Advantages of depletion mode FET devices over enhancement MOSFETs

Question

I've grown the impression that JFETs and depletion mode MOSFETs are not used very much nowadays, especially in new designs.

I know that they need different bias networks, especially as discrete components, so I guess there might be some advantages in using them, but the current trend doesn't seem to corroborate that guess.

Are depletion mode devices being made obsolete by the development of the technology of enhancement MOSFETs? Are there still applications where the use of a depletion device gives distinct advantages, especially as discrete components (I know that depletion mode MOSFETs can be used as active loads in IC MOS technology, so they may still be needed there)?

On a related note, I'd like also to know whether JFETs are still actively used in new designs, since it seems that they could be replaced by depletion MOSFETs (assuming one really needed a depletion mode device for some reason).

Answer 1

Are depletion mode devices being made obsolete by the development of the technology of enhancement MOSFETs?

No, I wouldn't say so. Depletion mode devices, like most discrete devices, are being made obsolete by the development of integrated circuits. As time goes on, ICs are developed for progressively more specialized purposes, reducing the need for discretes of all sorts. This is particularly true for signal processing circuits.

Are there still applications where the use of a depletion device gives distinct advantages, especially as discrete components?

Yes. Depletion devices are awesome for startup circuits, to provide some small current during the circuit's power-up sequence and to be shut off when it is finished. Their "default-on" characteristic is what makes them useful here. You can find a number of high voltage depletion devices built for this purpose on the market.

It is also quite simple to make a "2-terminal" current source using a depletion-type device: it simply requires the FET and a resistor. Making a similar current source using enhancement MOSFETs is much clumsier.

On a related note, I'd like also to know whether JFETs are still actively used in new designs, since it seems that they could be replaced by depletion MOSFETs.

JFETs tend to have less flicker noise than depletion, and especially enhancement, MOSFETs, due to the fact that their conductive channel is buried deep within the silicon, far away from crystal defects at the surface. This makes them useful for low-noise amplifiers where the base current of a BJT would be unacceptable.

Answer 2

Regarding the ICs, I think it is simply because of the compatibility with the manufacturing processes used. Threshold voltage of a MOSFET is controlled by technological parameters (concentration of dopants in the channel and oxide thickness), which are shared among all devices on the chip and hopefully the whole wafer. Insertion of a depletion-mode device would introduce more steps, masks, etching etc., and would be simply expensive without gaining significant advantage over the enhancement-mode counterpart. For complementary logic you just don't need them.

JFETs are still sometimes used in the input stages of opamps due to their lower noise. As they require a good P-N juncion, it is probably difficult to shrink them to the dimensions used in modern IC technologies (a few tens of nanometers).

As a discrete part, I agree that I can't think of a single depletion-mode MOSFET and I have certainly never used it. As it would probably require negative voltage to bias it (or "more positive" than the supply rail), I think one hardly ever finds a good reason to use it instead of common e-mode MOSFET.