//()/./•: ISJECTIOX fX GERM AM! M 



.S37 



;itom, mav be so smuU because a liDle-elertroii i)air has (litVuully in satisfying:; 

 in the crystal the conditions somewhat analogous to conservation of energy 

 and momentum which hinder recombination of electrons and positive ions 

 in a gas discharge. Thus it has been pointed out that a hole-electron pair will 

 have a lowest energy state in which the two current carriers behave some- 

 thing like the proton and electron of a hydrogen atom/' Such a bound pair 

 are called an excitoii and the energy given up by their recombination is the 

 "exciton energv." In order to recombine they must radiate this energy in 



10 20 30 40 50 60 70 80 90 100 110 120 



TRANSIT TIME IN MICROSECONDS 



Fig. 7 — Tlie decay of injected holes in a sample of «-iype germanium. 



the form of a light quanta (photon) or a quantum of thermal vibration of the 

 crystal lattice (phonon). The recombination time for the photon recombina- 

 tion process can be estimated from the optical constants for germanium 

 and the theory of radiation density using the principle of detailed balancing, 

 which states that under equilibrium conditions the production of hole 

 electron pairs by photon absorption equals the rate of recombination with 

 photon emission; the lifetime obtained in this way is about 1 second at room 

 temperature indicating that the photon process is unimportant.^^ As has 

 been pointed out by A. \V. Lawson,'" the highest energy jihonon will have 



