June 1969 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Prior to the emergence of silicon-based semiconductors, selenium and copper(cuprous)-oxide rectifiers were the alternatives to vacuum tubes. Copper-oxide (Cu2O) was popular as a small signal detector since its forward voltage drop was similar to a Schottky type diode - typically around 0.2 V. Copper-oxide diodes were used in radios and test equipment meters. Selenium (Se) has a forward voltage drop of around 1 V, but its high reverse voltage withstanding of 20 V or more made it popular for voltage rectification, with as many layers as necessary being stacked serially as required. Selenium rectifier stacks could be found in most television sets beginning in the 1950s.
This edition of Electronics World ran a series of diode articles: Hot Carrier Diodes, Variable-Capacitance Diodes, Tunnel Diodes, Microwave Power Diodes, A Survey of Silicon Junction Diodes, and Light-Emitting Diodes.
A Survey of Silicon Junction Diodes
The author, Associate Professor of Electrical Engineering at Pratt Institute, received his B.E.E. cum laude from CCNY in 1951 and attended Columbia and Hofstra Universities (M.A. in Physics, 1958). His areas of interest ate solid-state electronics and computer logic. He is the co-author of "Semiconductor Fundamentals: Devices and Circuits" and is currently at work on a new book, "Electronic Circuit Analysis" which will be published very soon.
By A. H. Seidman / Contributing Editor
Notable progress has been made in developing higher-power rectifying, regulating, and switching diodes. Examples of what's available, their characteristics and cost, are given.
In the past decade silicon junction diodes for rectification, regulation, switching. and r.f. applications have moved to the forefront in the hierarchy of electronic devices. The technology has progressed to a point where the manufacturer has more control over his process, ensuring greater yields, increased reliability, and lower device costs. For example, applications specialists have available today solid-state rectifier assemblies with peak reverse (inverse) voltage (p.r.v.) ratings of up to 50 kV and Zener diodes that can dissipate as much as 300 watts of power. A variety of package types and diode arrays offer challenging design opportunities to the engineer.
There are more than a hundred firms manufacturing silicon junction diodes. Total sales in 1968 reached some $170 million. Rectifier diodes accounted for approximately $95 million of this sum; by 1972 this is expected to rise to $120 million. Rectifier assemblies, enjoying a $12 million volume in 1968, will probably reach $20 million in 1972.
Selenium and copper-oxide rectifiers represented sales of $15 million in 1968 but this figure is expected to drop to under $10 million by 1972. Although selenium rectifiers have self-healing properties and exhibit good surge suppression, the superior characteristics of silicon devices have resulted in their gradually replacing seleniums in most applications. Copper-oxide rectifiers, because of their small forward-voltage drop, find their greatest use as instrument rectifiers like those used in average-reading a.c. voltmeters. With the greater use of electronic and digital instrumentation, though, this picture can change drastically in the future.
Structure of Motorola multi-cell high-current rectifier. Each cell operates at 75 % maximum current capability.
About $56 million was spent for regulator (Zener) diodes in 1968. According to experts in this field, this will drop to $46 million by 1972. Reasons advanced for the decline are a saturated market for regulator diodes and the greater availability of integrated-circuit regulators. Low-level switching and r.f. diodes accounted for not more than $10 million in sales in 1968. This market will probably taper off considerably owing to the wider use of integrated circuits in digital and communications circuits. High-power switching and Zener diodes, however, should exhibit a steady growth in sales.
"Silicon rectifiers as a class used for bridges, full-wave rectification, single and polyphase, are essentially going through process refinement, product improvement. and cost reduction," according to Rein Rist, Marketing Manager for Thyristors and Rectifiers at RCA. This assessment of the rectifier industry is also shared by many knowledgeable individuals in the field. What are some of the "process refinements" and "product improvements" worthy of our attention?
Since 1957 virtually all silicon rectifier diodes have been either of diffused or epitaxial type construction. The alloy junction as well as germanium devices are virtually things of the past in this area. To ensure high reliability, diodes are passivated by a silicon-dioxide coating that surrounds the structure, or the chip is enclosed in glass (glass passivation). The latter process permits greater stability and improved hermetic sealing. producing better performance and allowing the low-cost plastic encapsulation to be employed with a high degree of reliability.
