Semiconductor Diodes and Transistors: Invention, Function, and Role in Computing

Semiconductor diodes and transistors revolutionized electronics by replacing bulky vacuum tubes with smaller, more efficient solid-state devices. Made from materials like silicon or germanium, they enabled the miniaturization and reliability that powered the second generation of computers in the late 1950s and early 1960s.

Semiconductor Diode

Invented in 1904 by John Ambrose Fleming (vacuum version) but realized in semiconductor form by Russell Ohl in 1940 at Bell Labs (p-n junction diode). Practical silicon diodes emerged in the 1950s.

Types and Function

Rectifier Diode: Basic p-n junction; allows current in one direction (forward bias) when voltage exceeds ~0.7V (silicon); blocks reverse flow. Used for AC-to-DC conversion.
Zener Diode: Operates in reverse breakdown for voltage regulation; maintains constant voltage drop.
LED (Light-Emitting Diode): Emits light in forward bias; used for indicators.
Schottky Diode: Metal-semiconductor junction; low forward drop (~0.3V), fast switching for high-frequency applications.

Function: Current flows when positive voltage is applied to p-side (anode) relative to n-side (cathode); exploits doping to create a depletion region that acts as a one-way valve.

Semiconductor Transistor

Invented in 1947 by John Bardeen, Walter Brattain, and William Shockley at Bell Labs (point-contact transistor); Shockley's bipolar junction transistor (BJT) followed in 1948. The MOSFET (field-effect transistor) was theorized in the 1920s but practical in 1960 by Dawon Kahng and Martin Atalla at Bell Labs.

Types and Function

Bipolar Junction Transistor (BJT): NPN or PNP; base current controls larger collector-emitter current (amplification/switching). Gain (ß) typically 100-300. Used in analog amps and digital logic.
Field-Effect Transistor (FET): JFET or MOSFET; voltage on gate controls channel conductivity. MOSFET (e.g., NMOS/PMOS) is most common in ICs—low power, high input impedance.
Other Types: Darlington (high gain composite), IGBT (power switching), phototransistor (light-sensitive).

Function: Acts as a switch (on/off in digital) or amplifier (proportional control in analog). In switching, base/gate signal toggles the device between cutoff (off) and saturation (on).

Advantages Over Vacuum Tubes

Size: Tiny (millimeters) vs. tubes (centimeters); enabled compact designs and integration into chips.
Power Efficiency: Low power consumption (mW vs. W per tube); no filament heating required.
Reliability and Longevity: Solid-state—no burnout; MTBF in years vs. hours for tubes.
Speed: Faster switching (nanoseconds vs. microseconds); cooler operation reduced thermal issues.
Cost: Mass-producible; prices dropped rapidly with silicon planar process (1959 by Fairchild).

First Use of Transistors in Computers

Transistors marked the "second generation" of computers (1955-1965).

Earliest: Bell Labs' TRADIC (1954-1955): First fully transistorized computer; 800 point-contact transistors; flew in a bomber for radar processing.
Commercial: IBM 608 (1957): First all-transistor calculator; followed by IBM 7090 (1959), a mainframe with over 50,000 transistors for scientific computing.
Impact: Smaller machines (room-sized vs. building-sized), lower power (kW vs. hundreds of kW), higher reliability (weeks between failures vs. daily). Enabled widespread adoption in business and science; paved way for integrated circuits.

Legacy

Transistors and diodes made modern computing possible, leading to Moore's Law and today's billions-of-transistor chips. From ENIAC's 18,000 tubes to the transistor's debut, they shrank computers from rooms to pockets while boosting speed and efficiency exponentially.

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