204 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1952 



mass ratio 1. An application of the resulting formula (43) to experimental mate- 

 rial has been published earHer. 



Evidence for the Noncuhic High Temperature Phase of BaTiOs . E. A. 

 WoodK Letter to the Editor. J. Chem. Phys., 19, p. 976, July, 1951. 



Seven-League Oscillator* F. B. Anderson^ Proc. I.R.E., 39, pp. 881- 

 890, Aug., 1951. 



A bridge-type RC oscillator is described wliich is continuously adjustable 

 over a frequency range of 20 cps to 3 mc in one sweep of a two-gang Unear 

 potentiometer control. Tracking requirements of the two-gang control are not 

 severe. The output is available in four phases, and the frequency is an approxi- 

 mately logarithmic function of the linear potentiometer setting. Practical hmits 

 of the frequency range are tentatively 0.01 cps and 10 mc. Accuracy of setting 

 of the order of one per cent is attainable with ordinary components. Frequency 

 stability is of the order of 2 per cent per db of tube gain variation. 



Semi-Conductor Surface Phenomena* W. H. BrattainK Semi-Con- 

 ducting Materials, H. K. Henisch, ecL, pp. 37-46. Proceedings of a con- 

 ference held at the University of Reading (July 10-15, 1950) London, 

 Butterworths, 1951. 



Developments in the understanding and interpretation of phenomena occur- 

 ring at the surface of a semi-conductor are reviewed. The development starts 

 with the Mott-Schottky theory of the space charge layer. Bardeen's concept of 

 a space charge layer due to 'surface states' explained the independence of recti- 

 fication on contact potential and Meyerhoff's small values for the contact poten- 

 tial between n- and p-tj'pe silicon. Shockley and Brattain observed that this 

 contact potential difference increased with impurity concentration. Illumination 

 of a silicon surface produced hole and electron pairs in the space charge layer. 

 The potential of the surface changed until the photocurrent was balanced by a 

 conduction current. The relation between photo-current and potential change 

 was of the same form as a forward characteristic for a rectifying contact. In an 

 experiment similar to Becquerel's, using water as an electrolyte, the surface 

 may be biased in the reverse direction. Wlien so biased the response to modu- 

 lated light gives the differential resistance of the space charge layer. This re- 

 sistance increases rapidly with reverse bias and the time constant of the layer 

 increases, both agreeing qualitatively with theory. This experimental method 

 of measuring changes in surface potential caused by illumination permits deter- 

 mination of the properties of the space charge layer at the free surface of a semi- 

 conductor. 



The Calculation of Traveling-Wave-Tube Gain* C. C. Cutler^. Proc. 

 T.R.E., 39, pp. 914-917, Aug., 1951. 



* A reprint of this article may be obtained on request. 

 1 Bell Tel. Labs. 



