Basic Semiconductor Devices and Atomic Structure – Vocabulary Flashcards

The Atom

  • All matter composed of atoms that typically contain electrons, protons, neutrons (normal hydrogen lacks a neutron)
  • Each chemical element → unique atomic structure defined by proton count (atomic number)
  • Early view: indivisible sphere ➔ superseded by models
    • Bohr (planetary) model: electrons orbit dense nucleus in discrete shells
    • Quantum model: more accurate, statistical (probability clouds) but harder to visualise; governed by
    • Wave-Particle Duality
    • Heisenberg Uncertainty Principle
    • Superposition Principle (e.g.
      Schrödinger’s cat thought experiment)
  • Key particles
    • Electron (−), Proton (+), Neutron (0)
    • Nucleus = protons + neutrons
  • Atomic number Z = #protons = #electrons (neutral atom)
  • Electron shells/energy levels numbered n=1,2,3,\dots; max electrons per shell N_e = 2n^{2}
    • Examples: 1st shell 2e⁻, 2nd 8e⁻, 3rd 18e⁻, 4th 32e⁻
  • Valence electrons = electrons in outermost shell → determine chemical & electrical properties
  • Ionisation: supplying ≥ ionisation energy allows valence e⁻ to escape → free electron + positive ion; reverse capture yields negative ion

Materials Used in Electronic Devices

  • Electrical categories
    • Insulators: very high resistivity; tightly-bound valence e⁻ (rubber, glass)
    • Conductors: single-element metals with 1 loosely-bound valence e⁻ (Cu, Ag, Au, Al) → abundant free e⁻
    • Semiconductors: between above; intrinsic Si, Ge, etc. have 4 valence e⁻
  • Band theory
    • Valence band (VB) vs Conduction band (CB)
    • Band gap E_g: energy difference VB→CB
    • Insulators: large E_g (only crossed under breakdown)
    • Semiconductors: moderate E_g (photon/thermal energy can excite e⁻)
    • Conductors: VB and CB overlap (no gap)
  • Silicon vs Copper atoms
    • Si core net +4, valence e⁻ in 3rd shell
    • Cu core net +1, valence e⁻ in 4th shell ➔ Cu e⁻ easier to free
  • Si vs Ge
    • Both 4 valence e⁻; Ge valence e⁻ in 4th shell (higher energy) → more temperature-sensitive
  • Covalent bond: sharing of valence e⁻ between atoms in crystal lattice (e.g. silicon crystal)
    • Intrinsic crystal = pure (no impurities)

Current in Semiconductors

  • At absolute 0\,K, CB empty
  • At room T, thermal energy generates electron–hole pairs (EHP)
    • Free/conduction electrons in CB
    • Corresponding hole in VB
  • Recombination: free e⁻ loses energy and falls into hole
  • Two current mechanisms under applied voltage V
    • Electron current (CB): free e⁻ drift toward + potential
    • Hole current (VB): valence e⁻ move into adjacent holes → holes drift toward – potential
  • Contrast: metals have only free-electron current (no holes)

Extrinsic Semiconductors – Doping

  • Doping adds controlled impurity atoms to increase carriers
  • n-type: add pentavalent (Sb, As, P) donors → extra free e⁻ (majority carriers = electrons, minority = holes)
  • p-type: add trivalent (B, Ga, In) acceptors → create holes (majority = holes, minority = electrons)
  • Doping terminology
    • Majority vs Minority carriers
    • Doping ≠ thermal EHP generation (minority carriers)
  • Intrinsic vs Extrinsic: pure vs doped material

PN Junction Fundamentals

  • Formed by adjoining p and n regions
  • Initial diffusion: e⁻ cross into p side & recombine with holes; holes diffuse opposite → leave charged ion cores
  • Depletion region: immobile ion layers; depleted of mobile carriers
  • Electric field across depletion ➔ barrier potential V_B
    • Typical V_B: Si ≈ 0.7\,\text{V}, Ge ≈ 0.3\,\text{V} (25 °C)
  • Equilibrium: diffusion current balanced by electric-field drift → no net current
  • Energy-band view: “energy hill” across depletion

