diode
**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.
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| Type | V_forward | Breakdown | Key property |
|---|---|---|---|
| Si rectifier | ~0.7 V | 50–1000 V | General purpose |
| Schottky | 0.2–0.4 V | 20–100 V | No minority-carrier storage, GHz switching |
| Zener | ~0.7 V fwd | 1.8–200 V (precise) | Voltage reference / clamp |
| Ge signal | ~0.3 V | ~75 V | Low Vf, low-level detection |
| Ge photodiode | reverse biased | — | Optical receiver at 1310/1550 nm |
| GaN Schottky | ~1.5 V | 650–1200 V | Power switching, 48 V rack |
**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.