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121B_1_bjt

2 04 06 08 1 xwb we see that if wb lb i cn i en jason

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Unformatted text preview: / L E - xE sinh W E / LE ) ( ( ) ) with x = - ( x + x E ) + WE ( x + xE ) p ( x ) p ( - x E ) exp LE if W E >> LE WE + ( x + x E ) p ( x ) p ( - x E ) WE Jason Woo if W E << LE EE121B Spring 2010 13 For the collector current, just like the case for PN junctions, it is given by the sum of diffusion currents and recombination currents. * J C = - J n ( x = WB ) - J p ( x = xC ) - J rec Diffusion Currents n p * J rec = - qDB + qDC - x x =W x x = xC 0 except when B breakdown Note the sign! Assuming the collector is long, we have (c.f. PN junction under reversed bias) qDC pC 0 p qDC (e =- LC x x = xC qVBC kT - 1) 0 (since VBC < 0 typically) - 1 i.e., qDB nB 0 W W qV qV JC = csch B exp BE - 1 - coth B exp BC LB kT kT LB LB 0 D p qV * - q C C 0 exp BC - 1 - J rec LC kT 0 Jason Woo 14 EE121B Spring 2010 Similarly for JE - J E = J n ( x = 0 ) + J p ( x = - x E ) + J rec n p = qDB - qDE x n x = 0 + J rec x = - xE Note the sign! W W qDB nB 0 qVBE qV coth B exp = - 1 - csch B exp BC LB kT kT LB LB 0 WE qDE pE 0 qVBE coth + exp - 1 + J rec LE kT LE - 1 qDE pE 0 WE << W = W E if W E * E Jason Woo LE 15 * WE = LE if W E >> LE EE121B Spring 2010 Also, J B = - JC + J E (Kirkoff's law) For practical B...
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