Diode Operation

  • Two-terminal pn-junction device
  • Forward Bias (FB)
    • n connected to −, p to +; V{BIAS} > VB
    • Depletion narrows; carriers cross; diode conducts
    • Forward voltage drop VF \approx VB + IF r'd (dynamic resistance small)
  • Reverse Bias (RB)
    • p to −, n to +; depletion widens; only tiny reverse current I_R (minority-carrier) flows
    • Reverse breakdown at V_{BR} → avalanche; must limit current

Voltage–Current (V-I) Characteristic

  • FB curve: knee at \approx 0.7\,\text{V} (Si). Above knee, IF increases exponentially; VF nearly constant
  • Dynamic (ac) resistance r'd = \Delta VF / \Delta IF, decreases as IF rises
  • RB curve: negligible IR until V{BR}, then steep increase
  • Complete V-I curve combines both regions; temperature ↑ ⇒
    • V_B decreases ≈ 2\,\text{mV}/^{\circ}!\text{C} (Si)
    • I_R increases

Diode Approximations

  1. Ideal: VF=0, r'd=0, I_R=0 (switch model)
  2. Practical: constant 0.7\,\text{V} drop (Si) in FB; RB open
  3. Complete: includes VB, r'd in FB, large r'R and IR in RB

Rectifiers

Half-Wave Rectifier (HWR)

  • Single diode + load
  • Conducts on one half-cycle ⇒ output pulsating dc (freq = input f)
  • Average output V{AVG}=\dfrac{Vp}{\pi} (≈31.8 % of V_p)
  • Peak inverse voltage \text{PIV}=V_p(\text{in})
  • Transformer coupling: V{sec}=n V{pri} (turns ratio n=N{sec}/N{pri})

Full-Wave Rectifier (FWR)

  1. Center-Tapped (CT): two diodes + CT secondary
    • Each diode conducts on alternate half-cycles
    • Output freq 2f
    • V{p(out)}=V{sec}/2 -0.7\,\text{V}
    • \text{PIV}=2V_{p(out)}+0.7
  2. Bridge: four diodes, no CT
    • V{p(out)}=V{sec}-1.4\,\text{V} (two diode drops)
    • \text{PIV}=V_{p(out)}+0.7 per diode
  • Full-wave average V{AVG}=\dfrac{2Vp}{\pi} (≈63.7 % of V_p)

Power-Supply Filters & Regulators

  • Capacitor-input (π-filter) most common
    • Capacitor charges to peak, discharges through RL ⇒ ripple voltage V{r(pp)}
    • For FWR approx: V{r(pp)} \approx \dfrac{V{p(rect)}}{f R_L C} ( f=120\,\text{Hz} )
    • Ripple factor r=V{r(pp)}/(2\sqrt{2}V{DC}); smaller r = better filtering
  • Surge current occurs at power-on while capacitor uncharged ⇒ use slow-blow fuse in primary
  • Voltage regulators (3-terminal IC)
    • Maintain constant V_{out} vs line or load changes
    • Line regulation: \Delta V{out}/\Delta V{in}
    • Load regulation: \dfrac{V{NL}-V{FL}}{V_{FL}}\times100\%

Zener Diode

  • Heavily-doped junction designed for operation in reverse breakdown at precise V_Z (≈ 1 V – 250 V)
  • Breakdown mechanisms
    • Zener (<5 V): field-ionisation
    • Avalanche (>5 V)
  • Zener equivalent
    • Ideal: constant voltage source V_Z (reverse)
    • Practical: includes small impedance ZZ = \Delta VZ/\Delta I_Z
  • Regulation limits: I{ZK} ≤ IZ ≤ I{ZM} (max via PD(max)=VZ I{ZM})
  • Temperature coefficient TC: \Delta VZ = VZ \times TC \times \Delta T
  • Power derating above 25 °C: P{max}(T)=P{25}-DF\,(T-25)
  • Applications
    • Voltage reference/regulator (series resistor limits current)
    • Limiter/clipper circuits

Varactor (Varicap) Diode

  • Reverse-biased pn junction acts as voltage-controlled capacitor: CT \propto 1/\sqrt{VR}
  • Doping profile & geometry set C{max}, C{min}, C_R (capacitance ratio)
  • Used in tuners, VCOs, filters; often back-to-back for symmetrical characteristics

