POINT-CONTACT TRANSISTOR SURFACE EFFECTS 783 



The other anomalous collector characteristics considered by Miller 

 have their origin in the relation between the transport factor and the 

 properties of the emitter at various operating conditions. In view of the 

 relations existing between the occurrence of these anomalies and the 

 Ico of the collector contact, there is some justification for classification 

 of these contacts as "over-formed." 



3. PROPERTIES OF UNFORMED POINT CONTACTS 



3.1 Physical Properties oj Metal- Semiconductor Contacts 



The classical ideas on the nature of the rectifying metal-semiconductor 

 contact have undergone substantial revision since the consideration by 

 Bardeen of the importance of surface states and the work on the point 

 contact transistory by Bardeen and Brattain. According to Bardeen's 

 model, the nature of the space charge layer at such a contact is to be 

 considered largely independent of the metal used for contact, and is pri- 

 maril}^ dependent on the charge residing in localized states at the ger- 

 manium surface. Thus the rectifying properties of the metal semiconduc- 

 tor contact in air are expected to be largely independent of the work 

 function of the contact metal. 



The question of the exact nature of the surface charges is not yet read- 

 ily answerable. Charges may arise which consist of electrons and holes 

 residing in surface states of the type proposed by Tamm.^' On the other 

 hand, other surface charges may arise as a result of adsorbed impurity 

 ions, or from adsorbed atoms or molecules having electrical dipole mo- 

 ments. Brattain and Bardeen^ have shown that the space charge layer 

 is dependent on the surrounding ambient and have indicated that charge 

 may reside on the outer surface of a film (presumably an oxide laj^er) 

 at the germanium surface as well as in surface states of the type men- 

 tioned above, which are presumably those responsible for surface re- 

 combination processes. 



Thus, it is the surface charge on the semiconductor, rather than the 

 nature of the metal, which primarily determines the nature of the po- 

 tential barrier which exists at a metal semiconductor junction. 



A schematic electron energy diagram for the contact between a metal 

 and an 7i-type semiconductor is shown in Fig. 9. The potential barrier 

 ^0 , and the nature of the space charge layer in the semiconductor are 

 determined by the surface charge system and the bulk properties of 

 the semiconductor. In turn, the surface charge system is dependent upon 

 such factors as the ambient at the germanium surface and the chemical 

 history of the surface. 



