H. Kroemer

Theory of a Wide-Gap Emitter for Transistors

In order to obtain a high current amplification factor, it is important in transistors that the ratio of the injected minority carrier current over the total emitter current, γ, be close to unity, or that the quantity 1-γ, called the injection deficit, be as small as possible. It is shown that the injection deficit of an emitter can be decreased by several orders of magnitude if the emitter has a higher band gap than the base region. This effect can be utilized either in addition to the commonly used high emitter doping in order to eliminate the alpha falloff with current, or to decrease the high emitter doping in order to obtain a lower emitter capacitance. Decreasing the emitter capacitance in high-frequency transistors may be utilized either to extend their frequency range or to increase their power capabilities by increasing the area.

Polar-on-nonpolar epitaxy

Abstract One of the most fundamental problems that must be solved if device-quality GaAs is to be grown on Si substrates is that of suppressing antiphase disorder. Recent experimental evidence shows that such disorder can be suppressed not only on the (211) orientation, but also on (100), contrary to earlier theoretical expectations. A detailed discussion is given of the mechanism by which this suppression takes place, through a combination of slight misorientation and a high-temperature surface anneal, which lead to the pairing of all Si surface steps into a particular kind of double-height steps. A recent model by Aspnes and Ihm explains the energetic preference for this kind of step by postulating a drastic reconstruction of the atomic configuration at the step edge through the formation of a π-bonded chain running along the step. Another unexpected puzzle is posed by the recent observation that on a given Si(100) surface antiphase disorder-free growth with both possible Ga—As sublattice allocations can be achieved, depending on initial nucleation conditions. A new detailed nucleation model is proposed that explains these observations, by drawing heavily on earlier considerations of Harrison et al. concerning the electrostatics of a polar-nonpolar interface.

Measurement of isotype heterojunction barriers by C‐V profiling

The Debye length smearing that occurs in C‐V profiling has precluded the use of C‐V profiling from an adjacent Schottky barrier to measure the magnitude of energy band discontinuities at barriers in isotype heterojunctions. It is observed, however, that in such a process both the number of the charge carriers and the moment of their distribution are conserved. This information permits the extraction of values for both the conduction band discontinuity ΔEc and any interface charge density. This technique and experimental results for an LPE‐grown n‐N GaAs‐Al0.3Ga0.7As heterojunction are described. We find ΔEc =0.248 eV, corresponding to about to 0.66ΔEg rather than Dingle’s commonly accepted value 0.85ΔEg . The difference is attributed to compositional grading during LPE growth.

Two integral relations pertaining to the electron transport through a bipolar transistor with a nonuniform energy gap in the base region

Abstract The two integral relations by Moll and Ross for the current flow through the base region of a bipolar transistor, and for the base transit time, are generalized to the case of a heterostructure bipolar transistor with a nonuniform energy gap in the base region.

Effects of interface layer sequencing on the transport properties of InAs/AlSb quantum wells: Evidence for antisite donors at the InAs/AlSb interface

Data are presented on the role of the InAs/AlSb interface in determining the electron transport in AlSb/InAs/AlSb quantum wells grown by molecular‐beam epitaxy. Because both anion and cation change across an InAs/AlSb interface, it is possible to grow such wells with two different types of interfaces, one with an InSb‐like bond configuration, the other AlAs‐like. Electron mobility and concentration were found to depend very strongly on the manner in which the quantum well’s interfaces were grown, indicating that high mobilities are seen only if the bottom interface is InSb‐like. An As‐on‐Al sites antisite defect model is postulated for bottom AlAs‐like interfaces. Such antisites were used in subsequent samples as donors in modulation‐doped high‐mobility InAs/AlSb quantum wells.

Molecular‐beam epitaxy growth of tilted GaAs/AlAs superlattices by deposition of fractional monolayers on vicinal (001) substrates

We report the successful growth of GaAs/AlAs superlattices having interface planes tilted with respect to the substrate surface plane. The amount of tilt and the superlattice period may be controlled by adjusting the growth parameters. The tilted superlattices (TSL’s) were produced by depositing fractional monolayer superlattices (GaAs)m(AlAs)n, with p=m+n≂1, on vicinal (001) substrates. We demonstrate the growth of quantum wirelike structures produced by placing short sections of TSL between horizontal layers of AlAs. Variations of the TSL period and tilt, both on uniform surfaces and on surfaces containing defects, yield insight to the growth kinetics and to the influence of variations in the growth parameters during molecular‐beam epitaxy growth.

