Forward / Reverse body biasing definitions?

[deltaDG]

I’m studying body effect and the forward body-bias technique in reducing the threshold voltage of a MOSFET. But I’m very confused by what it means to “forward body bias” a MOSFET. I search through the literature and it seems that most authors use the terms “Vbs” and “Vsb” interchangeably, without regards to the importance of sign convention, which confuses me even more when I try to put some numbers into equations.

These are my understandings. Can someone tell me whether I’m right or wrong?

1. When we talk about voltages in MOSFET, they are always referenced to the Source terminal, where the majority carriers originate from, right?

2. When you “forward body bias” a MOSFET, the voltage at the Body/Bulk terminal is higher than the Source terminal? So Vbulk – Vsource = Vbs is positive?

3. “Forward biasing the body lowers the threshold voltage…” If I apply the Shichman-Hodges equation from Wikipedia,

Vt=Vt0+γ(√(Vsb+2ΦF)-√(2ΦF))

Vsb (Vsource – Vbulk) actually is a negative value?

 

[metalgarri]

Hi,
Imagine to have an NMOS. The bulk is p-doped and S/D are n+ doped. In the bulk, the majority carriers are holes, that do not produce any current for a given Vds. On the other hand, in order to have a current. Vgs has to reach the so called threshold voltage. In this condition a depletion region is created, and at the same time under the oxide surface there is a channel made of electrons, whose concentration of charge(-) is equal to the concentration of dopants into the depletion region (called inversion region). The Vthreshold is the voltage at which it is possible to see this condition in the channel of a mosfet.

What happens if the Vbody increases in comparison to the Vsource?
The reverse bias of the body-source junction is reduced, so its depletion region is reduced as well, and this means that the number of dopants in the depletion region is less. Referring to the definition of Vth, this means that, in order to equate the number of dopants, less channel charge is needed. So, the final effect is to lower the Vth.

Let's take into account a source follower configuration. The Vth is increased because the body-source junction is more reverse-biased, and the depletion region is wider.

I hope this explaination could help (and I am sorry for my english ).

By the way, there is a very easy book that you can read, that is Physics of semiconductor device by Colinge. It is not so heavy but the concepts are well explained