Transistors
Transistors are the switches and amplifiers at the heart of all modern electronics. A single chip may contain tens of billions of them.
Invention
The transistor was invented at Bell Labs in 1947 by John Bardeen, Walter Brattain, and William Shockley, who shared the 1956 Nobel Prize in Physics. Replacing bulky, power-hungry vacuum tubes, transistors made portable radios, computers, and eventually integrated circuits possible.
Bipolar junction transistors
A bipolar junction transistor (BJT) has three regions, emitter, base, and collector, in NPN or PNP arrangements. A small base current controls a much larger collector current: \(I_C = \beta I_B\), where the current gain \(\beta\) is typically 50 to 300. For an NPN transistor to conduct, the base-emitter junction must be forward biased, about 0.7 V for silicon.
BJT operating regions
In the cut-off region, with too little base current, the transistor is off. In the active region, the collector current is proportional to base current, which is used for amplification. In saturation, the transistor is fully on, with a small collector-emitter voltage, about 0.2 V, which is used for switching. Designers choose resistor values to place the transistor in the intended region.
MOSFETs
Metal-oxide-semiconductor field-effect transistors (MOSFETs) control current between drain and source using the voltage on an insulated gate. Because the gate draws essentially no steady current, MOSFETs are efficient switches and dominate integrated circuits and power electronics. An n-channel enhancement MOSFET turns on when the gate-source voltage exceeds its threshold, often 1 to 3 V for logic-level parts.
CMOS
Complementary MOS (CMOS) logic pairs n-channel and p-channel MOSFETs so that, in steady state, one is always off, and almost no current flows except while switching. This low power use made CMOS the technology of virtually all processors and memories. Moore's law, the doubling of transistor counts roughly every two years, was driven by shrinking MOSFETs.
Transistors as switches
A microcontroller pin can supply only a few milliamperes, but a transistor lets it control motors, relays, and LED strips needing amperes. Driving the base through a resistor, or the gate directly with a logic-level MOSFET, switches the load on and off. Inductive loads need flyback diodes, and power transistors need heat sinks when they dissipate significant power.
Transistors as amplifiers
Biased in the active region, a transistor amplifies small signals: a small change in base current or gate voltage produces a larger change in collector or drain current, which a load resistor turns into a larger voltage. The common-emitter amplifier provides voltage gain with phase inversion; the emitter follower provides current gain with a voltage gain of about one, useful for driving loads.
Worked example
An NPN transistor with \(\beta = 100\) must switch a 100 mA lamp from a 5 V microcontroller pin. The base current needed is at least \(\frac{100}{100} = 1\) mA; designers use about 5 times more, 5 mA, to ensure saturation. The base resistor is \(R_B = \frac{5 - 0.7}{0.005} = 860\) \(\Omega\); a standard 820 \(\Omega\) resistor gives about 5.2 mA, within the pin's typical capability.
Key takeaways
The transistor, invented at Bell Labs in 1947 (Nobel Prize 1956), replaced vacuum tubes. BJTs (NPN, PNP) let a small base current control a collector current, I_C = beta I_B, with about 0.7 V base-emitter drop; they operate in cut-off, active (amplifying), or saturation (switching). MOSFETs control drain current with gate voltage and draw almost no gate current; logic-level parts switch at a few volts. CMOS pairs n- and p-channel MOSFETs for very low power and powers nearly all chips. Transistors switch large loads from small signals, with flyback diodes and heat sinks as needed, and amplify signals in configurations such as common emitter and emitter follower.
Flashcard
Bell Labs, 1947
Check your understanding
Why do MOSFETs dominate integrated circuits and power switching?