Re: PMOS device reversed

On 3 Jan 2006 07:42:20 -0800, "QQ" <q_q_404@xxxxxxxxxxx> wrote:

>Hi Jim,
>
>Thanks for your reply. Through my reading up on the net I was coming to
>the same conclusion but the information is quite hard to find. Would it
>help if this device was a "high voltage" pmos with a N buried layer?
>Would the N buried layer effectively cut off the vertical pnp
>transistor? (I read this somewhere..)
>
>> PMOS devices that need to handle bi-directional currents usually have
>> their well (body) tied to +VDD.
>
>I dont understand. How will this help to cut off the vertical pnp?. My
>source is supposed to be connected to the highest potential already. Is
>it sufficient to connect body to source?
>
>Will this problem occur in the case on NMOS device as well?
>
>Thanks
>qq

You originally indicated a desire to tie body to drain (rather than
source), which would provide the possibility of forward bias.

However, if you were really asking, "Are source and drain
interchangeable?"

The answer is yes (generally, some HV devices have different source
and drain structures), PROVIDED body is always most positive.

Draw some pictures to envision how the junctions are arranged.

>
>
>
>
>Jim Thompson wrote:
>> On 3 Jan 2006 00:18:31 -0800, "QQ" <q_q_404@xxxxxxxxxxx> wrote:
>>
>> >Thanks Graham, I was concerned about the same but I am not sure if it
>> >will or will not turn on any parasitic diodes given that all devices
>> >are sharing a common p-substrate. Any specific idea on what parasitics
>> >may be prone to turn on?
>> >
>> >Thanks
>> >QQ
>>
>> The "common p-substrate" will be at the body potential of all NMOS
>> devices... desirably the most negative potential on-chip.
>>
>> PMOS devices are built in an N-well, thus forward biasing the body
>> diode will activate a vertical PNP.
>>
>> If enough current flows to regenerate, this is called LATCH-UP.
>>
>> PMOS devices that need to handle bi-directional currents usually have
>> their well (body) tied to +VDD.
>>
>> ...Jim Thompson


...Jim Thompson
--
| James E.Thompson, P.E. | mens |
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| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
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