Diode is a two-terminal semiconductor device with strongly asymmetric current-voltage behavior: it conducts readily in one direction and blocks in the other. Diodes steer power and signals, establish voltage references, protect circuits from transients, and — in their photonic variants — convert between electrical and optical signals at the heart of modern AI chip interconnect.

<svg viewBox="0 0 760 470" xmlns="http://www.w3.org/2000/svg" role="img" aria-label="Diode — p-n junction, I-V curve, diode types for semiconductor and AI chip applications">
<defs>
<style>text{font-family:ui-monospace,'Cascadia Code',SFMono-Regular,Consolas,monospace;fill:#c9d1d9}.dim{fill:#8b949e}.hdr{font-size:12px;font-weight:700;fill:#e6edf3}.lrg{font-size:11px;fill:#e6edf3;font-weight:700}.med{font-size:10px}.sm{font-size:9px}</style>
<marker id="di_arr" markerWidth="7" markerHeight="7" refX="6" refY="3" orient="auto"><path d="M0,0 L6,3 L0,6" fill="#8b949e"/></marker>
<marker id="di_earr" markerWidth="7" markerHeight="7" refX="6" refY="3" orient="auto"><path d="M0,0 L6,3 L0,6" fill="#388bfd"/></marker>
<marker id="di_rarr" markerWidth="7" markerHeight="7" refX="6" refY="3" orient="auto"><path d="M0,0 L6,3 L0,6" fill="#f85149"/></marker>
</defs>
<rect width="760" height="470" fill="#0d1117"/>

<!-- ═══ PANEL 1: p-n Junction Cross-Section (x=8,y=8,w=234,h=290) ═══ -->
<rect x="8" y="8" width="234" height="290" rx="6" fill="#161b22"/>
<text x="125" y="27" text-anchor="middle" class="hdr">p-n Junction</text>

<!-- p-type region (left) -->
<rect x="18" y="36" width="90" height="80" rx="3" fill="#0d2d0d" stroke="#3fb950" stroke-width="1.2"/>
<text x="63" y="56" text-anchor="middle" class="med" fill="#3fb950" font-weight="700">p-type</text>
<text x="63" y="70" text-anchor="middle" class="sm dim">acceptors (B)</text>
<!-- Hole symbols -->
<circle cx="32" cy="84" r="5" fill="none" stroke="#3fb950" stroke-width="1.5"/>
<text x="32" y="88" text-anchor="middle" class="sm" fill="#3fb950" font-size="8">+</text>
<circle cx="50" cy="84" r="5" fill="none" stroke="#3fb950" stroke-width="1.5"/>
<text x="50" y="88" text-anchor="middle" class="sm" fill="#3fb950" font-size="8">+</text>
<circle cx="68" cy="84" r="5" fill="none" stroke="#3fb950" stroke-width="1.5"/>
<text x="68" y="88" text-anchor="middle" class="sm" fill="#3fb950" font-size="8">+</text>
<circle cx="86" cy="84" r="5" fill="none" stroke="#3fb950" stroke-width="1.5"/>
<text x="86" y="88" text-anchor="middle" class="sm" fill="#3fb950" font-size="8">+</text>
<circle cx="32" cy="100" r="5" fill="none" stroke="#3fb950" stroke-width="1.5"/>
<text x="32" y="104" text-anchor="middle" class="sm" fill="#3fb950" font-size="8">+</text>
<circle cx="50" cy="100" r="5" fill="none" stroke="#3fb950" stroke-width="1.5"/>
<text x="50" y="104" text-anchor="middle" class="sm" fill="#3fb950" font-size="8">+</text>

<!-- Depletion region (center) -->
<rect x="108" y="36" width="30" height="80" rx="0" fill="#1c2030" stroke="#d29922" stroke-width="1" stroke-dasharray="3,2"/>
<text x="123" y="57" text-anchor="middle" class="sm" fill="#d29922">de-</text>
<text x="123" y="68" text-anchor="middle" class="sm" fill="#d29922">ple-</text>
<text x="123" y="79" text-anchor="middle" class="sm" fill="#d29922">tion</text>
<!-- Space charge: negative on p-side -->
<text x="112" y="95" class="sm" fill="#f85149">&#8722;&#8722;</text>
<!-- Space charge: positive on n-side -->
<text x="126" y="95" class="sm" fill="#3fb950">++</text>

