Electronic – Is the MOSFET source always connected to bulk

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I'm trying to understand all the basic connections of a MOSFET. These questions may seem very basic but I have flipped every single textbook I own and googled every single website and never found a clear cut answer to the following questions. I would appreciate all the help I can get.

1)Why is the MOSFET source connected to bulk? Is there any specific situation where the bulk isn't connected to the source? From what I have gathered, the bulk is connected to the source to establish a stable threshold voltage. If so, could we connect the bulk of a PMOS to ground and NMOS to Vdd? What difference would it make?

2)A MOSFET can function as a diode when the gate is connected to the drain. I don't really understand the physical operation behind this. However, this does make sense mathematically, as the MOSFET will always be in saturation. The current is given by, uCox(W/2L)(Vgs-Vth)^2, which means that the current graph will be quadratic which is ALMOST but not identical as the exponential equation of that of a diode. My question is, how does the performance of a MOSFET connected diode compare to that of a PN diode?

3)Of late I have seen many Rectifier circuits in IC design use MOSFET connected diodes instead of PN diodes. Is there a specific reason for this? Wouldn't a MOSFET have a relatively large area overhead when compared to using PN diode in IC design?

Your help is very much appreciated. Thanksss!!!

Best Answer

The Source and Bulk do not have to be connected.

In power devices, and especially in discrete transistors, the S & B are built very close together and shorted. This improves the breakdown voltage performance of the transistor.

In an IC, in some CMOS processes, the B of NMOS devices is always substrate (ground), and so in structures such as NOR gates which have 2 NMOS in series, the 2nd NMOS doesn't have the S=B.

Generally performance (gm, current) is better with S=B, but some technologies don't allow the B to be separated from the substrate for NMOS devices. PMOS devices in an IC generally can have separate S & B connections.

If you connected the B of PMOS to GND, you would have a parasitic diode from S to B (GND), and so your supply would be shorted (unless you wanted to run on a very low supply voltage of << 0.6 V). Some very low voltage circuits do use this technique.

A MOSFET connected as a diode will generally have worse performance than a PN junction in terms of the 'sharpness' of the curve. However, FETs with low threshold voltage (say 0.4 V or lower) will turn on at a lower voltage than a diode will, and this can be useful in low voltage circuits. For the same reason that the B is always substrate in some CMOS ICs, there isn't the flexibility to use a PN junction as a diode in all circuit configurations. If the PN junction of a PMOS is used (P = Source, N = Bulk), then there are some additional parasitics that need to be considered that make this not useful generally.