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Method of Modulating Alloy Concentration in the Base of Heterojunction Bipolar Transistors

IP.com Disclosure Number: IPCOM000012071D
Original Publication Date: 2003-Apr-04
Included in the Prior Art Database: 2003-Apr-04
Document File: 5 page(s) / 111K

Publishing Venue

Motorola

Related People

James K Schaeffer: AUTHOR [+3]

Abstract

A method of enhancing the graded base structure of a high performance SiGe heterojunction bipolar transistor (HBT) through the use of Atomic Layer Deposition (ALD) is described. ALD allows for accurate placement of germanium and boron dopants within the graded base structure. Such improvements in the base profiles of HBTs can enhance the base-to-collector current amplification factor (b), improve collector currents (Ic), lower base resistance Rb, and increase operation frequencies (fmax), compared to HBTs fabricated with a silicon base.

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Method of Modulating Alloy Concentration in the Base of Heterojunction Bipolar Transistors

James K Schaeffer, Bich-Yen Nguyen, Shawn G Thomas

A method of enhancing the graded base structure of a high performance SiGe heterojunction bipolar transistor (HBT) through the use of Atomic Layer Deposition (ALD) is described.� ALD allows for accurate placement of germanium and boron dopants within the graded base structure.� Such improvements in the base profiles of HBTs can enhance the base-to-collector current amplification factor (b), improve collector currents (Ic), lower base resistance Rb, and increase operation frequencies (fmax), compared to HBTs fabricated with a silicon base.� �

Heterojunction bipolar transistors employ a heavily doped base region, for low base resistance, and a lightly doped emitter.� This doping scheme is opposite that of a traditional BJTs, which use a lightly doped base region and a heavily doped emitter region to maintain high emitter injection efficiency.� Unfortunately, homojunction bipolar transistors suffer from undesirably high base resistance due to their low base doping, limiting their usage in high frequency BJT applications.� For bipolar transistors in high frequency applications the emitter injection efficiency is increased without the heavily doped base requirements by using heterojunctions.� Since the emitter is a wider band gap semiconductor than the base, it is possible to make the barrier for electron injection (qVn) smaller than the hole barrier (qVp).� Carrier injection varies exponentially with barrier height, and thus a small difference between the two barriers can make a large difference in the transport of electrons and holes across the emitter junction.� The band gap offset at the emitter/base junction increases the barrier for injection of holes from the base to the emitter.� This increases b, the base-to-collector current amplification factor.� Other improvements obtained by using a SiGe base include higher collector currents (Ic), lower base resistance Rb, and higher operation frequencies (fmax), compared to HBTs constructed with a silicon base.

Further improvements in the high frequency behavior of heterojunction bipolar transistors can be realized by modulation of the alloy in the base region.� If the alloy composition in the base of an n-p-n transistor is varied such that the band-gap (Eg) decreases slightly from the emitter side to the collector side of the base, a built-in electric field accelerates electrons through the base region.� This field aided base transport is a major advantage of the heterojunction BJT.� The graded SiGe profile increases the Early voltage (Va) and reduces the base transit time (tb).

The graded base structure is traditionally formed via Chemical Vapor Deposition.� Typical graded base structures formed by CVD might utilize multiple, discrete steps in Ge concentration to achieve the net effect of a graded layer to achieve the desired built-in electric field (Fig. 1)...