Variation of L.F. noise figure of a junction transistor

Some results of measurement of l.f. noise component over a bandwidth of 300 c/s centred about a mid-band frequency channel of 1000 c/s are reported for a number of commercial transistors. Investigations cover a range of collector current (Ic) 0.3¿l.6 mA and a temperature range ¿20° to + 45°C. It is found that the noise increases with increasing values of Ic and decreasing values of temperature. Simultaneous measurement of lifetime ¿ of minority carriers in the base region shows that both 1/¿ and the l.f. noise vary in the same manner with temperature. Results are discussed in the light of the current ideas regarding the origin of 1.f. noise in semiconductor junction. It is shown that the major part of such noise in the transistors investigated may be attributed to surface recombination noise. It is tentatively suggested that the spectral distribution function ¿(f) of surface recombination noise varies exponentially as the surface recombination velocity. Further, the value of ¿(f) should, in general, depend on the current Ic and when this is taken into account the noise should vary as the square of Ic For a given Ic-value the l.f. noise figure depends very much on the functional relationship between ¿(f) and Ic.