SiGe HBT Multi-Finger Structure Reducing Junction Capacitance

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Solution Overview

Problem

SiGe HBT multi-finger devices face challenges with high junction capacitance and collector output resistance, limiting device speed and frequency characteristics due to large buried layers and uneven current distribution.

Innovation Solution

The SiGe HBT multi-finger structure is redesigned to share collectors between neighboring cells (C/BEBC/BEBC/.../C), using n-type pseudo buried layers and deep trench contacts to reduce capacitance and increase collector density, with phosphorus ion implantation and specific dielectric layer thicknesses to minimize ion implantation into active regions.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a large buried layer is used to connect collectors in multi-finger structure, then collector connection is achieved, but junction capacitance increases and device speed decreases

Engineering Contradiction:

Improvecollector connectionVSAvoiddevice speed

Core Design Contradiction:

ReliabilityVSSpeed

Solution Approach 1:

The patent divides the single large buried layer into multiple smaller buried layers, each serving a specific collector region. This segmentation reduces the total junction area and capacitance while maintaining proper collector connections. Each buried layer is localized under its corresponding collector, eliminating the need for a continuous large-area buried layer spanning the entire multi-finger structure.

Inventive Principle:

Principle #1Segmentation

2Reliability

If collector area is increased to reduce current density, then current distribution is improved, but junction capacitance increases

Engineering Contradiction:

Improvecurrent distributionVSAvoidjunction capacitance

Core Design Contradiction:

ReliabilityVSQuantity of substance

Solution Approach 1:

The patent applies local quality by creating collector regions with optimized local areas and doping concentrations. Each collector region is tailored to its specific current requirements, with buried layers positioned precisely where needed. This allows current density to be managed locally without requiring a globally large collector area, thus avoiding increased junction capacitance.

Inventive Principle:

Principle #3Local quality

3Productivity

If multi-finger structure is used to increase current driving capability, then current capacity is improved, but device area increases

Engineering Contradiction:

Improvecurrent driving capabilityVSAvoiddevice area

Core Design Contradiction:

ProductivityVSArea of stationary object

Solution Approach 1:

The patent utilizes vertical dimensionality by implementing multi-layer collector structures with buried layers at different depths and positions. This allows current paths to be routed through the vertical dimension rather than requiring proportional horizontal area expansion. The multi-finger structure is optimized by stacking collector regions and buried layers vertically, increasing current capacity without linearly increasing device footprint.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This configuration significantly reduces junction capacitance and collector resistance, improving cut-off frequency characteristics without increasing the device area, by distributing current more evenly and shortening the collector current path.

Implementation Method 1

The implantation impurity of the two pseudo buried layers 202 is phosphorous with a dosage of 1e14 ∼1e16 cm−2 and an energy of 2 ∼50 KeV

Methodology Applied

Scientific EffectIon implantation: Ion Implantation

Implementation Method 2

the emitter injection efficiency and current gain can be increased due to band gap difference of SiGe base to silicon emitter

Methodology Applied

Scientific EffectBand gap difference:

Implementation Method 3

base resistance can be reduced and fT can be increased by heavily doped SiGe base

Methodology Applied

Scientific EffectHeavy doping:

Data Source

PatentUS8227832B2SiGe heterojunction bipolar transistor multi-finger structure

Publication Date: 2012.07.24 SHANGHAI HUAHONG GRACE SEMICON MFG CORP

AI summary

The present invention provides a multi-finger structure of a SiGe heterojunction bipolar transistor (HBT). It is consisted of plural SiGe HBT single cells. The multi-finger structure is in a form of C/BEBC/BEBC/.../C, wherein, C, B, E respectively stands for collector, base and emitter; CBEBC stands for a SiGe HBT single cell. The collector region is consisted of an n type ion implanted layer inside the active region. The bottom of the implanted layer is connected to two n type pseudo buried layers. The two pseudo buried layers are formed through implantation to the bottom of the shallow trenches that surround the collector active region. Two collectors are picked up by deep trench contact through the field oxide above the two pseudo buried layers. The present invention can reduce junction capacitance, decrease collector electrode output resistance, and improve device frequency characteristics.