High Speed Transistor with Double Base Diffusion

A high speed bipolar transistor has been fabricated by using double base diffusion to reduce the base resistance R
B
. The base resistance forms two important time constants, R
B
C
D
and R
B
C
C
(R
L
/R
E
) with the emitter diffusion capacitance C
D
and collector capacitance C
C
dominating the switching delay of the circuits. We demonstrate that the base resistance of a single base diffused device can be reduced by a factor of four by using double base diffusion without affecting its cut-off frequency f
t
= 7 GHz. The double base diffusion also increases the punchthrough voltage of the device from 3 to 7V.