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I would like to know the difference between the transfer gate (TX) which is used in 4T CMOS Active Pixel Sensor (APS) as mentioned in the below image and a normal NMOS switch.

enter image description here

As far as I know, the transfer gate is used for transferring the accumulated charge from the pinned photodiode to the floating diffusion. But, this looks like a switch or an NMOS switch to me, where the gate of NMOS is switched on and the current flows in the channel. However, in the books, I see that the transfer gate is manufactured in relation with the pinned photodiode. Also, this concept of transfer gate was carried over from CCDs, which I noted down, but it was not enough to understand why this transfer gate is not just an NMOS switch.

Any guide/hint on the above would be helpful. Please also correct me if my understanding is wrong on the above and indeed if it can be seen as an NMOS switch. Thank you.

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    \$\begingroup\$ What do you mean by "manufactured in relation with..."? How exactly do you think this transistor is different from an "NMOS switch"? I don't understand what you are asking. \$\endgroup\$ Mar 7, 2020 at 14:34
  • \$\begingroup\$ @ElliotAlderson It's about the doping concentration during fabrication - its doped in such a way that complete charge transfer takes place. If it's a simple nmos switch, it should have been quoted in literature, which I couldn't find any. \$\endgroup\$
    – sundar
    Mar 7, 2020 at 17:39
  • \$\begingroup\$ @sundar Have you checked out this lecture: isl.stanford.edu/~abbas/ee392b/lect04.pdf#page=26 I'm not quiet sure it has your answer but it does go over the "photogate" transistor you are wondering about. \$\endgroup\$
    – eeintech
    Mar 9, 2020 at 0:01
  • \$\begingroup\$ @Cisco25 Well, yeah, in every literature, they claim it's a transfer gate but it looks like a simple Mos diode to me, \$\endgroup\$
    – sundar
    Mar 9, 2020 at 8:07

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The Transfer Gate (TX) is quite similiar with a NMOS. But TX needs to connect a Pinned photo diode which is buried under the Si surface.

So the TX drain side looks the same as a NMOS drain. But the TX source side is different from a NMOS source.

The TX source side is separate from the Photo diode N-region(N+). This Photo diode N-region(N+) is actually buried under the TX.

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