Differences in Construction of MOSFET and JFET

An N-channel JFET is demonstrated in figure (a). It comprises a thin N-type silicon bar with two islands of P-type semiconductor material embedded in the sides hence forming two PN junctions.  While in N-channel Depletion MOSFET, there  is  just one p-region instead of two. This region is called substrate. Two P-regions are embedded in it as shown in fig. (b)

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Since gate is insulated from the channel  by a thin layer of silicon dioxide, input impedance of MOSFET is very high (of the order of 104 to 109M?) while the input impedance of JFET is 100M?as for a reverse biased pn junction.

Unlike JFET, a MOSFET has no gate diode,  it forms a capacitor. Capacitor has  gate  and  channel  as  electrodes  and  oxide  layer  is  dielectric.   Due to this property, device can be operated with negative as well as positive gate voltages.