Basic question about creation of inversion layer and the bias effect

[deepak242003]

Hello All,

I have to simple doubts:

1) creation of inversion layer: above particular Vgs holes are rejpelled away from gate jujnction and immobile Negative ions are formed.. and when Vgs is increased further, inversion layer is formed.. ( electrons ) where these electrons comes from???


2) Bias effect :
If substrate is baised lower, more holes are repelled so less voltage should be required as part of first stage is already done..then threshold voltage shuould decrease... but it is said that it increases... sumwhere I read bcoz of depletion layer....


Any explanation or supporting documents will be highly appreciated.

 

[timof]

Re: basic device Physics

Hello All,

I have to simple doubts:

1) creation of inversion layer: above particular Vgs holes are rejpelled away from gate jujnction and immobile Negative ions are formed.. and when Vgs is increased further, inversion layer is formed.. ( electrons ) where these electrons comes from???


2) Bias effect :
If substrate is baised lower, more holes are repelled so less voltage should be required as part of first stage is already done..then threshold voltage shuould decrease... but it is said that it increases... sumwhere I read bcoz of depletion layer....


Any explanation or supporting documents will be highly appreciated.

Deepak - these are excellent questions. The answers are:

1. in nMOSFET the electrons are (quickly) coming from n+ doped source and drain regions. In a MOS capacitor, there may be no n+ regions, so the electrons are generated by thermal generation mechanism in the depletion regions - the generation process is much slower (than drift), but that depends on temperature.

2. Now, for back biased p-type substrate (i.e. negative voltage on p-type region with respect to n+ region source (overlapping with the gate)), if the substrate is already in inversion, further increase of negative bias would extend (enlarge) the depletion region (between the inversion layer and neutral substrate). This increase of depletion layer is accompanied by a negative charge buildup on one side of the depletion region (negative ions from depleted acceptors on the neutral substrate side), and positive charge buildup on the inversion region - i.e. the concentration of electrons in the inversion layer would decrease. So, negative back bias of the substrate (with respect to n+ doped source) results in a decrease of inversion layer charge (electron density), which is equivalent to increase of Vt.