IEEE TRANSACTIONS ON INDUSTRY AND GENERAL APPLICATIONS, VOL. IGA-7, NO. 2, MARCH/APRIL 1971

Design of a High-Power Zener Diode and Its EnergyAbsorption Capability

S. E. MILLER, JAMES E. REYNOLDS, MEMBER, IEEE, AND JAMES R. WASHBURN

Abstract-The structure and performance of a high-power zenerdiode to be used for transient suppression or voltage regulation aredescribed. A technique for accurately determining the energyabsorption capability for pulse durations down to 10 Ms is also de-scribed and used in rating the device.

DESIGN

THE MAJOR design problem consists in obtaininguniform avalanching which is tantamount to ob-

taining a uniform microplasma distribution in the junc-tion. Segregation of microplasmas will lead to hot spotsand may degrade the junction. Three of the chief factorsin achieving uniformity are as follows.

1) Radial Resistivity Gradient: Uniform doping of thebase silicon is desirable for uniform avalanching. Sincedoping gradients cannot be completely eliminated ineither float zone or Czochralski silicon, it is preferablethat the higher doping lie toward the center. This distri-bution ensures that avalanching will occur first in thecentral region, helping to prevent surface breakdown.

2) Dislocation Density: Lattice strain results in dis-locations which contribute to the energy available forionization. Device performance requires either an abun-dance of uniformly distributed dislocations or none at all.Evidence substantiating these requirements is found inthe literature [1]- [3].

3) Surface Control: It is necessary to treat the surfacesso that breakdown will not occur at the surface before itoccurs in the bulk. Adequate control of breakdown is

Paper 71 TP 2-IGA, approved by the Power Semiconductor Com-mittee of the IEEE IGA Group for publication in this TRANSAC-TIONS. Manuscript received January 20, 1971.

S. E. Miller and J. E. Reynolds are with Delco Electronics Divi-sion, General Motors Corporation, Kokomo, Ind.

J. R. Washburn is with Semiconductor Products Division,Motorola, Inc., Phoenix, Ariz.

achieved with shallow angle tapering through the junc-tion, with subsequent etching and edge coating.A cross-sectional view of the device structure is given in

Fig. 1. A p-type Czochralski silicon element of about 0.1-Q cm resistivity, with 1000-2000 dislocation/cm2, isphosphorus diffused. No enhancement layer is used.The silicon is alloyed between tungsten plates. Thesandwich is then bonded to Cu terminals using Au-Ge.The taper through the junction is about 100 from thehorizontal. After etching, varnish protection is affordedby DC-643. Encapsulation is completed by cold weldingin a DO-9 package (Fig. 2).

CHARACTERISTICSNominal values of Vz, IR, ZZT, and VFM are shown in

Table I for a 36-V (minimum) device.Power Derating for DC Regulation: This device may be

used either for voltage regulation in a dc mode or as atransient suppressor. A total power derating curve isgiven in Fig. 3. This power is the maximum allowable forany combination of reverse-forward operation at theindicated case temperatures.

HEAT DISSIPATION

The heat dissipation capabilities of zener diodes mustbe known in order to design networks for the absorptionof transient energy. Tests show that this zener diode canabsorb 4.3 kW for 50 ms. Absorption capabilities forshorter time periods, from about 1 ms down to 10 ,us,are difficult to measure directly. Here, thermal impedancesdetermined by an analog computer have been used topredict power absorption as a function of pulse durationfor pulses down to 10 ,us. It was shown that reliableoperation may be expected for at least 500 pulses at therated power absorption capability.

208

MILLER et al.: HIGH-POWER ZENER DIODE

TABLE I

Vz at 0.25 A, 30°C 36 V minimumI.e at 32 V, 30°C 5 mA maximum

at 32 V, 150°C 15 mA maximumZZT (dynamic impedance) at Iz = 2.3 A 0.15 51VFM at IFm = 800 A,

T. = 1750C 1.0 VeOc (steady state) 0. 18°C/W

Fig. 1. Device structure.

