Why are MOSFETs used for VLSI IC fabrication instead of JFETs?

Why are MOSFETs used for VLSI IC fabrication instead of JFETs? What are some reasons the use of JFETs is not common?

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3 Answers

The N channel JFET needs a negative voltage on its gate with respect to source therefore this complicates the power supply regime by requiring the addition of a negative rail. Similarly, for a P channel device its gate would have to rise above Vcc to be able to control it. Here's an n channel JFET pictorially: -

As you can see, the voltage on the gate has to be negative to the source in order to control it. Images taken from here

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The key determining factor is manufacturability.

To make an IC you must be able to produce transistors reliably and a key component of this is the ability to make transistors with the same characteristics across the whole wafer. This called "matching".

The JFET Pinch-off voltage is notoriously poorly matched and highly variable so much so that in CMOS processes if we put in a few JFet's they are used in only very particular locations and then a lot of circuitry is put around them to correct for them.

Other issues: JFets are not easily scalable to lower dimensions. In CMOS the key size parameter(gate length) is easily defined with lithography and etching. In a JFet the key dimension is a diffusion which is harder to control dimensionally. Also as we shrink sizes the gate dielectric material changes, in a JFet you're stuck with Si so you have little process control over your gate drive.

The nice things about JFets:

• They are lower noise than CMOS, since they are a bulk device (i.e. current runs under the surface). CMOS processes below 0.5 um are surface devices and the channels interacts with the Si/SiO2 interface in the channel under the gate.
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JFETs can only be operated in the depletion mode whereas MOSFETs can be operated in either depletion or in enhancement mode. In a JFET, if the gate is forward biased, excess- carrier injunction occurs and the gate current is substantial. Thus channel conductance is enhanced to some degree due to excess carriers but the device is never operated with gate forward biased because gate current is undesirable. MOSFETs have input impedance much higher than that of JFETs. MOSFET has low leakage current and better thermal stability than JFET. MOSFET technology is easily scalable and easier to manufacture. MOSFETs are also used for high power applications but JFETs are used for low power applications.

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