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BELL SYSTEM TECHNICAL JOURNAL 



that the current in the collector circuit is sensitive to and may be controlled 

 by changes of current from the emitter. In fact, when the emitter current 

 is varied by changing the emitter voltage, keeping the collector voltage 

 constant, the change in collector current may be larger than the change in 

 emitter current. As the emitter is biased in the direction of easy flow, a 

 small a-c. voltage, and thus a small power input, is sufficient to vary the 

 emitter current. The collector is biased in the direction of high resistance 

 and may be matched to a high resistance load. The a-c. voltage and power 

 in the load circuit are much larger than those in the input. An overall 

 power gain of a factor of 100 (or 20 db) can be obtained in favorable cases. 

 Terminal characteristics of an experimental transistor^^ are illustrated in 

 Fig. 3, which shows how the current-voltage characteristic of the collector 

 is changed by the current flowing from the emitter. Transistor characteris- 

 tics, and the way they change with separation between the points, with 

 temperature, and with frequency, are discussed in Section II. 



T >^ Vp Vr ^ I. 



SIGNALffU 



COLLECTOR 



LOAD 



-BASE 



T 



Fig. 1— Schematic of transistor showing circuit for ampUfication of an_a-c. signal and 

 the conventional directions for current flow. Normally h and W are positive, h and Vc 

 negative. 



The explanation of the action of the transistor depends on the nature of 

 the current flowing from the emitter. It is well known that in semi-con- 

 ductors there are two ways by which the electrons can carry electricity 

 which differ in the signs of the effective mobile charges.'^ The negative 

 carriers are excess electrons which are free to move and are denoted by the 

 term conduction electrons or simply electrons. They have energies in 

 the conduction band of the crystal. The positive carriers are missing or 

 defect "electrons" and are denoted by the term "holes". They represent 

 unoccupied energy states in the uppermost normally tilled band of the 

 crystal. The conductivity is called n- or p-type depending on whether 

 the mobile charges normally in excess in the material under equilibrium 

 conditions are electrons (negative carriers) or holes (positive carriers). 

 The germanium used in the transistor is n-type with about 5 X 10^^ conduc- 

 tion electrons per c.c; or about one electron per 10^ atoms. Transistor ac- 

 tion depends on the fact that the current from the emitter is composed in 



