question about depletion MOS

[overmars]

Hello!
I found in many applications, the depletion NMOS are used to be as clamp devices so that the power variation would not affect the ckt power supply. It is likely that the gate of this dep mos connect to GND and drain to VDD, voltage applied on source can be stable.
But can anyone tell me how does this Depletion NMOS function to realise this?
thank you.

 

[protonixs]

There are depletion-mode MOSFET devices, which are less commonly used than the standard enhancement-mode devices already described. These are MOSFET devices that are doped so that a channel exists even with zero voltage from gate to source. In order to control the channel, a negative voltage is applied to the gate (for an n-channel device), depleting the channel, which reduces the current flow through the device. In essence, the depletion-mode device is equivalent to a normally closed (on) switch, while the enhancement-mode device is equivalent to a normally open (off) switch.[2]

Due to their low noise figure in the RF region, and better gain, these devices are often preferred to bipolars in RF front-ends such as in TV sets. Depletion-mode MOSFET families include BF 960 by Siemens and BF 980 by Philips (dated 1980s), whose derivatives still AGC and RF mixers in front ends.

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