derive the current equation for an enhancement mode nmos transistor in saturation. how does the equation get modified for pmos transistor? is the same equation valid for depletion transistors?
Answers
Explanation:
In our previous tutorial about FET amplifiers, we saw that simple single stage amplifiers can be made using junction field effect transistors, or JFET’s. But there are other types of field effect transistors available which can be used to construct and amplifier, and in this tutorial we will look at the MOSFET Amplifier.
Metal Oxide Semiconductor Field Effect Transistor, or MOSFET for short, is an excellent choice for small signal linear amplifiers as their input impedance is extremely high making them easy to bias. But for a mosfet to produce linear amplification, it has to operate in its saturation region, unlike the Bipolar Junction Transistor. But just like the BJT, it too needs to be biased around a centrally fixed Q-point.
mosfet amplifier
A Typical
MOSFET Transistor
MOSFETS conduct through a conductive region or path called “the channel”. We can make this conductive channel wider or smaller by applying a suitable gate potential. An electric field induced around the gate terminal by the application of this gate voltage affects the electrical characteristics of the channel, thus the name field-effect transistor.
In other words, we can control how the mosfet operates by creating or “enhancing” its conductive channel between the source and drain regions producing a type of mosfet commonly called an n-channel Enhancement-mode MOSFET, which simply means that unless we bias them positively on the gate (negatively for the p-channel), no channel current will flow.
There are large variations in the characteristics of different types of mosfets, and hence the biasing of a mosfet must be done individually. As with the bipolar transistor common emitter configuration, the common source mosfet amplifier needs to be biased at a suitable quiescent value. But first lets remind ourselves of the mosfets basic characteristics and configuration.
Enhancement N-channel MOSFET
enhancement n channel mosfet configuration
Note that the fundamental differences between a Bipolar Junction Transistor and a FET are that a BJT has terminals labelled Collector, Emitter and Base, while a MOSFET has terminals labelled Drain, Source and Gate respectively.
Also the MOSFET differs from the BJT in that there is no direct connection between the gate and channel, unlike the base-emitter junction of the BJT, as the metal gate electrode is electrically insulated from the conductive channel giving it the secondary name of Insulated Gate Field Effect Transistor, or IGFET.
We can see that for the n-channel MOSFET (NMOS) above the substrate semiconductor material is p-type, while the source and drain electrodes are n-type. The supply voltage will be positive. Biasing the gate terminal positive attracts electrons within the p-type semiconductor substrate under the gate region towards it.
This over abundance of free electrons within the p-type substrate causes a conductive channel to appear or grow as the electrical