7.4 Electronic Semiconductors, Transistors, Diodes & Logic Gates

7.4 Electronic Semiconductors, Transistors, Diodes & Logic Gates

Quick Answer: Solid-state electronics are built from semiconductors (Silicon). Doping creates N-type (electron-rich) and P-type (hole-rich) materials. Combining them forms P-N Junction Diodes (one-way valves for AC rectification and voltage regulation) and Transistors (BJTs/MOSFETs used as switches and amplifiers). In digital systems, these components form Digital Logic Gates (AND, OR, NOT, NAND, NOR, XOR) that perform binary logic operations.

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1. Semiconductor Physics & Doping

Pure semiconductor materials—such as Silicon (Si) and Germanium (Ge)—have 4 valence electrons in their outer shell. At room temperature, pure semiconductors act as poor conductors.

                  [ Semiconductor Doping Process ]

     N-TYPE SEMICONDUCTOR                 P-TYPE SEMICONDUCTOR
  Doped with Pentavalent (Arsenic)     Doped with Trivalent (Boron)
      [ Extra Free Electron ]               [ Electron Hole (+) ]
      Majority Carrier: Electrons          Majority Carrier: Holes

To increase conductivity, pure silicon undergoes Doping (adding tiny controlled amounts of impurity atoms):

1. N-Type Semiconductor: Silicon is doped with pentavalent elements (5 valence electrons, e.g., Phosphorus, Arsenic, Antimony). This leaves extra free electrons ($N$ for Negative). Majority charge carriers are electrons.
2. P-Type Semiconductor: Silicon is doped with trivalent elements (3 valence electrons, e.g., Boron, Gallium, Indium). This creates electron deficiencies called holes ($P$ for Positive). Majority charge carriers are holes.

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2. Diodes & P-N Junctions

Connecting P-type and N-type silicon creates a P-N Junction Diode, a two-terminal electronic device that acts as a one-way valve for electric current.

                    [ P-N Junction Diode Biasing ]

            FORWARD BIAS                        REVERSE BIAS
     Current Flows (Diode ON)             Current Blocked (Diode OFF)

    + (Anode)      (Cathode) -           - (Anode)      (Cathode) +
    [ P-Type | N-Type ]                  [ P-Type | N-Type ]
    ---> Current Flow --->               x--- Current Blocked ---x

Diode Terminals & Symbol:

• Anode ($A$): Positive terminal (P-type side).
• Cathode ($K$): Negative terminal (N-type side, marked with a band/stripe).

Diode Biasing States:

1. Forward Bias: Anode connected to Positive (+), Cathode to Negative (-). Shrinks the central depletion region. Current flows freely once the barrier voltage is overcome:
   • Silicon Diode Barrier Voltage: $\approx 0.7\text{ Volts}$
   • Germanium Diode Barrier Voltage: $\approx 0.3\text{ Volts}$
2. Reverse Bias: Anode connected to Negative (-), Cathode to Positive (+). Widens the depletion region, blocking current flow entirely (except for negligible micro-ampere leakage current).

Specialized Diode Types:

• Rectifier Diode: Converts AC to DC. Types include Half-wave, Full-wave center-tapped, and Bridge rectifiers (uses 4 diodes).
• Zener Diode: Specially designed to operate safely in reverse breakdown mode at a precise voltage ($V_Z$). Used for DC voltage regulation.
• Light-Emitting Diode (LED): Emits photons (light) when forward-biased.
• Schottky Diode: Ultra-fast switching diode with a low forward voltage drop ($0.2\text{V} - 0.3\text{V}$).

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3. Transistors: BJTs and MOSFETs

Transistors are 3-terminal active semiconductor devices used for signal amplification and solid-state high-speed switching.

1. Bipolar Junction Transistors (BJTs)

BJTs are current-controlled devices consisting of three layers of doped semiconductor material:

                 [ NPN vs PNP Transistor Symbols ]

          NPN Transistor                       PNP Transistor
            Collector                            Collector
                |                                    |
       Base ----|---\(Arrow Out)            Base ----|---/(Arrow In)
                |                                    |
             Emitter                              Emitter
     (Mnemonic: Not Pointing iN)          (Mnemonic: Points iN)

BJT Terminals:

• Base ($B$): The thin central control terminal.
• Collector ($C$): The terminal that collects charge carriers.
• Emitter ($E$): The terminal that emits charge carriers (identified by the arrow).

Fundamental BJT Equations:

$\text{Emitter Current: } I_E = I_B + I_C$ $\text{DC Current Gain (Beta, } \beta \text{ or } h_{FE}\text{): } \beta = \frac{I_C}{I_B} \quad \implies \quad I_C = \beta \times I_B$ A small base current ($I_B$) controls a much larger collector current ($I_C$). Typical $\beta$ values range from 50 to 300.

BJT Operating Modes:

• Cutoff Region: Base current $I_B = 0 \implies$ Transistor is fully OFF (Open Switch).
• Active Region: Transistor operates linearly as a Signal Amplifier ($I_C = \beta I_B$).
• Saturation Region: Base current is high $\implies$ Transistor is fully ON (Closed Switch, $V_{CE} \approx 0.2\text{V}$).

2. Field-Effect Transistors (FETs / MOSFETs)

FETs are voltage-controlled transistors featuring extremely high input impedance. Terminals are Gate ($G$), Drain ($D$), and Source ($S$). Gate voltage controls current flowing between Drain and Source.

4. Digital Logic Gates & Truth Tables

Digital circuits represent information using binary logic levels: HIGH ($1$ / $+5\text{V}$) and LOW ($0$ / $0\text{V}$). Logic Gates are fundamental digital building blocks that process these binary inputs.

Master Logic Gate Reference & Truth Tables

Universal Logic Gates: NAND and NOR gates are called universal gates because any logic gate or digital computer circuit can be constructed using only NAND or only NOR gates.

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5. AFCT EI Semiconductor & Logic Gate Summary Table