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Modulation-Doped Superlattice Base Heterojunction Bipolar Transistor (Modhbt)

IP.com Disclosure Number: IPCOM000035921D
Original Publication Date: 1989-Aug-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 3 page(s) / 63K

Publishing Venue

IBM

Related People

Patton, GL: AUTHOR [+3]

Abstract

Disclosed is an approach for obtaining a simultaneous enhancement of the vertical and horizontal transport of minority and majority carriers, respectively, in the base region of a bipolar transistor by tailoring of bandgap, doping, and lattice constant. (Image Omitted)

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Modulation-Doped Superlattice Base Heterojunction Bipolar Transistor (Modhbt)

Disclosed is an approach for obtaining a simultaneous enhancement of the vertical and horizontal transport of minority and majority carriers, respectively, in the base region of a bipolar transistor by tailoring of bandgap, doping, and lattice constant.

(Image Omitted)

In a bipolar transistor, the collector and base currents flow perpendicular to each other in the base region: the collector current flows vertically through the base (in a vertical device) and the base current flows laterally along the base- emitter junction. For high frequency applications, the mobility in both of these directions should be high: the transit time of minority carriers in the base is reduced by a high mobility/diffusivity in the vertical direction, while the base series resistance is reduced by a high mobility in the lateral direction.

To reduce the base series resistance, the use of a modulation- doped superlattice structure [1] for the base of a heterojunction bipolar transistor is disclosed here. The superlattice structure, which is illustrated in Fig. 1, is a multilayered heterojunction formed of alternating layers of wide-gap and narrow- gap material, each layer having a typical thickness of 5 to 10 nm. In this structure, only the wide-gap material is doped. Due to the band discontinuity, mobile carriers move from the wide-gap to the narrow-gap material and are confined there. Because the carriers are physically separated from the parent atoms, majority carrier mobility is greatly increased, especially at low temperature where impurity scattering is normally dominant. Such a structure could be fabricated using low temperature epitaxy (i.e., molecular beam epitaxy).

A special embodiment of the idea of the MOdulation-Doped superlattice-base Heterojunction Bipolar Transistor (MODHBT) is that the superlattice structure enhances, rather than degrades (as would be expected), the vertical transport of minority carriers through the base. In the case of an NPN bipolar transistor, the presence of any type of barrier or well in the conduction band of the base material would increase charge storage and reduce the collector current and gain of the device. Even in the case where these barriers are designed to permit resonant tunneling [2], the available output current is unacceptably low for VLSI applications.

To minimize these barriers and maximize the base transport factor, the superlattice base has the following key features:

1. The materials chosen for the superlattice base must result in a discontinuity in the valence band (NPN's) which is large enough to confine the carriers which spill into the...