Physical Principles Involved in Transistor Action* 



By J. BARDEEN and W. H. BRATTAIN 



The transitor in the form described herein consists of two-point contact elec- 

 trodes, called emitter and collector, placed in close proximity on the upper face 

 of a small block of germanium. The base electrode, the third element of the 

 triode, is a large area low resistance contact on the lower face. Each point 

 contact has characteristics similar to those of the high-back-voltage rectifier. 

 When suitable d-c. bias potentials are applied, the device may be used to am- 

 plify a-c. signals. A signal introduced between the emitter and base appears m 

 amplified form between collector and base. The emitter is biased in the positive 

 direction, which is that of easy flow. A larger negative or reverse voltage is 

 applied to the collector. Transistor action depends on the fact that electrons in 

 semi-conductors can carrv current in two different ways : by excess or conduc- 

 tion electrons and by defect "electrons" or holes. The germanium used is n-type, 

 i.e. the carriers are conduction electrons. Current from the emitter is composed 

 in large part of holes, i.e. of carriers of opposite sign to those normally in excess 

 in the body of the block. The holes are attracted by the field of the collector 

 current, so that a large part of the emitter current, introduced at low impedance, 

 flows into the collector circuit and through a high-impedance load. There is a. 

 voltage gain and a power gain of an input signal. There may be current ampli- 

 fication as well. 



The influence of the emitter current, /,, on collector current, h, is expressed in 

 terms of a current multiplication factor, a, which gives the rate of change of h 

 with respect to h at constant collector voltage. Values of a in tv-pical units 

 range from about 1 to 3. It is shown in a general way how a depends on bias 

 voltages, frequency, temperature, and electrode spacing. There is an influence 

 of collector current on emitter current in the nature of a positive feedback which, 

 under some operating conditions, may lead to instability. 



The wav the concentrations and mobilities of electrons and holes in germa- 

 nium depend on impurities and on temperature is described briefly. The theory 

 of germanium point contact rectifiers is discussed in terms of the Mott-Schottky 

 theorv. The barrier laver is such as to raise the levels of the filled band to a 

 position close to the Fermi level at the surface, giving an inversion layer of p-type 

 or defect conductivity. There is considerable evidence that the barrier layer is 

 intrinsic and occurs at the free surface, independent of a metal contact. Poten- 

 tial probe tests on some surfaces indicate considerable surface conductivity 

 which is attributed to the p-type layer. All surfaces tested show an excess 

 conductivity in the vicinity of the point contact which increases with forward 

 current and' is attributed to a flow of holes into the body of the germanium, the 

 space charge of the holes being compensated by electrons. It is shown why such 

 a flow is to be expected for the type of barrier layer which exists in germanium, 

 and that this flow accounts for the large currents observed in the forward direc- 

 tion. In the transistor, holes may flow from the emitter to the collector either 

 in the surface laver or through the body of the germanium. Estimates are 

 made of the field produced by the collector current, of the transit time for holes, 

 of the space charge produced by holes flowing into the collector, and of the feed- 

 back resistance which gives the influence of collector current on emitter current. 

 These calculations confirm the general picture given of transistor action. 



I — Introduction 



T 



HE transistor, a semi-conductor triode which in its present form uses a 

 small block of germanium as the basic element, has been described briefly 



*This paper appears also in the Physical Review, April 15, 1949. 



239 



