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High Speed PNP Transistors

IP.com Disclosure Number: IPCOM000083892D
Original Publication Date: 1975-Aug-01
Included in the Prior Art Database: 2005-Mar-01
Document File: 3 page(s) / 41K

Publishing Venue

IBM

Related People

Malaviya, SD: AUTHOR

Abstract

Lateral transistors are the only PNP devices available in the present technologies which primarily use vertical NPN devices. However, normal process tolerances lead to poor Betas with corresponding slower speeds and lower gains.

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High Speed PNP Transistors

Lateral transistors are the only PNP devices available in the present technologies which primarily use vertical NPN devices. However, normal process tolerances lead to poor Betas with corresponding slower speeds and lower gains.

Fig. 1 is an improved lateral PNP transistor which is compatible with the standard NPN process. The Schottky contact for the collector permits closer lateral spacing, by eliminating the out-diffusion tolerance that is associated with conventional P+ diffusions, helping to improve Beta.

The P- emitter region (which corresponds to the base of an NPN device) is surrounded by a Schottky barrier contact to the N- epitaxial layer. The Schottky collector contact is surrounded by a ring of shallow base diffusion (corresponding to the emitter diffusion of an NPN device). A subcollector is not required for this device. The standard techniques of isolation (P+ and/or SiO(2)) can also be used here.

Holes emitted by the emitter are swept away by the surrounding depletion region under the Schottky contact, which is held at a negative voltage. Holes that are not collected by the Schottky become base current by recombining with electrons in the epitaxial layer.

The base tends to make the upper portion of the epitaxial layer are forward biased than the lower portion, which increases the current density near the surface where there is a better chance of its being swept away by the Schottky collector, thereby improving Beta. Also, holes emitted by the bottom surface of the emitter have less chance of ending up at the collector than those emitted from the sides, especially from the sides near the top surface of the device.

Thus, it is desirable to use a shallow P- diffusion for the emitter with a large sidewall-to-bottom surface ratio. This can be obtained by using a ring-like geometry, as in Fig. 2, where the emitter diffusion 10 comp...