© 2012 Eric Pop, UIUCECE 340: Semiconductor Electronics ECE 340 Lecture 41 Other Modes of BJT Operation; Common-Emitter Gain So far we’ve studied BJT only in “normal active” mode (for PNP, V EB >0 forward biased, V CB < 0 reverse biased). Here, well-designed devices have high gain (β = ); most useful. What about operation in other bias conditions? “Inverted” mode: CB forward-biased, EB reverse-biased. “Cut-off” mode: both junctions reverse-biased. Negligible currents (~1 pA), nearly an open circuit  useful. 1

© 2012 Eric Pop, UIUCECE 340: Semiconductor Electronics “Saturation” mode: Both EB and CB junctions are forward-biased. Base is flooded with minority carriers, i.e. _________ for a PNP.  No isolation, slow recovery, in practice  avoid.  But, interesting to study because it is a superposition of “forward” and “inverted” modes of operation We can still write down the minority carrier concentration in the Base, at the E and C side: Δp E = Δp C = Where p n = __________. 2

© 2012 Eric Pop, UIUCECE 340: Semiconductor Electronics So I can write the hole diffusion current in the Base: And the Base current due to recombination: Key observation: minority carrier diffusion equation is linear, so all BJT operation modes can be written as a superposition of the “normal active” and “inverted” modes. 3 (note that I C and I E can have either sign depending on voltages.)

© 2012 Eric Pop, UIUCECE 340: Semiconductor Electronics Ex: Common-Emitter configuration. Draw circuit: Draw current-voltage diagram:  At low V EC both CB and EB diodes are _________________. Consequently the transistor is in ________________.  At higher V EC, CB diode goes into deeper and deeper reverse bias, and EB stays forward-biased with roughly constant V EB ~ _________. 4

© 2012 Eric Pop, UIUCECE 340: Semiconductor Electronics As the CB diode is pushed into deeper reverse bias, the CB depletion region _____________. This effectively modulates (decreases) the ___________. Which also modulates (increases) the _____________. 5 Experimental Data

© 2012 Eric Pop, UIUCECE 340: Semiconductor Electronics Ideally, we do not want the gain to change due to Base width modulation. During design, we have three options: 1) Increase the Base width, W B 2) Increase the Base doping, N B 3) Decrease the Collector doping, N C Which one is the most acceptable in practice? What if we really push V EC until the transistor breaks?  Punch-through (W B  0)  Avalanche breakdown