Transistor Collector Doping for Reduced Capacitance
Original Publication Date: 1983-Jul-01
Included in the Prior Art Database: 2005-Feb-07
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.