Reverse breakdown - avalanche vs Zener — Hardware notes

WHY does breakdown happen at all?

WHY do we care? Under reverse bias the depletion region widens and holds nearly the whole applied voltage. This region has a huge electric field. Physics offers only two ways for that field to suddenly conjure lots of free carriers — and which one wins depends on how heavily the junction is doped.

Mechanism 1 — Avalanche (impact ionization)

WHY lightly doped? Light doping ⇒ wide depletion region ⇒ the carrier has a long runway to accelerate and gain the ionization energy $E_i$ before colliding.

Deriving the condition (first principles)

A carrier travelling distance $\ell$ (the mean free path) in field $E$ gains energy: $\Delta W = qE\ell$

Why this step? Work = force × distance, and force on charge is $qE$ . To ionize an atom it must reach the ionization energy $E_i$ : $qE\ell \ge E_i \;\Rightarrow\; E \ge \frac{E_i}{q\ell}$

This defines a critical field $E_{crit}$ . Now — how does that relate to voltage? For a wide depletion width $W$ , the voltage needed is roughly $V_{BR}\approx E_{crit}\,W_{\text{eff}}$ , and since lightly-doped junctions have large $W$ , $V_{BR}$ is large.

Temperature signature: Higher $T$ ⇒ more lattice vibration ⇒ carriers collide sooner (shorter effective $\ell$ ) ⇒ they gain less energy per collision ⇒ need a higher field ⇒ $V_{BR}$ increases with $T$ . So avalanche has a positive temperature coefficient.

Mechanism 2 — Zener (band-to-band tunneling)

WHY heavily doped? Heavy doping ⇒ narrow depletion region ⇒ even a modest voltage produces a colossal field ( $E=V/W$ , small $W$ ) ⇒ the tunneling barrier becomes thin enough to punch through.

Temperature signature: Higher $T$ shrinks the band gap $E_g$ slightly ⇒ easier to tunnel ⇒ breakdown happens at lower voltage ⇒ Zener has a negative temperature coefficient.

Figure — Reverse breakdown - avalanche vs Zener

The 80/20 comparison table

Feature	Zener	Avalanche
Mechanism	Tunneling	Impact ionization
Doping	Heavy	Light
Depletion width	Narrow	Wide
Typical $V_{BR}$	$< 5$ V	$> 6$ V
Temp. coefficient	Negative	Positive
The "crossover"	Around $\approx 5$ – $6$ V both occur; TC $\approx 0$	—

Common mistakes (steel-manned)

Flashcards

Reverse breakdown

Sudden large rise in reverse current when

V_R

reaches critical

V_{BR}

Avalanche mechanism

Impact ionization — fast carriers knock electrons out of bonds, a chain reaction.

Zener mechanism

Quantum tunneling of electrons across a narrow band gap under a very high field.

Avalanche needs which doping?

Light doping → wide depletion → long acceleration runway.

Zener needs which doping?

Heavy doping → narrow depletion → huge field → tunneling.

Temp coefficient of avalanche

Positive (

V_{BR}

rises with temperature).

Temp coefficient of Zener

Negative (

V_{BR}

falls with temperature).

Approx voltage where mechanisms cross over

~5–6 V (TC ≈ 0, very stable references).

Multiplication factor formula

M = 1/[1-(V_R/V_{BR})^n]

Is breakdown destructive?

No, if current is limited; destruction only from excess power/heat.

Why does avalanche

V_{BR}

rise with T?

More lattice vibration → shorter mean free path → carriers gain less energy → need higher field.

Why does Zener

V_{BR}

fall with T?

Band gap shrinks → easier tunneling → breakdown at lower voltage.

Recall Feynman: explain to a 12-year-old

Imagine a crowd trying to push through a fence (the diode blocking reverse current). Avalanche is like one runner sprinting fast down a long hallway, crashing into people and knocking them running too — soon there's a stampede. That needs a long hallway (lightly-doped, wide gap). Zener is when the wall is so thin that people just ghost straight through it like magic (tunneling) — that needs a super-thin wall (heavily-doped). Long hallway = high voltage before stampede; thin wall = low voltage to ghost through. And here's the neat trick: heating helps the ghosts (Zener down) but slows the runners (avalanche up).

Connections

Depletion Region Width — the master variable ( $W$ ) that decides which mechanism.
Doping & Carrier Concentration — heavy vs light doping sets $W$ .
PN Junction under Reverse Bias — where the field builds up.
Zener Diode Voltage Regulator — the practical application of non-destructive breakdown.
Quantum Tunneling — the physics behind Zener.
Temperature Effects in Semiconductors — origin of the opposite TCs.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, jab hum diode ko reverse bias karte hain, normally sirf tiny leakage current behta hai. Lekin agar voltage bahut zyada bada do, toh achanak current phat se badh jaata hai — isko reverse breakdown kehte hain. Ab yaha do alag physics chal sakti hai, aur dono ka symptom same dikhta hai (current suddenly up), par cause bilkul different hai.

Avalanche mein ek electron field se speed pakadta hai, phir atom se takra ke doosra electron nikal deta hai — ab do ho gaye, phir char, phir aath... chain reaction, jaise bumper cars ka stampede. Yeh lightly doped junction mein hota hai kyunki wahan depletion region chaudi hoti hai, electron ko lamba runway milta hai speed pakadne ke liye. Isliye avalanche ka $V_{BR}$ high hota hai (6V se upar).

Zener mein koi takkar nahi — junction itna heavily doped hota hai ki depletion region patli si ho jaati hai. Field itna intense ban jaata hai ki electron seedha barrier ke aar-paar tunnel kar jaata hai (quantum magic). Isliye Zener low voltage par (5V se neeche) hota hai.

Pehchanne ka best trick: temperature. Zener ka $V_{BR}$ garam karne par girta hai (negative TC, kyunki band gap chhota hota hai, tunneling easy). Avalanche ka $V_{BR}$ garam karne par badhta hai (positive TC, kyunki collisions jaldi hone lagti hain). Aur yaad rakho — breakdown diode ko kharaab nahi karta, jab tak current limit ho. Yahi principle Zener diode voltage regulator mein use hota hai. Exam ke liye: Zener = Zmall & Znegative.

Test yourself — Doping & PN Junctions

Connections