The Bipolar Junction Transistor (BJT)

William Shockley invented the modern bipolar junction transistor in 1951; being a semiconductor device with very low heat loss, it triggered a technological revolution that eventually let thousands of miniaturised transistors be packed onto a single integrated chip. A BJT is a semiconductor crystal (silicon or germanium) structured as three alternating regions: either an n-type layer sandwiched between two p-type layers (a PNP transistor) or a p-type layer sandwiched between two n-type layers (an NPN transistor), sealed inside a metal or plastic case for protection against moisture. The three regions are named EMITTER, BASE and COLLECTOR, brought out as three ohmic-contact terminals E, B, C. Because a BJT has two internal p-n junctions, it forms two depletion layers, across the emitter-base junction ($J_{EB}$) and the collector-base junction ($J_{CB}$). The circuit symbol carries an arrowhead on the emitter lead, pointing from p to n and showing the direction of conventional current -- in an NPN transistor the arrow points OUT of the emitter (n-to-p-to-n structure), in a PNP transistor it points INTO the emitter. Functionally the three terminals are distinct and NOT interchangeable, because of both their differing physical size and their differing doping level: the emitter is heavily doped, since its job is to SUPPLY majority carriers into the base; the base is very thin ($\sim10^{-6}$ m) and only lightly doped, since its job is merely to pass carriers through with minimal recombination; and the collector is moderately doped but made physically LARGER than the other two, since its job is to COLLECT the carriers the emitter supplied and it must dissipate more power in doing so. Suitable DC bias voltages across the terminals -- called transistor biasing -- set which of three operating modes the transistor is in: FORWARD ACTIVE (emitter-base junction forward biased, co …