Transit-time (intrinsic cutoff frequency)

Both logic switching speed and amplifier frequency response are limited by the transit time.

t = W_B²/h D_B + (C_JC + C_sub)r_C + X_C/2V_S

The unity-gain frequency ¦ _T is given by:

¦ _T = 1/2p t

The first time is called the base transit time (time required to move across the base). The second term is the delay associated with the various capacitances and the collector series resistance R_CA.

Basewidth Example

doping profile of bjt

W_B = electrical width of the base
W_met = metallurgical basewidth

The actual basewidth of the transistor is determined by reducing the metallurgical basewidth by the portions of the emitter-base and collector-base space charge regions which protrude into the base.

The space-charge region width is a function of voltage and doping. One must use the charts like in the figures below:

Layout Geometry

The geometry of the transistor determines the current rating and the actual frequency response. A set of design rules for a hypothetical bjt are based on making maximum use of lateral diffusion taking into consideration the IC processing minimum transistor layout.

Other Bipolar Isolation Techniques

 CDI - collector-diffusion-isolation structure was developed primarily for digital applications. The process eliminates the p-type isolation diffusion

V-Groove Isolation - Start out with <100> silicon and use anisotropic etching produces v-grooves which separate the transistor

Dielectric Isolation - anisotropic etch followed by oxidation