Alloyed Junction Avalanche Transistors

It has been shown recently that reverse biased silicon and germanium junctions break clown as a result of a multiplicative process which is analogous to multiplicative breakdown in a g a s . 1 , 2 ' 3 Minority carriers which are thermally or otherwise generated diffuse to the high field region of the reverse biased junction, where they are accelerated, producing hole-electron pairs by collision with the atoms of the crystal lattice. As in the case of ionization of a gas, the rate of pair production is dependent upon the electric field distribution; however, it has been found that in the case of semiconductors holes and electrons are comparably effective in producing additional current carriers. The holes and 883 884 T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , SEPTEMBER 1955 electrons which are produced by collision may themselves produce additional pairs, and so on, resulting in an avalanche. As the junction reverse bias is increased, the space charge layer widens, the maximum electric field increases, and the total rate of pair production increases. At a particular value of reverse voltage, called the breakdown voltage, the multiplication of the minority current carriers becomes essentially infinite. The current then increases very rapidly, being limited only by the external circuit resistance. In semiconductors the number of minority carriers which are multiplied may be augmented and controlled by means of an emitter junction which is placed in close proximity to the reverse-biased, multiplying collector junction.