Present device technology forces the manufacturer to make trade-offs; in general, the greater the p.r.v. the lower the average current rating. Single junction diodes are commonly available with p.r.v. ratings as high as 1000 volts. A representative example is Solitrons glass passivated chip (BF6-100C) with a p.r.v. of 1000 volts at 3 amperes. The device measures 0.09 inch in diameter, making it especially suited for hybrid integrated circuits. Many manufacturers are now providing small-size diode chips, having various characteristics and ratings for the hybrid market.
Table 1 - Summary of some representative rectifiers, Zener, and switching diodes, along with costs.
To obtain greater inverse voltage ratings for silicon rectifiers than are available from a single chip, individual diodes with highly uniform breakdown characteristics are strung in series. This procedure results in rectifiers with p.r.v. ratings of 50 kV and higher. Because of their variable reverse leakage current ratings, in the past it was necessary to hunt each diode in a string with an RC network to ensure that every diode would take its share of reverse voltage. This is not required today owing to the uniform diode characteristics obtained in manufacturing.
Some representative examples include Semtech's "Ministic" (SCMS 40K) where a number of junctions are metallurgically bonded at high temperature to provide p.r.v. ratings of 40 kV at 5-mA current. Solitron's "Solidpak " devices are rated up to 50 kV at 2.25 amperes. Motorola bas a new sub-miniature series, the "Surmetic," with p.r.v. ratings of 5 kV at 250 mA. The field of applications for high-voltage silicon rectifiers is growing and includes power supplies for electrostatic equipment, high-energy capacitor-discharge systems, cathode-ray tubes, as well as some types of x-ray equipment.
High-current rectifiers, like the Motorola MR 1219, rated at 100 amperes d.c. and a p.r.v. of 600 volts, make use of multi-cell construction (see photo). In this configuration, a number of matched single diodes are connected in parallel and housed in a single package.
Many diode manufacturers have available multiple matched diode assemblies for a variety of applications. In the power-supply field, examples include single-phase center-tap, single-phase bridge, three-phase bridge, and six-phase star. Typical of what can be obtained is Tung-Sol's rectifier stack (16F611S23-2UV2) for a six-phase star rated at 600 volts p.r.v. and 290 amperes d.c. under forced-air cooling.
Sylvania has available epoxy packages containing matched quads (M9316) rated at 150 volts p.r.v. and 200 mA and a ring modulator (M9330) rated at 50 mA. Other examples are Texas Instruments' (TID 25) 16-diode core drivers. Housed in a JEDEC TO-89 package, the diodes are epitaxial planar and are designed to drive magnetic cores, drums, tapes, and discs.
Typical (A) forward and (B) reverse characteristics of a representative rectifier diode at various operating temperatures. Device is one of Solitron's BF6 series.
Zener diodes, a term used to designate regulator and reference diodes, are offered in a large variety of ratings. The regulator diode, which enjoys the greatest application in power supplies, is available in Zener voltage from approximately 2 to 200 volts with dissipation ratings as high as 300 watts. The regulator diode is, however, temperature-sensitive. In applications where the output voltage must be confined to narrow limits as temperature or current varies, temperature-compensated diodes, called reference diodes, are used.
A forward-biased junction has a negative temperature coefficient of 2 mV/°C at 5.5 volts d.c. and 6 mV/°C at 10 volts d.c. By a careful combination of junctions it is possible to fabricate reference diodes with stable Zener voltages. For example, Motorola' s 1N946B reference diode, rated at a Zener voltage of 11.7 volts ±5% at 7.5 mA, has a maximum voltage change of 0.005 volt over a temperature range of -55° C to +150° C.
High-power Zener diodes find wide application in transient suppression, such as required for tactical radio equipment. Dr. J. Reynolds of Delco Radio has developed a Zener diode capable of dissipating 300 watts of power in a single chip at a case temperature of 100° C; the Zener voltage rating is between 30 and 50 volts. The transient peak power dissipated for a pulse width of 0.1 ms can be as high as 100 kW. Eventually, Zener voltage values greater than 200 volts at 300 watts dissipation may be realized.
