## ELE533 Bipolar Devices Final Exam

August 9, 2002

NAME___________________________________________________________

1. Consider a silicon npn transistor with a base width of 1.0 micron, base doping, Na = 1017 acceptors/cm3, emitter doping, Nde = 5x1019 donors/cm3, emitter width is 1.5 microns, electron and hole life times are 10 -6 sec. in the emitter and 10 -9 sec. in the base, the emitter area is 20 microns by 40 microns.

Find:

1. beta___180 _______

ß = (we/wb)(un/up)(NE/NAB)

Let Ne = Neff = 1018

In the base where doping is 1017 , un = 1200

In the emitter where doping is 5x1019 , up = 100

ß = (we/wb)(un/up)(Neff/NAB) = (1.5/1)(1200/100)(1018/1017 = 180

If the collector current is 1 mA, find:

1. gm______gm = Ic/VT = 1/26______

2. rpi , the incremental base-emitter resistance_____rpi = ß/gm = 4.68K__

3. Cd, the base-emitter diffusion capacitance__Cdiff = tbb gm = 6.2 pF_

where tbb = wb2/(2 Dn) = 10-8/(2*31) = 0.16 nSec.

Dn = VT un = 0.0259 * 1200

4. Assume the diffusion capacitance is much greater than the base emitter and base collector junction capacitances and find;
FT, the frequency at which the magnitude of beta is one___FT = 1/(2 pi tbb) = 995 MHz__