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Can a NMOS be used with a substrate terminal?
Some NMOS’s come with a substrate/bulk/body terminal so the “Source Voltage” referred to in literature then Substrate Voltage. Say then we short the gate to the drain and have the substrate take the input. We would then use this in the same NMOS V_DS bias. The higher voltage will go with drain and lower voltage to source.
When does the NMOS go active in a MOSFET?
Since N-channel MOSFETs need a gate-to-substrate positive differential relative to voltage threshold to go active, then in this configuration, when we pull the substrate terminal, relative to the threshold, the NMOS activates. Leave it within threshold or negative (above the gate), it doesn’t go active.
How does MOSFET connect body to source or ground?
In most applications this body terminal must be kept at a potential that guarantees that the drain-body and source-body diodes are kept reverse-biased so that the device can function as a FET. In most cases this can be guaranteed by connecting the body junction to the source, this creates a reverse-polarized drain-source diode.
How are NMOS and PMOS terminals the same?
First, there are diodes b/w source and drain leading to the substrate. In an NMOS they are 99.9% of the time tied to the most negative voltage, and for PMOS the most positive voltage. This prevents them from ever turning on unless somehow some signal exceeds the rails.
When do you apply voltage to a NMOS FET?
When you apply a voltage to the gate of an NMOS FET that is greater than the threshold voltage V t, a channel of electrons is formed under the gate. That channel connects the drain and source so that when you apply a voltage V d s, a current flows from between these terminals.
What is the effect of drain on NMOS?
This is due to an effect called channel length modulation. As V d s continues to increase, it cancels the inverted channel even further from the drain, leading to the channel shortening. This reduces the channel resistance (resistance is proportional to length) leading to higher current flow.
Can a NMOS be turned off by a PMOS?
In an NMOS they are 99.9% of the time tied to the most negative voltage, and for PMOS the most positive voltage. This prevents them from ever turning on unless somehow some signal exceeds the rails. So you would have a hard time getting this system to work, because to turn off the FET you would need to raise the body voltage (like a PMOS).
What is the substrate rule in PMOS circuit?
The junction would likely be burned out. The PMOS substrate rule: The substrate (body) should be connected to the highest voltage in the circuit – usually the positive power supply. Then the source and drain must both be at the same or lower voltages, and it will be impossible to forward-bias the diodes.
How does a negativevoltage work in a PMOS?
T), current can flow by applying a negativevoltage at the drain. This creates an electric field that will push holes from the source, through the inversion layer channel, and on into the drain. The moving holes represent a drain current flowing from source to drain. (Opposite the current for an NMOS!)