<!-- n-type region (right) -->
<rect x="138" y="36" width="90" height="80" rx="3" fill="#2d0d0d" stroke="#f85149" stroke-width="1.2"/>
<text x="183" y="56" text-anchor="middle" class="med" fill="#f85149" font-weight="700">n-type</text>
<text x="183" y="70" text-anchor="middle" class="sm dim">donors (P/As)</text>
<!-- Electron symbols -->
<circle cx="148" cy="84" r="5" fill="#f85149" opacity="0.7"/>
<text x="148" y="88" text-anchor="middle" class="sm" fill="#e6edf3" font-size="7">&#8722;</text>
<circle cx="166" cy="84" r="5" fill="#f85149" opacity="0.7"/>
<text x="166" y="88" text-anchor="middle" class="sm" fill="#e6edf3" font-size="7">&#8722;</text>
<circle cx="184" cy="84" r="5" fill="#f85149" opacity="0.7"/>
<text x="184" y="88" text-anchor="middle" class="sm" fill="#e6edf3" font-size="7">&#8722;</text>
<circle cx="202" cy="84" r="5" fill="#f85149" opacity="0.7"/>
<text x="202" y="88" text-anchor="middle" class="sm" fill="#e6edf3" font-size="7">&#8722;</text>
<circle cx="148" cy="100" r="5" fill="#f85149" opacity="0.7"/>
<text x="148" y="104" text-anchor="middle" class="sm" fill="#e6edf3" font-size="7">&#8722;</text>
<circle cx="166" cy="100" r="5" fill="#f85149" opacity="0.7"/>
<text x="166" y="104" text-anchor="middle" class="sm" fill="#e6edf3" font-size="7">&#8722;</text>

<!-- Built-in electric field arrow -->
<text x="125" y="130" text-anchor="middle" class="sm" fill="#d29922">Built-in field E&#x2080;</text>
<line x1="155" y1="138" x2="100" y2="138" stroke="#d29922" stroke-width="1.5" marker-end="url(#di_rarr)"/>
<text x="125" y="150" text-anchor="middle" class="sm dim">(n&#8594;p direction, opposes forward bias)</text>

<!-- Energy band diagram -->
<text x="125" y="168" text-anchor="middle" class="sm dim">Band diagram (equilibrium)</text>
<!-- Ec (conduction band) — steps down from p to n -->
<polyline points="20,178 108,178 138,192 228,192" fill="none" stroke="#388bfd" stroke-width="1.5"/>
<text x="22" y="176" class="sm" fill="#388bfd">Ec</text>
<!-- Ef (Fermi level) — flat at equilibrium -->
<line x1="20" y1="200" x2="228" y2="200" stroke="#d29922" stroke-width="1" stroke-dasharray="4,2"/>
<text x="22" y="198" class="sm" fill="#d29922">Ef</text>
<!-- Ev (valence band) — steps down from p to n -->
<polyline points="20,218 108,218 138,232 228,232" fill="none" stroke="#3fb950" stroke-width="1.5"/>
<text x="22" y="230" class="sm" fill="#3fb950">Ev</text>
<!-- Barrier annotation -->
<line x1="110" y1="178" x2="110" y2="192" stroke="#f85149" stroke-width="1" stroke-dasharray="2,2"/>
<text x="112" y="187" class="sm" fill="#f85149">qV&#x2070;</text>
<text x="125" y="250" text-anchor="middle" class="sm dim">V&#x2070; &#8776; 0.6&#8211;0.7 V (Si) &#183; 0.2&#8211;0.4 V (Schottky)</text>
<text x="125" y="264" text-anchor="middle" class="sm dim">Shockley: I = I&#x2080;(e^(qV/nkT) &#8722; 1)</text>
<text x="125" y="278" text-anchor="middle" class="sm dim">n=1 (diffusion) &#183; n=2 (recombination) &#183; I&#x2080;: saturation</text>


<!-- ═══ PANEL 2: I-V Characteristic (x=250,y=8,w=244,h=290) ═══ -->
<rect x="250" y="8" width="244" height="290" rx="6" fill="#161b22"/>
<text x="372" y="27" text-anchor="middle" class="hdr">I-V Characteristic</text>

<!-- Axes -->
<line x1="372" y1="40" x2="372" y2="265" stroke="#30363d" stroke-width="1.5"/>
<line x1="262" y1="190" x2="484" y2="190" stroke="#30363d" stroke-width="1.5"/>
<!-- Axis labels -->
<text x="487" y="194" class="sm dim">V &#8594;</text>
<text x="374" y="36" class="sm dim">I &#8594;</text>
<text x="370" y="194" text-anchor="end" class="sm dim">0</text>

