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Use of Heterojunctions as Emitter Contacts in High Gain, High Speed Si Transistors

IP.com Disclosure Number: IPCOM000089179D
Original Publication Date: 1977-Sep-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 2 page(s) / 34K

Publishing Venue

IBM

Related People

Dumke, WP: AUTHOR

Abstract

In high-speed Si transistors having shallow emitter-bade junctions, it is sometimes difficult to obtain sufficient current gain, particularly at low temperatures. The mechanism limiting the current gain is believed to be recombination of holes, injected from the base into the emitter, at the emitter contact. One method of preventing (reducing to a very low value) this injection would be to have a heterojunction emitter using a semiconductor with a larger band gap than Si in the emitter [2]. It is doubtful in such a device, however, that the abrupt emitter doping profiles achievable with As-diffused emitters could be obtained. These abrupt profiles seem to be essential to the speed of high performance Si logic bipolars.

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Use of Heterojunctions as Emitter Contacts in High Gain, High Speed Si Transistors

In high-speed Si transistors having shallow emitter-bade junctions, it is sometimes difficult to obtain sufficient current gain, particularly at low temperatures. The mechanism limiting the current gain is believed to be recombination of holes, injected from the base into the emitter, at the emitter contact. One method of preventing (reducing to a very low value) this injection would be to have a heterojunction emitter using a semiconductor with a larger band gap than Si in the emitter [2]. It is doubtful in such a device, however, that the abrupt emitter doping profiles achievable with As-diffused emitters could be obtained. These abrupt profiles seem to be essential to the speed of high performance Si logic bipolars.

The structure proposed here is a transistor with a shallow Si emitter and a larger bandgap semiconductor contacting the emitter in a n-n isotype heterojunction. The large bandgap semiconductor could be selected to have a lattice parameter closely matching that of Si as is the case with CaP, AlP, and ZnS [3]. GaP [4] would be a particularly good candidate since it appears to have the same electron affinity as Si and makes ohmic n-n heterojunctions with Si even at 78 degrees K. Because of the higher bandgap, holes injected into the emitter would see a confining barrier at the heterojunction without encountering the usual extremely high surface recombination velocity ch...