Determination of valence and conduction‐band discontinuities at the (Ga,In) P/GaAs heterojunction by C‐V profiling

The valence and conduction band discontinuities for the lattice matched (Ga,In)P/GaAs heterojunction have been determined by capacitance‐voltage (C‐V) profiling. Both p‐p and n‐n heterojunctions were profiled, in order to obtain separate and independent values for both the valence‐band‐edge discontinuity (ΔEv) and the conduction‐band discontinuity (ΔEc). The band lineup is found to be of the straddling type with the valence‐ and conduction‐band discontinuities 0.24 and 0.22 eV, respectively, with an estimated accuracy of ±10 meV. Computer reconstruction of the C‐V profiles was used to check the consistency of the data. The band offset data indicate that the (Ga,In)P/(Al,Ga)As system should be staggered for a certain range of Al compositions.

Electron concentrations and mobilities in AlSb/InAs/AlSb quantum wells

We present data on the electron concentrations and mobilities in deep (≊1.3 eV) AlSb/InAs/AlSb quantum wells grown by molecular‐beam epitaxy. High electron sheet concentrations of the order 1012 cm−2, found in the not‐intentionally doped wells, indicate the presence of a deep donor in the AlSb barriers. Typical mobilities are between 22 000 and 28 000 cm2/V s at room temperature, increasing with decreasing temperature, and leveling out below 50 K at values between 175 000 and 330 000 cm2/V s. The temperature‐independent low‐temperature mobilities indicate a nonthermal scattering mechanism, possibly interface roughness scattering. Under illumination the wells exhibit a strong negative photoconductivity, which is explained as a natural consequence of the band structure of the wells.

Improved efficiency in semiconducting polymer light-emitting diodes

We report visible light emission from metal-polymer diodes made from semiconducting polymers, with indium-tin oxide as the “ohmic” contact, and a variety of metals as the barrier metal. Our results, which confirm the discovery by the Cambridge group [Nature347, 539 (1990)], demonstrate that light-emitting diodes can be fabricated by casting the polymer film on indium-tin oxide from solution with no subsequent polymer processing or heat treatment required. Electrical characterization reveals diode behavior with rectification ratios greater than 105 at sufficiently high voltages. Use of an electrode material with low work function leads to more than an order of magnitude improvement in the room-temperature efficiency of the devices. For example, the most efficient devices made with calcium as the rectifying contact display efficiencies of 0.01 photons per electron.

HETEROSTRUCTURE DEVICES: A DEVICE PHYSICIST LOOKS AT INTERFACES

The band offsets occurring at abrupt hetero-interfaces in heterostructure devices serve as potential steps acting on the mobile carriers, in addition to the macroscopic electrostatic forces already present in homostructure devices. Incorporation of hetero-interfaces therefore offers a powerful device design parameter to control the distribution and flow of mobile carriers, greatly improving existing kinds of devices and making new kinds of devices possible. Unusual device requirements can often be met by band lineups occurring in suitable semiconductor combinations. Excellent theoretical rules exist for the semi-quantitative (< ±0.2 eV) prediction of band offsets, even unusual ones, but no quantitatively accurate (< ±l kT) purely theoretical predictive rules are currently available. Poorly-understood second-order nuisance effects, such as small interface charges and small technology-dependent offset variations, act as major limitations in device design. Suitable measurements on device-type structures can provide accurate values for interface physics parameters, but the most widely used measurements are of limited reliability, with pure I—V measurement being of least use. Many of the problems at interfaces between two III/V semiconductors are hugely magnified at interfaces between a compound semiconductor and an elemental one. Large interface charges, and a strong technology dependence of band offsets are to be expected, but can be reduced by deliberate use of certain unconventional crystallographic orientations. An understanding of such polar/nonpolar interfaces is emerging; it is expected to lead to a better understanding and control of III/V-only device interfaces as well.