bulk terminal

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Posted by: cree_sha2002 6/17/2004 12:05:36 AM Category: CMOS
Questions posted: 4 Comments Posted: 0

Why bulk terminal of nmos is connected to vss and pmos to vdd?

Posted by: unsubscribed 5/5/2008 12:48:30 AM

why source of a pmos connected to VDD and source of nmos to VSS?

Posted by: ram_vlsi 7/9/2004 1:58:44 AM
Comments Posted:29 Questions Posted:13

Hi anonymous,
Bulk terminal of NMOS ( PMOS ) is connected to GND ( VDD ) to make sure that Vsb = 0.
This is because NMOS (PMOS) transistors source is connected to GND ( VDD ).

This is done to avoid effects of BODY EFFECT,variation of Vt with Vsb.

Further discussion:

( But causes prob when there is astack of transistors ( NMOS or PMOS ) as their sources are no longer at GND ( NMOS ) or VDD ( PMOS ). they will be at some other potential and the bulks will be clamped to VDD or GND resulting in threshold voltage drift. See Weste chap.2

It is better to connect BULK terminal to source terminal if this is not desired but leads to capacitance problems. If confused see Allen,Hollberg book ANALOG CMOS INTEGRATED CIRCUITS ).
bye
RAM :-)

Posted by: vikas 6/17/2004 1:24:55 AM
Comments Posted:54 Questions Posted:2

Channel is tappered in between source and drain because the body of device is connected with sourse which is further connected at ground potential, so vgs( Vg-Vs= 5v-0v=5v)at source is equal to that of gate voltage and hence the electric field produced by gate voltage is able to collect the electrons from substrate. But if we go to drain side, drain is at some positive potential so vg-vd( voltage at gate - voltage at drain)is some what less and the electric feld produced by gate voltage is somewhat weak. that is why less no. of electrons will come in taht region. We can broadly speak that the drain terminal and substrate make a pn junction doide which is reverse biased adn the created depletion will inhibit the electrons to come in that region.

Posted by: vikas 6/17/2004 1:15:47 AM
Comments Posted:54 Questions Posted:2

Why channel of MOSFET between source and drain is tappered ?

vikas from bhiwani india

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