MOS capacitor transient response

[hasib_eee]

Hello.

Suppose we have a MOS capacitor (n-substrate). When we give negative voltage the majority carrier (electrons) are repelled from the oxide-substrate interface and move towards the bulk.

Say the bias is -0.3 volts. The substrate is depleted and in steady-state. Suddenly if the bias is changed from -0.3 to -0.6, I know that the width of depletion region will increase definitely.

My question is how long will it take to reach the new steady state? Does it depends on some form of life time or response time? I found typical example in textbooks for optical step excitation where they showed (1-exp(t/tao)) like shape to reach steady-state.

How can I model the movement of majority carriers (electrons) under step bias?

Please give me some information.

Thanks.

M. Satter.

 

[ranger]

Hi hasib_eee,

I think I may have found an article that answers (to a certain degree) your question. Among the test conditions were step like bias voltages. I assume that your school gives you free access to various scientific journals. Here is the abstract:

We present an experimental study of charge transfer in polymer thin-film field-effect devices. The rearrangement of the charge-carrier density in the transistor channel upon a gate-voltage swing has been monitored in real time and space by means of noncontact scanning potentiometry. The experimental results are in excellent agreement with a simple theory, in which the charging currents are assumed to be driven by drift in the self-induced electric field. The charge density exponentially approaches its final value with a time constant given by L²/µ|Vg|pi², where L is the characteristic device dimension, µ the field-effect mobility, and Vg the final gate voltage.

See:
L. Bürgi, R. H. Friend, and H. Sirringhaus. Applied Physics Letters; 3/3/2003, Vol. 82 Issue 9, p1482, 3p