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• Early view: indivisible sphere ➔ superseded by models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