Charge storage diode for magnetic memory application

Abstract

CHARGE STORAGE DIODES are a class of diodes utilizing controlled minority carrier storage'. Such a diode has been developed for use as a transient bidirectional current steering element in sub-microsecond magnetic memory access circuits2.The electrical requirements for this diode have been derived from considerations associated with driving a 1024-word 200-400-nsec waffle-iron memory3. The memory requires a word current waveform for the read-write necessary to avoid a very fast snapback to high imped-cycle as shown in Figure 1. To prevent ringing, it is ance at the end of the write cycle. A forward turn-on transient voltage of less than 1.5 = 0.5 v 1s required. A using the maximum allowable. memory disturb current capacitance requirement of 3 pf for the diode is derived through an unselected crosspoint. For general appllcation, the breakdown voltage should exceed 60 v. We will consider a planar epitaxial pin diode structure voltages of 100 v or greater, forward voltage drops of as shown in Figure 2. This. deslgn obtains breakdown mately 2 pf a t zero volts. 1.5 i 0.5 v (at 500 ma), and capacitance of approxifunction of the minority carrier storage in the diode. The charge extracted during the write cycle is a This stored charge is expressed mathematically by the equation for the continuity of charge: -+ -= l ( t ) ? dQ Q d t r where Q is the total stored charge, T is the effective minority carrier lifetime, and I ( t ) is the diode current as a function of time. Integrating equation . ( l ) over the read-write current waveform for the condltlon of no net charge remainlng and solving for the constant current phase (tu) of the reverse conduction cycle yields [ t , = 7127In this equation, If and, I, are the forward !read) and reverse conduction (write) currents, respectlvely, tf 1s the effective duration of the forward current pulse, and t, is the time constant of the reverse conduction current recovery phase i.e., ITecoDe7?1 = I, exp (--tit,).The length of the write cycle is most nearly represented by t,, = t, f t,. With this definition of write cycle duration, then t,, can be expressed as t,, = r l n [ l +Zf(I -exp(-tf/7j)/Irl ( 3 )with less than 5% for the read-write conditions of interest here and where t, 5 0.2 T. For the read-write cycle shown in Figure 1 where t,, ranges from 80 nsec IMoll, J. L., Krakauer, S., and Shen, R., "P-N Junction 2 Melmed, A., and Sherlin, R., "Diode Steered Magnetic Core 3Finch, T. R., and Bobeck, A. H., "The Waffle Iron Store," __ Charge Storage Diodes," Proc. 1964 International Solid-State Circuits Conference nsec (maximum time allowable if recovery 1s to be (time required to set a certain amount of flux) to 140 sufficient to allow 400 nsec cycle) the minority carrier lifetime must be held within the range of 75 to 170 nsec. to prevent either a long recovery tail or a fast step. To The recovery phase of the write cycle is controlled effect a .suitable recovery it is necessary to establish to the junction. Diffusion profiles approaching those used some bullt-in field that confin...