PRIXCIP/.ES OF TRAXSIS'IOR ACTION 241 



The first experiments performed to measure this effect indicated that most 

 of the induced charge was not mobile. This result, taken along with other 

 unexplained phenomena such as the small contact j)otential difference be- 

 tween n- and p- type silicon* and the independence of the rectifying proper- 

 ties of the point contact rectifier on the work function of the metal point, 

 led one of the authors to an explanation in terms of surface states.^ This 

 work led to the concept that space charge barrier layers may be present at 

 the free surfaces of semi-conductors such as germanium and silicon, inde- 

 pendent of a metal contact. Two experiments immediately suggested 

 were to measure the dependence of contact potential on impurity concen- 

 tration^'^ and to measure the change of contact potential on illuminating 

 the surface with light." Both of these e.xperiments were successful and 

 confirmed the theory-. It was while studying the latter effect with a silicon 

 surface immersed in a liquid that it was found that the density of surface 

 charges and the field in the space charge region could be varied by applying 

 a potential across an electrolyte in contact with the silicon surface. '^ While 

 studying the effect of field applied by an electrolyte on the current voltage 

 characteristic of a high-back-voltage germanium rectifier, the authors were 

 led to the concept that a portion of the current was being carried by holes 

 flowing near the surface. Upon replacing the electrolyte with a metal 

 contact transistor action was discovered. 



The germanium used in the transistor is an n-t}^e or excess semi-conductor 

 with a resistivity of the order of 10-ohm cm, and is the same as the material 

 used in high-back-voltage germanium rectifiers.'^ All of the material we 

 have used was prepared by J. C. Scaff and H. C. Theuerer of the metallurgi- 

 cal group of the Laboratories. 



While different metals may be used for the contact points, most work has 

 been done with phosphor bronze points. The spring contacts are made 

 with wire from .002 to .005" in diameter. The ends are cut in the form of a 

 wedge so that the two contacts can be placed close together. The actual 

 contact area is probably no more than about 10~^ cm-. 



The treatment of the germanium surface is similar to that used m making 

 high-back-voltage rectifiers.'* The surface is ground flat and then etched. 

 In some cases special additional treatments such as anodizing the surface 

 or oxidation at 500°C have been used. The oxide films formed in these 

 processes wash off easily and contact is made to the germanium surface. 



The circuit of Fig. 1 shows how the transistor may be used to amplify 

 a small a-c. signal. The emitter is biased in the forward (positive) direc- 

 tion so that a small d-c. current, of the order of 1 ma, flows into the ger- 

 manium block. The collector is biased in the reverse (negative) direction 

 with a higher voltage so that a d-c. current of a few milliamperes flows 

 out through the collector point and through the load circuit. It is found 



