TAILIEUCHUNG - The VLSI Handbook P2

The preferred profile to achieve a good compromise between a too high field at the base-collector junction and suppression of the Kirk effect at high current densities is obtained by a retrograde collector profile [30]. For this profile the SIC implantation energy is chosen to obtain a low impurity concentration near the base-collector junction and then increasing toward the subcollector. The thickness of the epilayer exhibits large variations among different device designs, extending several micrometers in depth for analog bipolar components, whereas a high-speed digital design typically has an epilayer thickness around 1 µm or below, thus reducing the total. | 1-8 The VLSI Handbook 1021 1020 1019 1018 1017 1016 Depth m FIGURE Vertical impurity doping profile with a SIC for a 50 GHz fT fm Di Si-bipolar BiCMOS technology from Malm B. G. et al. IEEE Trans. Electron Dev. vol. 52 1423 2005 . The preferred profile to achieve a good compromise between a too high field at the base-collector junction and suppression of the Kirk effect at high current densities is obtained by a retrograde collector profile 30 . For this profile the SIC implantation energy is chosen to obtain a low impurity concentration near the base-collector junction and then increasing toward the subcollector. The thickness of the epilayer exhibits large variations among different device designs extending several micrometers in depth for analog bipolar components whereas a high-speed digital design typically has an epilayer thickness around 1 pm or below thus reducing the total collector resistance. As a result the transistor breakdown voltage is sometimes determined by reach-through breakdown . full depletion of penetration of the epicollector . The thickness of the collector layer can therefore be used as a parameter in determining BVCEO which in turn is traded off against fT. In cases where fmax is of interest the collector design must be carefully taken into account. Compared with fT the optimum fmax is found for thicker and less doped collector epilayers 32 33 . The vertical collector design will therefore to a large extent determine the trade-off between fT and fmax. Base Region The width and peak concentration of the base profile are two of the most fundamental parameters in vertical profile design. In a conventional Si bipolar process the base width is limited by the implantation energy and to some extent the collector doping since an implanted profile will have a Gaussian tail toward the collector. The base width WB is normally in the range pm whereas a typical base peak concentration lies between 1017 and 1018 .

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