Optical Diodes

  • Light-Emitting Diode (LED)
    • Forward bias → e⁻-hole recombination releases photons (electroluminescence)
    • Materials dictate colour (GaAs IR, GaAsP red–yellow, GaN blue, InGaN white with phosphor)
    • VF ≈ 1.2–3.2 V; IF 10–30 mA
    • Radiation patterns vary (indicator vs high-intensity)
    • Applications: indicators, 7-segment displays, IR remote, traffic lights (series-parallel arrays), lighting
  • OLED: organic layers produce light; printable; flexible displays
  • Quantum dots: nano-crystals, size-dependent bandgap → colour tuning; used in LED filters, bio-imaging
  • Photodiode
    • Operates in reverse bias; photon absorption generates current proportional to irradiance
    • Dark current (no light) very small
    • Used in detectors, opto-isolators, fiber optics
  • Laser Diode: similar to LED but with resonant cavity → coherent monochromatic light; used in CD/DVD, fibre-optic, laser printers

Other Special-Purpose Diodes

  • Schottky (hot-carrier): metal-semiconductor junction; V_F ≈ 0.3 V; fast switching
  • PIN diode: intrinsic layer between p and n ➔ variable resistance (forward) or capacitance (reverse); RF switches, attenuators
  • Step-Recovery: graded junction; sharp turn-off → harmonic generation
  • Tunnel diode: very heavily doped; exhibits negative resistance region; microwave oscillators
  • Current-Regulator Diode (CRD): maintains constant current IP over wide V{AK} range

Bipolar Junction Transistor (BJT) Structure

  • Three doped regions: Emitter (heavily), Base (light, thin), Collector (moderate)
  • Two pn junctions: Base-Emitter (BE), Base-Collector (BC)
  • Types & symbols: npn (arrow out), pnp (arrow in); arrow indicates conventional emitter current

Basic BJT Operation

  • Forward-reverse bias for linear use: BE junction F-B, BC junction R-B
  • Current flow (npn)
    • Majority e⁻ injected from emitter to base; few recombine (IB); most swept into collector (IC)
  • Kirchhoff: IE = IC + I_B

Key Parameters

  • DC current gain \beta{DC}=h{FE}=IC/IB (20–200+)
  • \alpha{DC}=IC/I_E\approx \beta/(\beta+1)

DC Bias Analysis

  • With bias sources V{BB},V{CC} and resistors RB,RC
  • Approximate V_{BE}=0.7\,\text{V} (Si)
  • IB=(V{BB}-0.7)/RB; IC=\beta I_B
  • V{CE}=V{CC}-IC RC; V{CB}=V{CE}-0.7

Collector Characteristic Curves

  • Family of IC vs V{CE} for various I_B
    • Regions
    • Cutoff (IB≈0, IC≈0, VCE≈VCC)
    • Active/Linear (IC=βIB, BC reverse-biased)
    • Saturation (both junctions forward, IC at I_{C(sat)})
    • Breakdown (avoid)
  • I{C(sat)}=(V{CC}-V{CE(sat)})/RC (neglect V_{CE(sat)} small)
  • I{B(min)}=I{C(sat)}/\beta; design with IB≫I{B(min)}
  • DC load line: line between cutoff point (V{CE}=V{CC}, IC=0) and saturation (V{CE}=V{CE(sat)}, IC=I_{C(sat)}) on characteristic graph

Temperature & Ratings

  • β increases with temperature; device parameters vary
  • Maximum ratings: V{CEO(max)}, V{CBO(max)}, I{C(max)}, P{D(max)}
  • PD = V{CE} I_C; derate above 25 °C using datasheet factor (mW/°C)

BJT as a Voltage Amplifier

  • AC superimposed on DC bias
  • Small-signal emitter resistance r'e ≈ 25\,mV/IE (at 25 °C)
  • Voltage gain (common-emitter, unbypassed) Av = - RC / r'_e (negative sign ⇒ inversion)
  • Output Vo = Av V_{in}

BJT as a Switch

  • Two states
    • Cutoff: IB=0 ⇒ IC\approx0, V{CE}\approx V{CC}
    • Saturation: IB \ge I{B(min)} ⇒ V{CE}\approx V{CE(sat)}\,(\sim0.1–0.3\,V), IC=I{C(sat)}
  • Application example: driving LED; ensure IB > 2 I{B(min)} for safe saturation