International Radiation Detectors, Inc.

Noise Characteristics

Spontaneous fluctuations of current and voltage in electronic devices are called noise. When superimposed to a given signal, noise determines the lowest limit below which no signal can be reliably measured or amplified. Thermal noise is the predominant noise source when IRD photodiodes are used without any external bias. When these are used with an external bias, shot noise will be the predominant noise source.

The thermal rms noise current, It, is given by

I_t=(4kTΔf/R)^.5 [A]

where K is the Boltzmann constant (1.3807 x 10^-23J/K), T is the absolute temperature [K], R is the shunt resistance [Ω], and Δf is the bandwidth of measurement [Hz]. For 1cm x 1cm area diode with 1 GΩ shunt resistance at 22°C the noise current will be 4.1 fA/(Hz)^.5. In the literature the thermal noise is also called Nyquist noise and Johnson noise.

When the diodes are used with external bias, the rms shot noise current, Is, is given by

I_s=(2q{I_d+I_ph}Δf)^.5 [A]

where q is the elemental electron charge, I_d is the dark current, I_ph is the photogenerated current and Δf is the bandwidth of the measuring equipment.

Thus, for a dark current value of 10 nA and photogenerated current of 100 nA the shot noise current will be 1.876 x 10^-13 A/Hz^.5