200.

10I"100*

D so o10Case Temperature, IC

150 175

Fig. 3. Total power derating curve.

-X

TemP-.002 0 +.002 °

I

C}NJunction

Fig. 2. Completed assembly. Fig. 4. Temperature profile for 20-,us pulse in infinite length ofsilicon.

ANALOG TREATMENT

Mathematical Model

Assuming an infinite silicon cylinder with the p-n

junction plane as the heat generator, Fourier's equationbecomes unidirectional:

aT/at = a(a2T/0x2)

where To is the initial temperature, the appropriate solu-tion to (1) for unidirectional heat flow from a planar source

of heat after Jakob [4, p. 258] is

-To = (CV\at)u {(2/Vfir) (exp - u2) du

(1)

where

T absolute temperaturet timex distance from p-n junctiona thermal diffusivity.

For the boundary conditions of no peripheral heat loss

and

T = ToT = T

aT/ax = -C,

at t = 0

at x 4

at x = 0

+ [(exp - U2)//lr u]- 1} (2)

where

u = x/2V/oat.

Equation (2) allows the temperature gradient to becalculated as a function of distance from the heat source.

Such a calculation shows that the diffusion of heat inabout 20 ,us or less is restricted to 4 0.002 in from thep-n junction (see Fig. 4). Since the junction is of theorder of 0.002 in from the silicon surface in an actualdevice, the heat is contained within the silicon and the

DC Mode

209

IEEE TRANSACTIONS ON INDUSTRY AND GENERAL APPLICATIONS, MARCH/APRIL 1971

preceding model is quite rigorous. The model is also use-able with small error up to about 10 ms.

Construction and Calibration of Analog

With validity of the model shown, an analog computer[4, p. 409] was constructed, a schematic of which is shownin Fig. 5. For additional information on the structure ofthe device see Fig. 1. Note the intermediate nodal pointin the network representing the p-n junction.

It can also be shown from (2) that

TJ- TO = constant /t (3)

where

TJ junction temperaturet duration of pulse.

By placing a variable resistor and a variable capacitorat the junction node and making appropriate adjust-ments, the analog circuit was calibrated with a 20-,uspulse to give the parabolic TJ excursion indicated in (3).

Transient Thermal Impedance 0Jc(t)Electrical energy pulses were injected at the nodal

point in the analog circuit corresponding to the p-njunction in the zener diode. Hence the voltage gradientsgenerated in the analog circuit represent temperaturegradients in the zener diode. Using the AT informationwith the heat energy derived from its equivalent electricalenergy, thermal impedance was calculated, and values aregiven in Fig. 6.

Transient Energy Absorption CapabilitiesAn important reference point was established by pulsing

35 zener diodes 500 times each with 4.3 kW for 50 ms.A 10-s rest period was allowed between pulses for thermalrecovery. One failure occurred at 169 cycles.To allow this reliability to be related to pulses shorter

than 50 ms, it was assumed that reliability is limited bythe maximum junction temperature; this maximum isidentical with a TJ(max) found experimentally for 50-mspulses. Maximum allowable power was then calculatedfrom

P(max)(W) = TJ(max) (OC)6JC(t) (0C/W)

(4)

The resulting data are plotted in Fig. 7 and give theengineer a zener diode performance curve around whichtransient suppressor networks can be designed.

Operating life and environmental tests are listed inTable II. Other tests passed included thermal shock(-550C to +1000C, 5-min soak) for 5 cycles, moistureresistance as per MIL Std 202B-106A, mechanical shock,and vibration.

Previously, the usage of single zener diodes has beenlimited to fairly small loads. The zener diode device

01U<)c0

E 30j-a0.E

'

*

I-

c 50.o00._WDc0

Fig. 5. Zener diode analog circuit.

16o 000 ,0.o00Duration of Square Wave , t (ps)

Fig. 6. DC junction to case transient thermal impedance 0JC(t).