<!-- Forward bias curve (right side, exponential rise) -->
<!-- Points: V=0.3 x=400, small I y=188 -->
<!--         V=0.5 x=420, moderate y=165 -->
<!--         V=0.7 x=440, large   y=100 -->
<!--         V=0.9 x=460, very large y=45 -->
<path d="M372,190 C385,189 400,185 420,165 C440,140 455,100 470,45" fill="none" stroke="#3fb950" stroke-width="2.5"/>
<text x="455" y="85" class="sm" fill="#3fb950">Forward</text>
<text x="455" y="97" class="sm" fill="#3fb950">bias</text>

<!-- Threshold voltage marker -->
<line x1="430" y1="175" x2="430" y2="200" stroke="#d29922" stroke-width="1" stroke-dasharray="2,2"/>
<text x="432" y="210" class="sm" fill="#d29922">V&#x2098; &#8776; 0.7V</text>
<text x="432" y="220" class="sm" fill="#d29922">(Si)</text>

<!-- Reverse bias: small saturation current (right above axis on left side) -->
<line x1="372" y1="193" x2="295" y2="193" stroke="#8b949e" stroke-width="1.5"/>
<text x="285" y="191" text-anchor="end" class="sm dim">&#8722;I&#x2080;</text>

<!-- Breakdown -->
<path d="M295,193 C292,193 288,200 284,240 C280,265 274,275 268,282" fill="none" stroke="#f85149" stroke-width="2.5"/>
<text x="260" y="260" class="sm" fill="#f85149">Break-</text>
<text x="260" y="272" class="sm" fill="#f85149">down</text>
<!-- Breakdown voltage marker -->
<line x1="295" y1="185" x2="295" y2="200" stroke="#f85149" stroke-width="1" stroke-dasharray="2,2"/>
<text x="278" y="183" class="sm" fill="#f85149">&#8722;V&#x2071;</text>

<!-- Region labels -->
<text x="315" y="175" text-anchor="middle" class="sm dim">Reverse</text>
<text x="315" y="185" text-anchor="middle" class="sm dim">bias</text>
<text x="410" y="250" class="sm dim">&#9632; Si: V&#x2098;&#8776;0.7V, V&#x2071;&#8776;&#8722;50..&#8722;1000V</text>
<text x="410" y="262" class="sm dim">&#9632; Ge: V&#x2098;&#8776;0.3V, lower noise</text>
<text x="410" y="274" class="sm dim">&#9632; Schottky: V&#x2098;&#8776;0.2V, fast recovery</text>
<text x="410" y="286" class="sm" fill="#f85149">&#9632; Zener: V&#x2071; precise, used as ref</text>


<!-- ═══ PANEL 3: Diode Types (x=502,y=8,w=250,h=290) ═══ -->
<rect x="502" y="8" width="250" height="290" rx="6" fill="#161b22"/>
<text x="627" y="27" text-anchor="middle" class="hdr">Diode Types</text>

<!-- Rectifier / Signal diode -->
<rect x="512" y="34" width="230" height="44" rx="4" fill="#0d1117" stroke="#3fb950" stroke-width="1.2"/>
<text x="520" y="50" class="sm" fill="#3fb950" font-weight="700">Rectifier / Signal</text>
<text x="520" y="63" class="sm dim">1N4007 (1A, 1000V) &#183; 1N4148 (signal, fast)</text>
<text x="520" y="74" class="sm dim">AC&#8594;DC conversion, clipping, clamping</text>

<!-- Zener -->
<rect x="512" y="84" width="230" height="44" rx="4" fill="#0d1117" stroke="#d29922" stroke-width="1.2"/>
<text x="520" y="100" class="sm" fill="#d29922" font-weight="700">Zener</text>
<text x="520" y="113" class="sm dim">Precise breakdown: 1.8V&#8211;200V &#183; used as voltage ref</text>
<text x="520" y="124" class="sm dim">Power supply rail clamping, IC bandgap reference</text>

<!-- Schottky -->
<rect x="512" y="134" width="230" height="44" rx="4" fill="#0d1117" stroke="#388bfd" stroke-width="1.2"/>
<text x="520" y="150" class="sm" fill="#388bfd" font-weight="700">Schottky (metal-semiconductor)</text>
<text x="520" y="163" class="sm dim">V&#x2098;&#8776;0.2&#8211;0.4V &#183; no minority-carrier storage</text>
<text x="520" y="174" class="sm dim">GHz switching &#183; BEOL clamp, power converters</text>

