TRANSISTORS AND JUNCTION DIODES 



829 



rectifying cells, there are two directions of current flow, forward and 

 reverse. Each diode has a positive and a negative terminal, and we de- 

 tine the positive terminal as that terminal towards which forward current 

 flows within the diode. LikeA\ise, the negative terminal is that terminal 

 towards which reverse current flows within the diode. In Fig. 1(a), 

 terminal 1 is the negative terminal and terminal 2 the positive. The 

 circuit convention for the diode is a shorthand method of indicating the 

 polarity of the diode to the engineer. If a battery is connected to a diode 

 as shown in Fig. 1(b), forward current A\ill flow, and if connected per 

 Fig. 1(c), reverse current \\\\\ flow. If the battery is replaced by a source 

 of alternating current, forward current will flow through the diode during 

 the half cycle that terminal 1 is positive, and reverse current \\\\\ flow 

 during the half cycle that terminal 2 is positive. The rectifier is said to 

 "conduct" during the first half cycle and to "block" during the second 

 half cycle, for the resistance in the conducting direction is very much 

 less than the resistance in the blocking direction. 



The figure of merit of a diode is a measure of this ease of conduction 

 and the effectiveness of the blocking action. The ease of conduction can 

 readil}' be determined on a static basis by applying a dc voltage to the 

 diode as shown in Fig. 1(b) and plotting forward current through the 

 diode as a function of applied voltage. Like^^^se, the blocking charac- 

 teristic can be determined if a circuit per Fig. 1(c) is employed. 



2.12. Typical Junction Rectifiers 



Fig. 2 is a photograph of several sizes of typical junction diodes. The 

 diodes sho^^^l have a range of forward current from several milliamperes 

 (Diode I) to hundreds of amperes (Diode IV). Diode I is made from a 

 crystal of silicon and the balance are made from germanium. Most rec- 

 tifying diodes have a particular field of use dictated mainly by their 

 power handling capacity in the forward direction of current flow, al- 

 though Diode I is of interest because of its unusual reverse or blocking 

 characteristic, as will be pointed out later in this paper. 



(-)o. 



2, 



DIFFICULT 

 DIRECTION OF CURRENT FLOW 



(a) 



FORWARD 

 CURRENT FLOW 



(b) 



Y'lg. 1 — Rectifier terminology. 



REVERSE 

 CURRENT FLOW 



(c) 



