ELECTRICAL MEASUREMENTS 177 



If a grid or screen of fine wires is placed between the cathode and the 

 plate (as in Fig. 13-2b) and this grid is made slightly positive or negative, 

 the small charge will interfere with the free movement of electrons 

 across the space. Variations in the grid voltage are reflected in the cur- 

 rent passing through the vacuum tube to the plate. The cathode-to-plate 

 voltage is much higher than the grid voltage, so any "signal" or variation 

 in grid voltage is thus amplified. The "triode" amplifier tube, with all 

 the variations and improvements which have been made, is responsible 

 almost by itself for our whole electronic world. Sometimes other elec- 

 trodes are introduced, as in tetrodes and pentodes, but usually these are 

 included to improve the performance of the basic triode. 



Semiconducting devices, or transistors, can perform some operations 

 similar to those performed by vacuum tubes. These solid state materials 

 differ only in degree from conducting materials (metals) or from in- 

 sulating materials. Crystals of germanium or silicon possess a definitely 

 ordered structure in which pairs of electrons are shared by adjacent 

 atoms. Traces of impurities may fit into the crystal structure but intro- 

 duce extra electrons which are not needed in the crystal bonds and thus 

 are free to migrate. Other impurities may fit the crystal pattern, but with 

 deficiencies of electrons or "holes." Such "slightly impure" crystals 

 become the basis of transistors. A crystal of germanium (4 valence elec- 

 trons) with a trace of arsenic or phosphorus (5 valence electrons) pos- 

 sesses an excess of electrons and is called N-type (for negative) material. 

 A similar crystal containing traces of gallium or indium (3 valence 

 electrons) would be called P-type from its excess of "holes" or posi- 

 tive charges. If a piece of N-type and a piece of P-type crystal are 

 joined and then a voltage is applied, electrons flow in one direction 

 more easily than in the other. "Holes" flow more easily in the opposite 

 direction. This N-P junction becomes a rectifier (see Fig. 13-3a). 



A combination of N-P-N layers is a junction transistor, which can 

 best be explained with the help of Fig. 13-3b. The size of the current 

 flowing across the N-P and P-N junctions depends upon a number of 

 factors. Electrons cross easily from the negatively-charged emitter to the 

 P-type base, filling some of the holes in the base. The positively-charged 

 collector withdraws electrons from the base, across the P-N junction, 

 creating new holes. These new holes migrate across the base, eventually 

 being filled with electrons from the emitter. The number of new holes 

 formed— and therefore the current flowing in the base-collector half of 

 the transistor— depends upon the number of electrons injected into the 

 base from the emitter. The transistor can thus accept a signal and con- 



