To measure the current noise of the diodes under an applied bias an amplifier
designed by the Politecnico di Milano was used
[5]. This had a large bandwidth ( 
), high
gain ( 
), low noise
( 
) and an adjustable input bias offset. The amplifier had
a large input impedance (>1G 
)
and a low output impedance ( 
). This amplifier was used as an
operational amplifier with a feedback resistor Rf and
a feedback capacitance Cf. The output of the amplifier was passed through
the high impedance input of an AC coupled
oscilloscope and out through the 'scope's 
output. The 'scope thus
acted as a low noise voltage amplifier. The output of
the 'scope was sent to a HP signal analyser with input impedance.
The feedback resistor was chosen to maximize the gain of the amplifier (large
Rf),
minimize the background noise due to the thermal noise of the feedback
resistor (large Rf, see equation (8)) and to make sure that the
amplifier did not saturate (limits Rf).
Before the current noise density spectra of the diodes were measured the transfer function and the background noise of the amplifier were determined. The transfer function was required so that the measured voltage noise could be referred back to the input as a current noise. Background noise measurements were made to show that the noise contribution from the 'scope was negligible and that the overall background noise present in normal operation was close to the thermal noise of the feedback resistor.
Noise spectra were obtained for both diodes, at zero bias, under reverse bias, at the same leakage current for both diodes and under forward bias for the small diode at three leakage currents equivalent to those used for the reverse bias case.