THE FIRST TRANSISTOR

Two junctions, three layers

1947 Bardeen, Brattain and Shockley at Bell Labs build the first transistor, a bipolar one, on germanium

Structure An emitter, a very thin base and a collector: n–p–n or p–n–p

Current-controlled A small current into the base steers a much larger current from collector to emitter

Bipolar Both electrons and holes take part, one as the main carrier and one in the base current

A MOSFET is controlled by a field. A bipolar transistor is controlled by a current.

HOW IT WORKS

Inject, cross, collect

I_C = I_S · e^{V_BE ÷ V_T}

A forward-biased emitter junction injects electrons into the base. Because the base is much thinner than the distance electrons travel before recombining, almost all of them reach the collector, which sweeps them out. The few that recombine form the base current, so I_C = βI_B with β often 50 to 200. The collector current grows exponentially with V_BE.

Ten times the current for every 60 mV, at any current.

GAIN PER MICROAMP

Where the bipolar wins

g_m = I_C ÷ V_T

Because the current is exponential, a bipolar transistor's transconductance depends only on the current, not on the geometry: about 39 mA/V per milliamp. A MOSFET in strong inversion delivers a smaller g_m for the same current. The BJT is therefore the choice for the highest-gain, lowest-noise analog stages.

The exponential is the bipolar's strength, and also why it draws constant base current.

WHERE IT LIVES TODAY

Bipolar is not dead

RF and millimeter-wave SiGe heterojunction bipolar transistors (HBTs) reach hundreds of GHz in radar and 5G front ends

BiCMOS Bipolar and CMOS on the same chip: bipolar for the fast analog, CMOS for logic

Voltage references The predictable V_BE of a bipolar is the heart of the bandgap reference inside nearly every chip

Hidden bipolars Every CMOS chip contains parasitic bipolar transistors, and they cause latch-up if the layout is careless

MOSFETs run the logic, but bipolars still run the stable analog corners.

CHECK THE FAMILIES

BJT or MOSFET?

Bipolar: current-controlled, high g_m. MOSFET: field-controlled, almost no gate current.

UNIT 20 STUDY COMPLETE

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Ready for the Fab Challenge?

You've met the bipolar transistor, the device that started it all, and seen where it still beats the MOSFET.