121B_1_bjt

In the common emitter case under normal mode

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Unformatted text preview: ICEO i.e., when M (VCB )T = M ( BVCEO )T = 1 OF = 1 With the expression for M, we have M ( BVCEO )T Jason Woo 1 = m T 1 - ( BVCEO / BVCBO ) 42 =1 EE121B Spring 2010 We therefore have: BVCEO = BVCBO (1 - T )1/ m as T = o 1 BVCEO < BVCBO Since o 1 o = 1 - o 1 - o BVCEO 1 BVCBO o 1/ m we have Jason Woo 43 EE121B Spring 2010 Jason Woo 44 EE121B Spring 2010 Dynamic Behavior of Bipolar Transistors AC model of a BJT: hybrid- model RL C B v rbb B E b rbb rbe C b c b c C b e ib rbc gm be ro e Jason Woo 45 EE121B Spring 2010 and I BE kT kT rbe = = = qI BE qI C VBE C b e Q B = VBE -1 with QB = QBdep + QBf + QBimg QBdep is the base-emitter depletion charge. QBf = Aq WB 0 ndx is the charges stored in the quasi-neutral base region 2 i Since Jason Woo qVBE qn DB exp kT J n = qn ( x ) ( x ) = N BEWB 46 EE121B Spring 2010 QBf = AJ n QBf VBE WB 0 1 dx ( x) and qI C = B kT where B 0 WB 1 dx ( x) is the base transit time. For uniformly doped base n ( 0 ) ( W B - x ) n ( x ) = WB n ( 0 ) J n = qDn WB Jason Woo 47 EE121B Spring 2010 i.e., QBf Jn = Dn (W WB 0 B - x )dx 2 J nWB = 2 Dn WB2 B = 2 Dn For non-uniformly doped base, since the built-in E-field actually increases the electron velocity across the base 2 WB B = Dn with > 2 Jason Woo 48 EE121B Spring 2010 QBing is the "hole charge" stored in the base, due to the electrons in the base-collector depletion region. First, the electrons stored in the depletion region is Qelecdep = AJ n WB + xc WB 1 dx ( x) AJ n xc sat The hole c...
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