Input capacitance scaling related to short-channel noise phenomena in MOSFET´s

In optical (charge) amplifier design, it is common practice, to size the input MOSFET so that the amplifier´s input capacitance is approximately equal to the value of the photodiode capacitance. When plotted versus capacitance, the input equivalent noise current reaches its minimum value, for a given drain bias current, in a curve widely known and characterized as a shallow one. For high bit rate photocurrent amplifiers, which employ short-channel MOSFET´s as the input device, the observed sensitivity degradation is due to the increased input referred noise attributed to the MOSFET´s short-channel operation. It is shown here that for short-channel MOSFET´s the electron warming in the channel, the voltage fluctuations due to the gate polysilicon resistance, and the induced thermal noise at the gate, lead to a considerably lower value for the optimum input capacitance. In this case, the noise power versus the capacitance curve becomes steeper and the minimum is more prominent