<!-- Photodiode -->
<rect x="512" y="184" width="110" height="44" rx="4" fill="#0d1117" stroke="#a5d6ff" stroke-width="1.2"/>
<text x="520" y="200" class="sm" fill="#a5d6ff" font-weight="700">Photodiode</text>
<text x="520" y="213" class="sm dim">reverse-bias: photon</text>
<text x="520" y="224" class="sm dim">&#8594; e&#x207B;h&#x207A; pair &#8594; current</text>

<!-- LED -->
<rect x="628" y="184" width="114" height="44" rx="4" fill="#0d1117" stroke="#d29922" stroke-width="1.2"/>
<text x="636" y="200" class="sm" fill="#d29922" font-weight="700">LED</text>
<text x="636" y="213" class="sm dim">forward-bias: e&#x207B;h&#x207A;</text>
<text x="636" y="224" class="sm dim">recombine &#8594; photon</text>

<!-- PIN / Avalanche -->
<rect x="512" y="234" width="110" height="58" rx="4" fill="#0d1117" stroke="#8b949e" stroke-width="1"/>
<text x="520" y="250" class="sm" fill="#c9d1d9" font-weight="700">PIN diode</text>
<text x="520" y="263" class="sm dim">p-intrinsic-n &#183; wide i-layer</text>
<text x="520" y="276" class="sm dim">RF switch, attenuator</text>
<text x="520" y="287" class="sm dim">high reverse V handling</text>

<rect x="628" y="234" width="114" height="58" rx="4" fill="#0d1117" stroke="#f85149" stroke-width="1"/>
<text x="636" y="250" class="sm" fill="#f85149" font-weight="700">Avalanche / TVS</text>
<text x="636" y="263" class="sm dim">ESD protection in ICs</text>
<text x="636" y="276" class="sm dim">clamp transient spikes</text>
<text x="636" y="287" class="sm dim">I/O pad protection</text>


<!-- ═══ CARDS (y=305, h=158) ═══ -->
<!-- Card 1: Forward/Reverse Physics -->
<rect x="8" y="305" width="234" height="158" rx="6" fill="#161b22"/>
<text x="125" y="322" text-anchor="middle" class="lrg">Junction Physics</text>
<text x="16" y="338" class="sm" fill="#3fb950">Forward bias: applied V opposes built-in field</text>
<text x="16" y="352" class="sm dim">&#8594; depletion region narrows &#8594; diffusion current flows</text>
<text x="16" y="366" class="sm dim">&#8594; exponential rise: I = I&#x2080; exp(qV/nkT)</text>
<text x="16" y="382" class="sm" fill="#f85149">Reverse bias: applied V adds to built-in field</text>
<text x="16" y="396" class="sm dim">&#8594; depletion widens &#8594; only tiny drift current (I&#x2080;)</text>
<text x="16" y="410" class="sm dim">&#8594; breakdown: Zener (tunneling) or avalanche (impact ion.)</text>
<text x="16" y="426" class="sm dim">Zener: below 5V &#183; Avalanche: above 5V</text>
<text x="16" y="440" class="sm dim">Carrier storage: minority carriers stored &#8594; reverse</text>
<text x="16" y="454" class="sm dim">recovery time trr &#183; Schottky: no storage &#8594; fast</text>

<!-- Card 2: Key Parameters -->
<rect x="250" y="305" width="244" height="158" rx="6" fill="#161b22"/>
<text x="372" y="322" text-anchor="middle" class="lrg">Key Parameters</text>
<text x="258" y="338" class="sm dim">V&#x2098; : forward voltage drop (temp &#8722;2 mV/&#176;C)</text>
<text x="258" y="352" class="sm dim">I&#x2098; : max forward current (thermal limit)</text>
<text x="258" y="366" class="sm dim">V&#x2071; / BV: breakdown voltage (reverse)</text>
<text x="258" y="380" class="sm dim">I&#x2080; : reverse saturation current (doubles/10&#176;C)</text>
<text x="258" y="394" class="sm dim">C&#x2C7C; : junction capacitance (depletion-width dependent)</text>
<text x="258" y="408" class="sm dim">trr: reverse recovery time (Schottky: ~0 ns)</text>
<text x="258" y="422" class="sm" fill="#d29922">n: ideality factor (1=ideal, 2=recombination dom.)</text>
<text x="258" y="436" class="sm dim">Pdiss = V&#x2098; &#215; I&#x2098; &#8594; must stay below thermal limit</text>
<text x="258" y="450" class="sm dim">SPICE model: Is, n, Rs, Cj0, Vj, M, tt, BV, IBV</text>

