TRANSISTORS AND JUNCTION DIODES 833 



ambient air is 40°C, which permits the junction temperature to be 25 to 

 30°C above the air temperature before the critical vakie is reached. 

 ^ ^A typical load-current versus air velocity curve is shown in Fig. 4. 

 The curve is based on a G5°C junction temperature measured by thermo- 

 couples attached to the radiating structure near the junction and 40°C 

 ambient air. Notice that the curve is taken with a working circuit com- 

 posed of six diodes in a three-phase full wave bridge arrangement. In 

 general, engineers developing rectifier circuits find that curves showing 

 the properties of combinations of rectifying diodes an; more useful than 

 single diode characteristics, except where the properties of the diode are 

 such as to make it useful as a valve, or as a reference standard, as is the 

 case of Diode I in Fig. 2. This leads directly to a more detailed considera- 

 tion of the blocking or reverse characteristics of junction rectifiers. 



2.2. Reference Voltage Diodes 

 2.21. General 



In the case of sihcon junction diodes it has been possible to reduce the 

 reverse current to a very low value for reverse voltages up to a value 

 called the "saturation voltage." When the saturation voltage is reached 

 the electrons and/or holes which comprise the leakage current are given 

 sufficient energy to create other electron -hole pairs which add to the 

 original reverse current. This process is cumulative and leads to large in- 

 creases in current for small further increases in voltage. The effect is 

 illustrated by the reverse voltage-current characteristic for Diode I 

 in Fig. 3. This curve shows the reverse current to be quite low for volt- 

 ages less than 22 volts. This portion of the characteristic is called the 

 "high resistance region." As voltage is further increased the curve goes 

 through a "transition region" to the "saturation voltage region" at 23 

 volts where voltage is nearly constant over a wide range of current. 

 The voltage saturation characteristic makes the diode suitable for use as 

 a source of reference potential in the control of power. Those readers 

 who ^\ish to study the basis of these properties will find the theory cov- 

 ered elsewhere in the literature.^ -^ 



The rectifier selected for study in this Section is Diode I. This is a p-n 

 junction rectifier made from silicon. It has been constructed to obtain 

 a reasonably constant saturation voltage as shown in Fig. 5. In order 

 to show the wide range of current values where this voltage is sub- 

 stantially constant, Fig. 5 is plotted to a logarithmic scale. In this con- 

 nection it is interesting to note that the saturation voltage can be con- 

 trolled in manufacture from a few volts to several hundred volts. This 



