SEMICONDUCTOR DEVICE TYPES

Forward bias: External voltage shrinks depletion region, current flows.
Reverse bias: Depletion widens, blocks current (only leakage).

Key physics: Thin base lets most injected electrons reach collector.
Gain: I_C = β × I_B  (β ~ 50–300).
PNP: Same structure reversed — holes are injected instead.

Heavy doping: Thin depletion → Ev(p) aligns with Ec(n) → quantum tunneling.
Breakdown voltage V_Z: Typically 2–6V. Sharp, stable, reversible.
Usage: Voltage regulation, reference, overvoltage protection.

Radiative recombination: e⁻ (minority) injected into P recombine with h⁺ → photon with E = E_g.
Direct bandgap required: GaAs, GaN, InGaN (not Si — indirect gap, poor emitter).
Color = bandgap: Red (AlGaAs ~1.4eV) → Green (InGaN ~2.3eV) → Blue (GaN ~3.4eV).
N window: Thin N layer allows photons to escape with minimal reabsorption.

Photon path: Photons enter through N-type window layer, penetrate into depletion/P region where absorbed.
Absorption: hν > E_g creates e-h pairs in/near depletion zone → swept by E-field → photocurrent.
PIN structure: Intrinsic layer widens depletion → more absorption volume → faster response.
Usage: Fiber optics, cameras, solar cells (photovoltaic mode at V=0).

Key physics: Mirror of NPN — holes are injected from emitter through thin base to collector.
Gain: I_C = β × I_B (same principle, opposite carrier type).
Usage: Complementary to NPN in push-pull stages, level shifting, current sources.

Trigger: Gate pulse injects carriers → regenerative feedback latches both internal transistors ON.
Latch: Stays on even after gate removed. Must reduce I below I_H to turn off.
J2 is the key junction — reverse-biased in blocking, collapses when triggered.

Depletion mode: Channel exists at V_GS=0. Negative gate voltage widens depletion, narrows channel.
Pinch-off: At V_GS = -V_P, channel fully depleted → no current.
Voltage controlled: Gate draws no DC current (high input impedance).

Enhancement mode: No channel at V_GS=0. Gate voltage > V_th creates inversion layer.
Oxide insulated gate: Zero DC gate current → ultra-high input impedance.
Scaling: Dominant transistor type in digital ICs since ~1980s.

Enhancement mode: No channel at V_GS=0; gate voltage > V_th creates e⁻ inversion layer in P-substrate.
Pull-down switch: Connects output to GND when ON; used in CMOS pull-down network.
Applications: Analog amplifiers, power switching, digital CMOS pull-down.

Enhancement mode: No channel at V_GS=0; gate voltage < -|V_tp| creates h⁺ inversion layer in N-well.
Pull-up switch: Connects output to V_DD when ON; paired with NMOS for CMOS complementary logic.
Applications: CMOS pull-up network, load switches, power management.

Structure: Simplest MOS device — no source/drain, just gate-oxide-semiconductor stack.
C-V behavior: Accumulation → depletion → inversion as V_G sweeps.
Usage: Test structure for oxide quality, threshold voltage extraction, DRAM storage cell.

Linear region: Small V_DS → channel is uniform → device behaves as voltage-controlled resistor.
R depends on: Channel length L (longer = more R), gate voltage (higher V_GS = less R).
Usage: Active loads in amplifiers, analog MUX switches, programmable resistors.

3D structure: Channel is a vertical "fin" wrapped by gate on 3 sides → superior electrostatic control.
Short-channel effects: Greatly reduced vs. planar MOSFET at same node.
Successor: Gate-All-Around (GAA) / nanosheet at 3nm and below.

Vertical cavity: Light bounces between top/bottom Distributed Bragg Reflector mirrors, emits from surface (not edge).
Distributed Bragg Reflector mirrors: Distributed Bragg Reflectors — alternating λ/4 layers give >99% reflectivity.
Oxide aperture: Confines current to small area → low threshold current (<1 mA).
Advantages: Circular beam (easy fiber coupling), 2D array fabrication, wafer-level testing.

Edge emission: Light amplified along the cavity axis (hundreds of μm), emits from cleaved crystal facet.
Facet mirrors: Cleaved semiconductor-air interface gives ~30% reflectivity; rear facet HR-coated for higher R.
Ridge waveguide: Etched ridge confines current injection and optical mode laterally.
High power: Long gain path → higher output power than VCSEL; elliptical beam needs corrective optics.
Key application: Analog CATV Directly Modulated Laser (DML) — FP cavity provides high linearity for analog signals.

Structure: Bragg grating etched directly into the top of the active/guide layer before P-cladding regrowth.
Single mode: Grating selects one wavelength — no mode hopping.
Fabrication: Simplest DFB process — one regrowth step after grating etch.

Buried active: Narrow MQW mesa (~1.5 μm wide) buried in P-InP/N-InP current-blocking layers.
No ridge needed: Blocking layers confine current to the mesa — broad area top contact works.
Best performance: Low threshold, high efficiency, excellent thermal dissipation.
Complex fabrication: Requires mesa etch + two regrowth steps — expensive but worth it for telecom.

Separated grating: Grating etched into separate confinement layer above (or below) active region.
Evanescent coupling: Optical mode tail overlaps grating — index perturbation provides feedback.
λ/4 phase shift: Quarter-wave defect at cavity center breaks degeneracy → guaranteed single mode.
Both facets AR-coated: Grating alone provides all feedback — no need for HR mirror.
Key applications: DWDM telecom, coherent detection, tunable laser arrays, gas sensing.