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Emitter Contacts for High Current Gain in Bipolar Transistors With Shallow Emitter Junctions

IP.com Disclosure Number: IPCOM000088768D
Original Publication Date: 1977-Jul-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 2 page(s) / 47K

Publishing Venue

IBM

Related People

Dumke, WP: AUTHOR [+2]

Abstract

In bipolar transistors with shallow emitter junctions, the dominant mechanism limiting the achievable current gain is the recombination of minority carriers (holes in an p n transistor) at the emitter contact. The usual assumption is that the surface recombination velocity at this surface is infinite, and for the commonly used ohmic contacts, this assumption would appear to be a good approximation.

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Emitter Contacts for High Current Gain in Bipolar Transistors With Shallow Emitter Junctions

In bipolar transistors with shallow emitter junctions, the dominant mechanism limiting the achievable current gain is the recombination of minority carriers (holes in an p n transistor) at the emitter contact. The usual assumption is that the surface recombination velocity at this surface is infinite, and for the commonly used ohmic contacts, this assumption would appear to be a good approximation.

If a contact could be made to the emitter which had a low surface recombination velocity, it would be possible to obtain a much higher current gain, particularly in devices with shallow emitters. This improvement could remove the inadequacy in the DC gain of bipolar transistors at low temperatures.

The usual band profile for a contact to n type Si is shown in Fig. 1. It is seen that there is in the Si close to the interface a well which can trap electrons, holding them there until they recombine with an electron from the metallic contact. This type of contact effectively gives an infinite surface recombination velocity.

In Fig. 2, an alternate structure is shown which has two features. The first is a thin layer of an insulator, such as SiO(2), roughly 20 angstroms thick. The second is a metal with a low work function, such as Mg. Because of the low work function of the Mg, the Si will have a small electron enhancement layer at the interface. The position of the Fermi level in t...