0.00-

00

a.-

10

Ito

100 1000 10,000

Duration of Square Wave, t (ps)

Fig. 7. Peak reverse power rating for variable pulsewidth.

TABLE II

Test Conditions Failures

Pulse; 120-A reverse, 1 at 169500 cycles, 50-ms square wave, cycles35 units 10-s intervals

Half-wave 1750C, noneblocking life; 500 h,80 units 29 V

Temperature -65°C to +150'C nonecycling; for 10 cycles;15 units 1-h soak

10,000

100.000

210

10

MILLER et al.: HIGH-POWER ZENER DIODE 211

described here is satisfactory for suppression of transientssuch as those encountered in tactical radios. It is nowbeing produced for military tracked vehicles. This ap-plication has been described by Reich [5], [6].

ACKNOWLEDGMENT

The authors would like to thank G. M. Morgan forconsiderable help in the analog computer work.

REFERENCES[1] A. G. Chynoweth and G. L. Pearson, J. Appl. Phys., vol. 29,

1958, p. 1103.[21 B. Senitzky and J. L. Moll, Phys. Rev., vol. 110, 1958, p. 612.[3] R. L. Batdorf et al., J. Appl. Phys., vol. 31, 1960, p. 1153.[4] M. Jakob, Heat Transfer, vol. 1. New York: Wiley, 1949.[5] B. Reich, "Zener diodes quell power-supply transients," Electro-

Technol., Jan. 1968, p. 71.[6] -, "Protection of semiconductor devices, circuits, and

equipment from voltage transients," Proc. IEEE, vol. 55,Aug. 1967, pp. 1355-1361.

S. E. Miller was born in Beecher City, Ill., on September 28, 1933. He received the B.S.degree in physics and mathematics from Eastern Illinois University, Charleston, in 1960and the M.S. degree from Purdue University, Lafayette, Ind., in 1971.He was employed by the P. R. Mallory Semiconductor Division as a quality control and

production control engineer. Since 1961 he has been with Semiconductor Device Engineeringat Delco Electronics Division, General Motors Corporation, Kokomo Ind.

James E. Reynolds (A'56-M'61) received the B.S. degree from the University of Alabama,University, in 1944, the M.S. degree from the University of Missouri, Rolla, in 1947, and theD.Sc. degree from the Massachusetts Institute of Technology, Cambridge, in 1953, all inmetallurgical engineering.

In 1944 he taught in the ASTP program at the University of Alabama. After two years inNaval Electronics, he became Plant Metallurgist for the Continental Gin Company, Bir-mingham, Ala., and then served for one year as an Instructor in the Department of Metallurgyat the Missouri School of Mines. He was awarded an instructorship and several scholarshipswhile attending M.I.T., where he also taught naval officers in post graduate work. From 1953to 1956 he worked at Battelle Memorial Institute on diffusion in titanium and the corrosion_of nuclear fuel elements. He was in semiconductor work as Product Manager, RectifierDiffusion Department, Mororola, Inc., Phoenix, Ariz., and Manufacturing Superintendent,Westinghouse Semiconductors, Youngwood, Pa., before joining Delco Electronics Division,General Motors Corporation, Kokomo, Ind., in 1963. He is now in charge of the research,development, and production assistance activities for rectifiers, thyristors, and zener diodes.

Dr. Reynolds is a member of Sigma Xi.

James R. Washburn was born in Marion, N. C., on July 20, 1940. He received the B.S. degreein applied physics from North Carolina State University, Raleigh, in 1964.

In 1965 he joined the Delco Radio Division, General Motors Corporation, Kokomo Ind.,as a Physicist in semiconductor research and development. His principal activities were inmethods of characterization of silicon power rectifiers, transient heat absorption, and semi-conductor surface studies. Since 1969, he has been with the Semiconductor ProductsDivision, Motorola, Inc., Phoenix, Ariz., where his responsibilities have been in thyristordiffusion and wafer processing.