121B_1_bjt

# Pnp characteristics can be derived similarly first

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Unformatted text preview: milarly. First, let us look at the DC characteristics. Contact IE E n+ e h IB B p e n+ n Contact IC C I E = I En + I Ep I C = I Cn + I CB 0 I B = I E - I C (Kirkoff's Law) Note that, Jason Woo I Cn I En if the base is small few electrons recombine in the base 6 EE121B Spring 2010 The key action is therefore in the quasi-neutral base, consider a uniformly doped NPN BJT, the equation that governs the electron flow under low level injection is: J n = qDB n x This is very similar to deriving the diffusion currents in PN junction under forward bias. Now in steady state, the continuality equation is: J n = qR = q n n B x where Combined the two equations, we have 2 n n = 2 2 LB x Jason Woo LB = DB B LB is the diffusion length of the minority carriers (electrons) in the base. 7 EE121B Spring 2010 Similarly, in the quasi-neutral emitter region: p J p = -qDE x and J p p = -qR p = q E x 2 p p = 2 2 LE x LE = DE E Combined the two equations, we have where LE is the diffusion length of the minority carriers (holes) in the emitter. Jason Woo 8 EE121B Spring 2010 To solve the equations, we ne...
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