DIGITAL PRESSURE TRANSDUCER STUDY 



A. E. SNYDER and A. D' ONOFRIO 



Pratt and Whitney Company, Inc. 



West Hartford, Connecticut 



ABSTRACT 



The tunnel diode has a characteristic per- 

 formance curve which can he utilized for under- 

 water transducer applications. One very impor- 

 tant application of this component would be as a 

 pressure transducer providing a direct digital 

 output reading. A tunnel diode and appropriate 

 electronic circuitry has been examined and evalu- 

 ated for this purpose. The design, analysis, 

 test and evaluation results of this study are 

 presented. 



INTRODUCTION 



The discovery of the tunneling effect in 

 heavily doped P-N junctions by Esaki in 1958 

 has made available a component which exhibits 

 characteristics that are useful in areas ranging 

 from computer logic to communications . A great 

 many papers have been published since 1958 

 regarding tunnel diode characteristics and appli- 

 cations. 



This paper will describe an investigation, 

 conducted in the Research Laboratories of the 

 Pratt and Whitney Company to evaluate a digital 

 output tunnel diode transducer for oceanographic 

 pressure measurement. The possibility that 

 future oceanographic instrument systems may be 

 computer oriented establishes the requirement for 

 a digital output pressure transducer capable of 

 measuring pressures to 15,000 psi with an accuracy 

 of +0.5$> of full scale. . The approach taken 

 during this investigation has been to use the 

 tunnel diode in a unique hybrid oscillator cir- 

 cuit rather than in the conventional amplifier 

 or oscillator modes. 



TUNNEL DIODE AS A PRESSURE TRANSDUCER 



The reported works of Mason and Sikorski and 

 Andreatch3 demonstrated clearly that the charac- 

 teristic I-E curve of the tunnel diode is pres- 

 sure sensitive. Curve "A" of Fig. 1 represents 

 the I-E characteristics of a germanium tunnel 

 diode at atmospheric pressure. The peak current 

 and voltage are designated by I p and Ep respec- 

 tively. The negative conductance, -gd, is the 

 slope of the I-E tunnel diode curve between I_ 



MILLIVOLTS 



Fig. 1. Tunnel diode characteristic curve. 



and I v where I v is the valley current and E^. is 

 the valley voltage. Germanium differs from sili- 

 con tunnel diodes in that the peak current 

 decreases with pressure. 3 The change in the I-E 

 curve, represented by curves "B" and "C", results 

 from the application of hydrostatic pressure. 

 The applied pressure stresses the semiconductor, 

 affecting the energy gap and effective mass 

 (ratio of effective mass of the electron to its 

 mass in free space) which are related to the tun- 

 neling probability. 2 The tunneling probability 

 is related to the current through the junction 

 resulting in the change observed in the I-E curve. 

 Mitchell^ has derived the tunnel diode curve 

 utilizing quantum mechanics and the energy level 

 relationships . 



Superior numbers refer to similarly numbered references at the end of this paper. 



189 



