Normally on (depletion mode): the gate voltage pinches the channel off | Usually normally off (enhancement mode); depletion types also exist |
| Gate current | Small reverse leakage; it rises sharply if the gate is forward-biased | Almost zero in steady state, because of the insulator |
| Input impedance | High | Very high |
| Noise | Low noise, especially at low frequency | Higher low-frequency (1/f) noise |
| ESD robustness | Better, because there is no thin gate oxide to break down | Sensitive: the gate oxide can be destroyed by static discharge |
| Integration | Rarely used for large-scale integration | The basis of CMOS: billions of transistors per chip |
| Typical uses | Low-noise preamplifiers, op-amp input stages, audio and RF front ends, analog switches | Digital logic, memory, power switching and most modern ICs |
Figures are typical orders of magnitude and vary by process generation and product.