Physical Operation of BJTs Structure review Voltage vs. position in an npn structure –cutoff –active –saturation Designing for high  The Early effect Parasitic capacitance (C , C  ) From Prof. J. Hopwood

The npn Bipolar Junction Transistor collectoremitter p-typen + -typen-type base integrated circuit BJT metal silicon oxide doped silicon wafer (chip) EBC npn structure

Voltage inside the npn structure x V(x) ~0.7 volts (for Si) At zero bias (v BE =0, v BC =0), currents balanced, no net current flow  i B =i C = 0 (cutoff) pBpB n+En+E nCnC EBJ=off CBJ=off

Voltage inside the npn structure x V(x) ~0.7 volts (for Si) When (v BE =0.65, v C >v B ) electrons and holes overcome the built-in voltage barrier between the base and emitter i B > 0 and i E > i B (due to n + emitter doping) p (B) n + (E) n (C) EBJ=on CBJ=off iBiB -i E >> i B

Voltage inside the npn structure x V(x) ~0.7 volts (for Si) If the base region is very thin, the electrons injected by the emitter are collected by the positive voltage applied at v C i C  i E >>i B (active region) p (B) n + (E) n (C) EBJ=on CBJ=off iBiB -i E >> i B vCvC -i C

Voltage inside the npn structure x V(x) ~0.7 volts (for Si) If the base region is too thick, the electrons injected by the emitter are lost by recombining with holes in the base before the voltage applied at v C can collect them (another component of base current): i C < i E (active region with low ,  ) p (B) n + (E) n (C) EBJ=onCBJ=off iBiB i E >> i B vCvC iCiC

Voltage inside the npn structure x V(x) If v C drops such that the CBJ is forward biased, the collector no longer is able to gather the injected electrons! These uncollected electrons exit through the base! i B is large and i C is small  Saturation p (B) n + (E) n (C) EBJ=onCBJ=on iBiB -i E >> i B vCvC -i C -i B

How do we achieve high  ? make the base region thin (typ. <1 micron) –this makes the collection efficiency of injected electrons high and decreases the chance of these electrons recombining in the base region make the emitter heavily doped –i E /i B  n (emitter) /p (base)  (emitter doping concentration)/(base doping conc.)   These two quantities are difficult to control precisely! Therefore, the current gain is not uniform among BJTs (except when the BJTs are all made on the same chip... an integrated circuit)

The Early Effect As V C increases, the depletion width of the B-C junction becomes wider. This make the base width more narrow This increases the collection efficiency Finally, i C /i B increases (higher  ) V CE ICIC p (B) n + (E) n (C) p (B) n + (E) n (C)

Parasitic (unwanted) Capacitance Each junction forms a parasitic capacitor: semiconductor/depletion/semiconductor p (B) n + (E) n (C) C  C  At high frequency, (1/j  C  0), these capacitors become short circuits and prevent the BJT from proper operation. This is a fundamental limit on high frequency circuits. CC CC