ELE533 Bipolar Devices
Final Exam

August 13, 1998

Question 1 Answer

Consider a silicon npn transistor with a base width of 1.0 micron, base doping, Na = 1017 acceptors/cm3, emitter doping, Nde = 5x1018 donors/cm3, emitter width is 1.5 microns, electron and hole life times are 10-6 sec, the emitter area is 20 microns by 50 microns.

Find:

  • the emitter injection efficiency___99.8%_____

    efficiency = Inc/(Inc + Ip) = 1/(1 + 1/ß)

    ß = (We/Wb)(un/up)(Nde/Na) = (1.5)(800/100)(5x1018/1017) = 600

    efficiency = 600/601 = 99.8%

  • beta_____600____

    •  If the collector current is 1 mA, find:

  • gm = Ic/Vt = 1/26____

  • rb, the incremental base input resistance_rb = ß/gm = 600*26 = 15.6K__

  • Cd, the base emitter diffusion capacitance___Cdd = gm*tdd = 9.3 pF__

    tdd = wb2/2Dn = wb2/2 Vt un = 10-8/2*800*.0258 = 0.24 nSec

    Cdd = gm * tdd = 0.24X10-9/26 = 9.3 pF

  • 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_663 MHz__

    FT = 1/(2pi tdd) = 1/(2*3.1415*0.24x10-9 = 663 MHz