INTRODUCTION TO SONAR 



-JUNCTION 



LEAD 



mm^ — 



->--- 



LEAD 



P-TYPE N-TYPE 



CRYSTAL CRYSTAL 



+ - 



20.23:71.98 

 Figure 10-20. — Junction diode. 



germanium, P-type m»\terial is formad, which 

 produces the flow of positive (hole) current. (The 

 theory of operation of seni!.conductors , which is too 

 involved for this text, can be found in Introduction 

 to Elec tronics , NavPers 10084.) 



When N- and P-type materials are combined, 

 they form what is called a junction diode, shown 

 with its schematic symbol in figure 10-20. Because 

 a diode will not ampUfy a signal, a third semicon- 

 ductor section is added, forming a junction tran- 

 sistor, which acts like a vacuum tube triode . (Other 

 transistor types include the tetrode and the power 

 transistor.) The transistor actually is two junction 

 diodes placed back to back, with the center element 

 common to both junctions, and are of either the 

 PNP or the NPN type. Internal currentflowis the 

 result of hole conduction in the PNP type, and of 

 electron conduction in the NPN type. External cur- 

 rent in a transistor circuit is always electron flow. 



The elements of a transistor are called the 

 emitter, base, and collector. They correspond 

 respectively to a vacuum tube's cathode, control 

 grid, and plate. Figure 10-21 shows the two types 

 of transistors, their schematic symbols, and their 

 relation to the triode. The prim^xry difference be- 

 tween the operation of a vacuum tube and a tran- 

 sistor is that the tuba is voltage-operated and the 

 transistor is current-operated, resulting in low 

 power requirements for transistors. 



Transistors, like vacuum tubes, come in a 

 variety of types, each withits own characteristics. 

 Following are some of the data found on a tran- 

 sistor specification sheet published by the manu- 

 facturer. 



1. The kind of semiconductor (PNP, NPN, 

 diode, etc.), type of material used, and type 

 of construction, 



2. Common applications (audioampllfier, rec- 

 tifier, etc.). 



3. Absolute m.aximiun ratings of voltages and 

 collector current. These ratings must never 

 be exceeded. 



4. Collector power dissipation factor. 



5. Beta (gain) of the transistor. 



6. Collector cut-off (leakage) current. 



You should know the semJ conductor specifications 

 before attempting quality tests. 



Transistors normally are more rugged than 

 electron tubes, and do not often require replace- 

 ment. Their life expectancy is 40,000 hours or 

 more. Should It become necessary for you to 

 replace atransistor, however, certain precautions 

 must be observed. Before removing the old tran- 

 sistor, note the orientation of the transistor's 

 base, emitter, and collector leads to ensure proper 

 insertion of the new device. Cut the leads of the 

 new transistor to the proper length to prevent 

 undue stress. When soldering the new transistor 

 into the circuit, use the proper solder, soldering 

 iron, and a heat sink. Transistors are very suscep- 

 tible to heat, r-f radiation, and electric shock. One 

 of the most frequent causes of damage to a tran- 

 sistor is the electrostatic discharge from the 

 human body when the device is handled. Such 

 damiOge may be avoided by discharging your body 

 to the chassis before handling the transistor. 

 Detailed information on how to replace atransis- 

 tor, handling precautions, and transistor lead 

 coding methods is given in Servicing Techniques 



COLLECTOR 



(C) 



EMITTER .=- 

 (E) T + 



1.280A(20A) 

 Figure 10-21. — Corresponding elements in triode 

 and transistor. 



170 



