vbulk

Vbulk, often written vbulk, is a term used in semiconductor device modeling to denote the electric potential of the bulk (substrate) node of a transistor or diode. It represents the voltage of the substrate with respect to the circuit reference and affects device behavior through the body effect, junction depletion, and leakage currents. In MOSFET models, vbulk is the bulk node that may be tied to ground or biased independently to implement body biasing.

The body effect describes how the threshold voltage of a transistor depends on the source-to-bulk voltage, Vsb. When the bulk and source are at different potentials, the depletion region at the source-substrate junction changes, altering the threshold. A common expression for the threshold shift is Vth = Vth0 + γ(√(|φf| + Vsb) − √|φf|), where γ is the body-effect coefficient and φf is the Fermi potential. Thus, vbulk plays a central role in predicting and controlling device behavior, especially in analog design and process variations.

In circuit design practice, vbulk is often tied to the source to minimize body bias or is deliberately biased to achieve a desired Vth through body-biased operation. Some processes provide a separate bulk contact to enable independent biasing, while others require the bulk to be fixed during fabrication. In simulations, vbulk is treated as a node voltage and is solved alongside other circuit variables; accurate modeling may require including its effects on leakage and subthreshold conduction.

See also: body bias, MOSFET, SPICE, semiconductor device physics.