According to Dr. Reynolds, a major problem in attaining high dissipation ratings in Zener devices is a nonuniform microplasma distribution in the junction. A microplasma is a unit of avalanche current equal to 100 μA. Segregation of microplasmas leads to hot spots and the ultimate degradation of the junction. This phenomenon has limited the dissipation ratings of single-chip devices in the past. Dr. Reynolds has licked the problem by careful processing and bonding techniques. A special p-type silicon element of 0.1 ohm-cm resistivity with 1000-2000 dislocations/cm2 is phosphorous-diffused. No enhancement layer is used and the silicon is alloyed between tungsten plates. The junction is tapered by approximately 10 degrees.
Other manufacturers use stacks of Zener diodes to obtain greater dissipation ratings. Typical of this approach is Motorola's series of MPZ5 transient suppressors. Using six matched Zener diodes, ratings of 350 watts dissipation at 25 °C at minimum Zener voltages of 16 to 180 volts are available. The transient peak power rating is 40 kW for a pulse width of 0.1 ms.
Switching and R. F. Diodes
Example of a representative temperature-derating curve for h-v rectifier diode. Unit is a Motorola Type MR990.
Because of the increasing use of integrated circuits, the market for low-level discrete switching and r.f. diodes is dwindling. Exceptions to this trend are special arrays for functions such as ring modulators and core drivers, and microwave diodes. The latter enjoyed a market of $24 million in 1968 and is on the ascent. High-power discrete switching diodes also find a viable market. An example is Unitrode's UTX-4120 miniature fast-recovery diode. Rated for a p.r.v. of 200 volts, the device switches 4 amperes with a recovery time (turn-off time) of less than 100 nanoseconds.
A new development, intended to replace the hydrogen thyratron in such applications as line-type modulators, is Westinghouse's Type 423 reverse-switching rectifier (r.s.r.). When reverse-biased, the device initially blocks and is non-conducting. At a critical voltage and current, the switch begins to conduct, rapidly reaching a low-impedance level. It remains conducting until the voltage or current is reduced essentially to zero. The minimum energy required to switch the device is 150 microjoules in the form of a high-voltage, fast-rising pulse.
The switch is capable of handling a maximum peak pulse current up to 2000 amperes with a typical di/dt rating of 10,000 amperes per microsecond. The switching voltage rating is as high as 1200 volts. In low-power (50 kW) applications, the device will replace the 5C22 thyratron. For higher switching powers, series-parallel strings of r.s.r. devices are employed.
In selecting high-power diodes, the current-temperature derating curve is of paramount importance. The temperature scale for the curve can either be designated as case or ambient temperature and should be noted when calculating heat-sink requirements. Another item to consider is surge current; the manufacturer normally furnishes plots showing allowable peak surge current as a function of pulse width or frequency.
Because of the cost of many high-voltage stacks and high-current diodes of multi-cell construction, the engineer may be tempted to string individual diodes in series or parallel to achieve the higher ratings. Extreme care must be exercised.
In series stringing of diodes, parallel RC combinations across each diode are generally required to ensure equal distribution of voltage drops across the diodes when they are exposed to a reverse voltage. One technique used in paralleling diodes is to derate the current rating of each device by approximately 25 percent.
For most applications requiring Zener-type diodes, the regulating diode will suffice. In critical applications, however, the reference diode must be considered. Device dissipation is an important factor in all cases. If the zener diode is used for transient suppression, a common application for a high-wattage device, the surge-power rating becomes especially important.
Performance of a typical reference diode over a wide temperature range. This particular diode is Motorola 1N946B.
The reverse recovery time is the significant parameter for switching diodes. Forward recovery time, which is a function of the driving current and its waveform, is generally not of concern in the majority of applications. Peak reverse voltage and maximum switching current are other parameters to consider in selecting switching diodes.
A real challenge facing the solid-state diode manufacturer is the development of single-chip devices with greater voltage, current, and power-dissipation ratings. Instead of using a string of diodes to provide a high p.r.v. rating or a number of diode cells in parallel to achieve current ratings in the hundreds of amperes, there are advantages to having this accomplished on a single chip. Undoubtedly costs would be reduced and greater reliability realized.
Integrated circuits will have an ever-increasing impact on small-signal, low-level discrete components, like switching diodes. This segment of the industry will concentrate on the development of high-power switches with faster recovery times. Single chips with high power ratings should become more plentiful for application in hybrid circuits.
Over-all, the junction-diode industry appears to be in a healthy condition and likely to remain so for many years to come.
Posted February 5, 2018