IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-30, NO. 11, NOVEMBER 1983 1565

modeling work has discussed two-dimensional current spreading effects, base transit time effects and transmission line effects. Going beyond the more conventional static descriptions, such as holding current and holding voltage, recent experiments have investigated transient triggering and latching behavior.

Techniques used to prevent latchup fall into two categories. The first deliberately spoils the injection or transport properties of the parasitic bipolar transistors. The second decouples the two (or more) transistors required to have latchup. While the end goal of both is degraded gain, the technological requirements can be quite different.

Over the past decade emphasis has shifted from the first to the second. This trend will accelerate as CMOS becomes the dominant VLSI technology and as the parasitic bipolar characteristics become so good that these devices will be harnessed rather than harassed. The advent of epi CMOS and the promise of trench isolation offers good hope for total latchup immunity.

The talk will review recent modeling and characterization work and discuss the new understanding such work has provided. It will also review the process innovations contributing to latchup immunity and discuss their efficacy for scaled CMOS.

1-4 Gain and Loss Processes in InGaAsP/InP DH Lasers-Manfred H. Pilkuhn, Stuttgart University, Germany F.R.

During the recent development of InGaAsP/InP double heterostructure (DH) lasers for long wavelength (1.3-1.6hm) optical communication, the strong temperature sensitivity of threshold current ("To-problem") and quantum efficiency has been a major concern: High Temperature CW operation becomes more difficult than in comparable short wavelength GaAs structures.

The physics of the various gain/loss mechanisms in quaternary lasers is discussed in this paper:

1) carrier losses due to non-radiative auger recombination and optical losses due intervalence band absorption depend on band structure, doping level and temperature. Theoretical and experimental values of auger

coefficients are relatively large (1-3x: 10-29cm 6s-1) for InGaAsP, and they have a strong influence on the temperature sensitivity of the optical gain and the threshold current. Intervalence band absorption processes at high hole concentration and high temperature determine the optical losses and the quantum efficiency of InGaAsP lasers. Recent experimental evidence for the influence of the split-off valence band will be discussed.

2) carrier leakage over the heterobarriers is another loss mechanism which becomes important at high temperatures (T=340K), and it may be documented through emission from the cladding InP or from additional InGaAsP layers in multilayer structures. For a quantitative analysis, information on carrier temperature, fermi1 level position, and density of states is desired.

The relative importance of the various loss mechanisms and their influence on the performance of InGaAsP lasers will be discussed.

Finally, in connection with possibilities for improvements, quantum well lasers will be mentioned briefly.

IIA-1 (GaAl) As/GaAs Heterojunction Dipolar Transistors with Graded Composition in the Base-D.L. Miller, P.M. Asbeck, R.J. Anderson, and F.H. Eisen, Rockwell International/Microelectronics Research and Development Center, 1049 Camino Das Rios, Thousand Oaks, CA 91360, (805) 498-4.545.

We report here the first fabrication and high speed operation of three terminal heterojunction bipolar transistors in which the semiconductor bandgap ' varies continuously across the base region. Devices of this type have demonstrated a cutoff frequency, ft, up to 16 GHz, which is comparable to the highest previously reported value for bipolar transistors [ l l . The devices utilize decreasing bandgap across the base region to provide a built-in potential gradient which assists the transport of minority carriers from emitter to collector, as first discussed by Kroemer [21; phototransistors employing this principle have been reported recently 13 1.

Heterojunction bipolar transistors having A1,3Ga.7 emitters and graded A1,Gal-,As bases were fabricated from material grown by

1566 IEEE TRANSACT1 3lNS ON ELECTRON DEVICES, YOL. ED-30, NO. 1 1 , NOVEMBER 1983

molecular beam epitaxy. The composition of the base region was graded from x=.05 at the emitter to x-0 at the collector, which provides a potential gradient of about 9 ~ 1 0 ~ v/cm across the 800A thick base. Emitter contacts were 2.5pm wide. The devices were fabricated on semi-insulating substrates utilizing a buried n+ layer to contact the collector. Beryllium ion implantation, together with thermal pulse: annealing 141, was used to make contact to the: base region from the wafer surface. The: intrinsic base sheet resistance was onky 1300 /O.

For high speed measurements, the devices were mounted in conventional ceramic microwave transistor packages; their cutofl' frequencies were determined from S-parameta measurements over the range 2 to 6 GHz. The: high frequency current gain of the device!, increased with increasing collector curren'. density, J,, up to the high value of J, = 4x10' A/cm2 (VcE = 3V) without the onset of thl: Kirk effect. The maximum observed cutofi frequency, ft, was 16 GHz, which correspond; to a total emitter to collector transit time of 1I) psec. According to devices modeling, thi 5 measured value is dominated by times associated with package-related parasitics and depletion layer charging times. Calculatiorls indicate that with a design optimized to reduc:: the collector junction capacitance and 1.3

reduce emitter and collector series resistance!;, graded bandgap base heterojunction bipoh I

transistors should be capable of attaining a. cutoff frequency ft approaching 100 GHz.

'D. Ankri, W. Schaff, P. Smith, C.E.C:. Wood, and L.F. Eastman, IEDM 1982 Tech. Dig. p 708.

2H. Kroemer, RCA, Rev. 18, 332 (1957). 3F. Capasso, W.T. Tsang, C.G. Bethe 1,

A.L. Hutchinson, and B.F. Levine, Applicid Phys. Lett. 42, [ ( l ) ] , 93 (1983).

4P.M. Asbeck, D.L. Miller, E.J. Babcock, and C.G. Kirkpatrick, to be published in Elec. Dev. Lett.

IIA-2 Double Heterojunction &.&a& s / GaAs Bipolar Transistors with Improv :d Interfaces--. Fishcer, S.L. Su, D. Arnold, J. Klem, and H. Morkoc, W.G. Lyons and 13. Tejayadi, Department of Electric a1 Engineering and Coordinated Scie~r~ce Laboratory, University of Illinois, 1101 747.

Springfield Avenue, Urbana, IL 61801, (217) 333-0722.

Heterojunction bipolar transistors are very promising candidates for high speed logic devices because of their potential to provide both very fast switching and large fan-out. The use of a large bandgap emitter allows the emitter and base dopings to be selected to minimize emitter capacitance and base resistance without adversely affecting the emitter injection efficiency. In addition, a large bandgap collector can make the emitter and collector junctions symmetrical and eliminate the collector turn-on voltage inherent in single heterojunction devices.

Although liquid phase epitaxy has been used to prepare heterojunction bipolars with extremely high current gains, the nonuniformities across the wafer this technique provides makes it unsuitable for preparation of integrated circuits. In addition, the common emitter current gain of LPE grown bipolars show a strong dependence on the collector current, which indicates a large interface recombination current. Molecular beam epitaxy on the other hand can produce uniform layers, but has in the past suffered from low current gains, e.g. -100-200.

We have performed a study of double heterojunction bipolar transistor structures prepared by molecula! beam epitaxy. Base thicknesses from 500A to 2000A and base dopings of from 4x10'7-5x'8cm-3 were investigated. The device performance was correlated to growth conditions, particularly the substrate temperature, to the use of graded and abrupt heterojunction, and to interface properties. As a result of this study, we have obtained double heterojunction bipolar transistors with current gains as high as 1700.' Further, improvements in emitter and collector junction interface properties have allowed current gains which are nearly constant over almost two decades of collector current to be obtained.

This presentation will concentrate on the novel interface concepts employed to obtain almost constant current gain as well as the effects of growth conditions and materials properties on the device performance.

'High Current gain device has been submitted to EDL for publication.