<!-- Card 3: AI/Chip Applications -->
<rect x="502" y="305" width="250" height="158" rx="6" fill="#161b22"/>
<text x="627" y="322" text-anchor="middle" class="lrg">AI Chip Applications</text>
<text x="510" y="338" class="sm" fill="#f85149">ESD protection: TVS/avalanche on every I/O pad</text>
<text x="510" y="352" class="sm dim">Clamp HBM/CDM events &#8594; protect gate oxide</text>
<text x="510" y="366" class="sm dim">Latch-up prevention: substrate / well diodes</text>
<text x="510" y="380" class="sm dim">Bandgap reference: Zener + BJT &#8594; PTAT+CTAT</text>
<text x="510" y="394" class="sm dim">&#8594; 1.2V reference for voltage regulators on die</text>
<text x="510" y="408" class="sm" fill="#a5d6ff">Silicon photonics: Ge photodiode at 1310/1550 nm</text>
<text x="510" y="422" class="sm dim">&#8594; optical I/O &#8594; Co-packaged optics (CPO)</text>
<text x="510" y="436" class="sm dim">Power: Schottky in VRM &#8594; high-freq, low-drop</text>
<text x="510" y="450" class="sm dim">GaN Schottky: 650V, fast &#8594; 48V rack converters</text>
</svg>

p-n junction physics — when p-type (hole-rich) and n-type (electron-rich) silicon are joined, electrons diffuse toward the p-side and holes toward the n-side, leaving behind ionized dopants that create a built-in electric field pointing from n to p. This field sweeps majority carriers back, establishing equilibrium with a depletion region devoid of free carriers. The built-in potential V₀ (≈0.6–0.7 V for Si) is the barrier that forward bias must overcome.

Shockley equation — I = I₀(exp(qV/nkT) − 1) where I₀ is the reverse saturation current, n is the ideality factor (1 for pure diffusion, 2 when recombination in the depletion region dominates), and kT/q ≈ 26 mV at room temperature. The exponential dependence means a 60 mV increase in forward voltage roughly doubles current (for n=1). I₀ doubles roughly every 10 °C, making leakage current strongly temperature dependent — a critical concern for AI chips operating at high power density.

Reverse bias and breakdown — under reverse bias the depletion region widens and only the tiny drift current I₀ flows until breakdown. Zener breakdown (below ~5 V) is quantum tunneling through the narrow, highly doped depletion region; avalanche breakdown (above ~5 V) is impact ionization — energetic carriers knock loose additional electron-hole pairs. Zener diodes exploit the precision and sharpness of this onset for voltage reference and protection applications.

Schottky diodes replace the p-n junction with a metal-semiconductor interface. The absence of minority-carrier storage eliminates reverse-recovery time (trr ≈ 0 ns vs. tens of nanoseconds for p-n), enabling GHz switching. The lower forward voltage (0.2–0.4 V) reduces conduction losses. Both properties make Schottky diodes essential in high-frequency voltage-regulator modules (VRMs) and as clamping diodes in mixed-signal and RF circuits.

ESD protection in ICs — every I/O pad on an AI chip is guarded by an ESD network: typically a pair of diodes (one to VDD, one to VSS) to clamp transient voltages within a safe range, plus a larger TVS or avalanche structure for human-body-model (HBM) and charged-device-model (CDM) events. At 3 nm nodes, gate oxide is only ~1 nm thick — a millisecond overvoltage of 5 V can permanently rupture it.

Silicon photonics — germanium photodiodes integrated in silicon photonics platforms absorb 1310 and 1550 nm photons (bandgap 0.67 eV) to generate photocurrent, enabling optical-to-electrical conversion in co-packaged optics (CPO) transceivers. The same platform hosts silicon ring-modulator diodes that shift the refractive index under forward bias to modulate the optical carrier. These devices are increasingly the bridge between the AI chip's electrical domain and the fiber-optic scale-out fabric.

Read the diode through a junction-barrier lens rather than a switch lens: the exponential I-V and all the failure modes (thermal runaway, latch-up, ESD rupture) arise from the physics of that depletion-region barrier — understanding the barrier is understanding the device.