956 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



Samples, referred to as "bridges", have been cut from thin slabs of 

 single crystal germanium, by a technique devised by W. L. Bond, 

 often of a form shown in Fig. 4. Side arms for both the current and 

 the noise measuring electrodes have been found necessary to avoid 

 spurious noise at the electrodes. A large inductance in the bias circuit 

 greatly reduces the effect of any noise voltage generated at the bias 

 electrodes. The spurious noise power from this source is seldom more 

 than a few per cent of that being measured. It should be noted that the 

 contact area for the noise measuring electrodes should not be on a por- 

 tion of the specimen carrying bias current, otherwise spurious noise may 

 be generated at these electrodes. Typical dimensions for the straight 

 central filmanent of the bridge are 0.05 x 0.05 x 0.7 cm. The side arms 

 have sandblasted surfaces to suppress holes or electrons injected at the 

 electrodes. The central portion may be etched, sandblasted, or other- 

 wise treated at will. The enlarged circular areas are rhodium plated to 

 provide good contacts to each side arm. 



Measurements of the noise spectrum in such bridges with several dif- 

 ferent etching treatments and with sandblasted surfaces are charac- 

 terized by the 1/f spectrum over a wide frequency range.* Fairly ex- 

 tensive measurements have been made in the audio frequency range, 

 and a few covering the range from 20 cycles to 1 megacycle. A typical 

 spectrum is shown in Fig. 5. 



The current dependence of the noise is shown in Fig. 6 for a number 

 of samples, mostly n-type, one p-type, and with various resistivities. 

 The outstanding feature is that noise voltage always increases with dc 

 bias voltage. In many cases there is direct proportionality at the lower 

 bias values, increasing to a square law at higher biases. There are some 



axj 



LENGTH 

 .a BOUT 7 MM 



TO NOISE 

 MEASURING 

 AMPLIFIER 



Fig. 4 — Filament with side arms cut out of a single crystal of germanium. 



* Departures from the 1/f spectrum at frequencies of the order of 100 kilo- 

 cycles and above were first discovered by G. B. Herzog and A. Van der Ziel. 

 See Reference 13. 



