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CBEC Configuration for High Speed HBT's

IP.com Disclosure Number: IPCOM000202449D
Publication Date: 2010-Dec-15
Document File: 5 page(s) / 104K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a layout structure for a SiGe HBT that reduces the device footprint but increases device speed. The structure has the contact configuration CBEC. Performance of CBEC for various emitter geometries ranging from 0.12 umx0.52 um to 0.12 um x 20 um is demonstrated.

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CBEC Configuration for High Speed HBT's

The cut-off frequency of an HBT is usually increased by increasing the doping density in the collector. This is used to tune fT over a large range from a few GHz to hundreds of GHz. This approach always results in a corresponding degradation of the breakdown voltage (BVCEO), while the fT.BVCEO product remains unchanged. Various layout scenarios whereby the fT can be considerably increased while the BVCEO is unaffected for a smaller device footprint is reported. In this investigation, results of an increase in fT are reported for various device geometries for three layout configurations.

A thorough analysis is done to see the influence of various capacitances and resistances on fT.

Based on these calculations, a contact configuration is proposed which is expected to improve the fT characteristics over geometry.

Fig 1 shows the fT-Ic characteristics for a CBE, CBEBC and CBE-min geometries- here a CBE structure is equivalent in foot print to a CBEBC structure except that the second base and collector contacts are removed. The CBEmin structure is designed to minimum ground rules. The increase of fT for the CBE-min device is attributed to a reduction of Cbc - this is despite the fact that there will be an increase in Rcx and Rb due to the absence of the second B and C contacts. As can be seen, this configuration results not only in an increase in the peak fT but also operates at lower currents for a given frequency. Fig 2 shows that the breakdown voltage is not affected by a CBEmin layout. This means that fT has been increased while preserving the BVCEO.

As the emitter length increases, Figure 3 shows, however, that the peak fT becomes higher for the CBEBC configuration. This is because, the Rcx starts to become more and more important compare...