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.