# MOSFET operation

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#### Anagha_elex

##### New Member
In theory books while explaining the working of MOSFETS its written that a temporary channel is formed joining the two layer at Source and drain. this newly formed channel has a lower density of carriers.......

As per the concept it contradicts to the theory that In a bar of semi- conductor the density of carrier is always uniform i mean following the example of two water tanks connected wid a pipe can never hav diff water levels.

Can any one clarify abt this doubt of mine?

##### Banned
In a solid bar of semi conductor maybe. A mosfet is not a solid bar, there are multiple layers with different dopant concentrations.

#### Claude Abraham

##### Member
Please forget the water tanks and flow analogy. At the atomic level water flow cannot explain semiconductor physics. Quantum mechanics (QM) and statistical thermodynamics, as well as e/m field theory are needed to understand what is going on. Even then, nobody really understands QM. We just accept it.

The density of carriers is high in the drain (D) and source (S) regions. When the gate to source is biased positive (n-channel device) from an external source, an inversion layer is formed in between the D and S. The substrate is p-channel, but the inversion layer consists of electrons injected into the substrate by the external source. The substrate now has more electrons then holes, so that inversion is achieved and this conducting layer is n-channel.

By varying the value of gate to source voltage (the gate current is varying at the same time), the charge density in the substrate is varied so that signal amplification takes place. When the G-S is turned on hard, and kept on, the substrate electron density is maximized and the channel has minimum value of resistance, called Rdson. At lesser values of Vgs, the density of electrons in the channel is lesser. When Vgs is 0V, there are no external electrons being injected into the substrate and the channel reverts to p polarity. Hence there is no conduction.

Depending on the value of Vgs, the density of e- carriers in the channel can vary greatly. It doesn't have to equal the same value as the density in the D & S regions. Another example is the familiar bjt device. The base & emitter regions are adjacent. Yet the doping density, hence carrier density, differs for B & E.

A good semiconductor physics text, along with QM, e/m field, & statistical thermodynamics background will explain in detail what I've just covered in a sketchy manner. Does this help?

#### Anagha_elex

##### New Member
thanx... i think i see where i was confusing things......

#### Anagha_elex

##### New Member
I had another quey regarding High Freequency analysis of BJT amplifiers.....

While doing HF analysis we take coupling n bypass caps to be short....
Where as while analysing the RC phase-shift oscillator we take them as short but consider those in RC ckt. Coupling caps are usually of order uf n those in RC are of order nf. is tht the reason? n we also conider junction caps of order pf in HF. nf n pf considered in d same range?

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