ELE533 Bipolar Devices
Final Exam

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 = (w_e/w_b)(u_n/u_p)(N_E/N_AB)

      Let N_e = N_eff = 10¹⁸

      In the base where doping is 10¹⁷ , u_n = 1200

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

      ß = (w_e/w_b)(u_n/u_p)(N_eff/N_AB) = (1.5/1)(1200/100)(10¹⁸/10¹⁷ = 180

   If the collector current is 1 mA, find:
   2. gm______gm = Ic/V_T = 1/26______

   3. r_pi , the incremental base-emitter resistance_____r_pi = ß/gm = 4.68K__

   4. Cd, the base-emitter diffusion capacitance__C_diff = t_bb gm = 6.2 pF_

      where t_bb = w_b²/(2 D_n) = 10^-8/(2*31) = 0.16 nSec.

      D_n = V_T u_n = 0.0259 * 1200

   5. Assume the diffusion capacitance is much greater than the base emitter and base collector junction capacitances and find;

      F_T, the frequency at which the magnitude of beta is one___F_T = 1/(2 pi t_bb) = 995 MHz__