Browse Prior Art Database

Transistor Collector Doping for Reduced Capacitance

IP.com Disclosure Number: IPCOM000046327D
Original Publication Date: 1983-Jul-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 2 page(s) / 33K

Publishing Venue

IBM

Related People

Dumke, WP: AUTHOR

Abstract

In the design of high-speed bipolar transistors there is usually a trade-off determining the allowable impurity concentration in the collector region. The doping must be high enough to avoid significant base stretching, known as the Kirk effect, yet reasonably light to avoid excessive collector capacitance. The doping profile which results from outdiffusion from polysilicon base contacts may be used to obtain lower values of collector capacitance (Cc) providing faster devices, without causing base stretching. When boron-doped polysilicon is used as the diffusion source for the extrinsic base diffusion in a "shallow" bipolar device, the extrinsic base is usually deeper than the collector junction under the emitter. This is shown in Fig. l.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 100% of the total text.

Page 1 of 2

Transistor Collector Doping for Reduced Capacitance

In the design of high-speed bipolar transistors there is usually a trade- off determining the allowable impurity concentration in the collector region. The doping must be high enough to avoid significant base stretching, known as the Kirk effect, yet reasonably light to avoid excessive collector capacitance. The doping profile which results from outdiffusion from polysilicon base contacts may be used to obtain lower values of collector capacitance (Cc) providing faster devices, without causing base stretching. When boron-doped polysilicon is used as the diffusion source for the extrinsic base diffusion in a "shallow" bipolar device, the extrinsic base is usually deeper than the collector junction under the emitter. This is shown in Fig. l.

The variation in depth of the collector junction is used to advantage by fabricating the device in an epitaxial (epi) layer having a donor concentration that is dependent on depth, as shown in Fig. 2 which is correlated with Fig. l. At the depth of the collector junction under the emitter, the donor concentration ND is high enough to avoid base stretching. Under the extrinsic base, the collector doping is low to reduce Cc . The profile of Fig. 2 would be obtained by varying the epi doping during growth.

1

Page 2 of 2

2

[This page contains 2 pictures or other non-text objects]