MOSFET structure (gate, source, drain, body)

2.4.8 · Hardware › Transistors: BJT & FET

MOSFET kis cheez se bana hota hai?

Ek n-channel MOSFET (NMOS) ke liye:

Body p-type hoti hai
Source aur drain n⁺ (heavily doped n-type) hote hain
Channel, jab banta hai, electrons carry karta hai

PMOS ke liye sab kuch ulta ho jaata hai: n-type body, p⁺ source/drain, aur holes current carry karti hain.

Figure — MOSFET structure (gate, source, drain, body)

Har terminal kyun exist karta hai (Feynman-style)

Channel kaise banta hai — first principles se derivation

Step 1 — MOS capacitor

Gate–oxide–body sandwich ek parallel-plate capacitor hai. Uski capacitance per unit area:

$C_{o x} = \frac{ε _{o x}}{t _{o x}}$

Step 2 — Threshold: channel kab banta hai?

NMOS par positive $V_{GS}$ apply karo. Gate ka positive charge oxide ke theek neeche p-body mein holes ko repel karta hai (depletion), phir, jab $V_{GS}$ kaafi bada ho jaata hai, surface par electrons ko attract karta hai. Jab itne electrons ikatte ho jaate hain ki body ke opposite type ki ek conducting sheet ban jaaye, tab hum kehte hain ki inversion ho gayi. Woh voltage jis par yeh theek hota hai, threshold voltage $V_{t h}$ kehlata hai.

$V_{GS} > V_{t h} ⟹ channel exists$

Step 3 — Induced channel charge

Jab $V_{GS} > V_{t h}$ ho, tab extra gate voltage jo threshold se zyada hai ( $V_{GS} - V_{t h}$ , jise overdrive $V_{O V}$ kehte hain) determine karta hai ki kitna mobile charge kheencha jaata hai. Channel ko capacitor ke stored charge $Q = C V$ ki tarah treat karte hue, per unit area electron (negative) sheet charge hai:

$Q_{c h} = - C_{o x} (V_{GS} - V_{t h})$

Minus sign physical hai: inversion layer electrons se bani hoti hai, isliye uski charge density negative hoti hai. Uski magnitude $∣ Q_{c h} ∣ = C_{o x} V_{O V}$ hai, aur woh magnitude hi channel conductance aur isliye $I_{D}$ determine karti hai.

Formula

$∣ Q_{c h} ∣ = C_{o x} V_{O V}$ kyun (aur minus sign kyun)? $V_{t h}$ se neeche, added gate charge zyaatar depletion region ko wide karta hai (fixed acceptor ions, koi mobile carriers nahi). $V_{t h}$ define ki gayi hai ki woh depletion cost pehle se include kare. Threshold ke upar, incremental gate charge incremental mobile electrons ke saath mirror hota hai → magnitude $C_{o x} (V_{GS} - V_{t h})$ . Sign negative hai kyunki electrons negative charge carry karte hain.

Approximation warning (steel-manned): Yeh " $V_{t h}$ saari depletion charge absorb karta hai, phir mobile charge par clean step" picture long-channel, constant-depletion-charge model hai. Reality mein, strong inversion mein bhi depletion region thoda aur wide hoti rehti hai aur ek chhota extra depletion voltage drop develop hota hai, isliye transition smooth hai, perfect step nahi. Yeh formula ek excellent first-order approximation hai, exact nahi.

Step 4 — Current phir kyun flow karta hai

Ab ek chhota $V_{D S}$ apply karo. Channel source se drain tak electrons ki ek patli resistive sheet hai. Horizontal field unhe drive karta hai: current flow hoti hai. Zyada overdrive ⇒ zyada mobile charge magnitude ⇒ lower resistance ⇒ zyada current. Yahi poora operating principle ek sentence mein hai.

Worked examples

Common mistakes (steel-manned)

Active recall

Recall Flashcards (jawaab dene ke baad reveal karo)

#flashcards/hardware MOSFET mein MOS letters ka matlab kya hai? ::: Metal–Oxide–Semiconductor (gate stack layers). Kaunsi layer gate current ≈ 0 banati hai? ::: Gate ke neeche patla oxide (SiO₂) insulator. NMOS ke liye body ka type kya hai aur S/D ka type kya hai? ::: Body p-type hai; source & drain n⁺ hain. Threshold voltage $V_{t h}$ define karo. ::: Woh $V_{GS}$ jis par ek inversion layer (channel) pehli baar form hoti hai, S aur D ko connect karti hai. Oxide capacitance per unit area ka formula? ::: $C_{o x} = ε_{o x} / t_{o x}$ . Threshold ke upar induced mobile channel (electron) charge per area? ::: $Q_{c h} = - C_{o x} (V_{GS} - V_{t h})$ (negative; magnitude $C_{o x} V_{O V}$ ). NMOS ke liye $Q_{c h}$ ka sign negative kyun hai? ::: Inversion layer electrons se bani hoti hai, jo negative charge carry karte hain. Kya $Q_{c h} = C_{o x} V_{O V}$ exact hai? ::: Nahi — yeh long-channel, constant-depletion-charge approximation hai; strong inversion mein ek chhota extra depletion drop develop hota rehta hai. NMOS ka source kaise identify karte ho? ::: Yeh lower potential par woh n⁺ terminal hai (electron supply). Body effect kya hai? ::: Nonzero $V_{S B}$ threshold voltage $V_{t h}$ ko increase karta hai. Patla oxide desirable kyun hai? ::: Bada $C_{o x}$ → zyada strong gate control → volt ke per zyada channel charge. "Field Effect" kya refer karta hai? ::: Gate ki electric field (injected current nahi) channel ko control karti hai.

Recall Feynman: 12-saal ke bacche ko explain karo

Ek garden hose (source se drain) imagine karo jisme beech mein ek squishy valve hai jo jelly se bana hai. Jelly ke upar ek kaaanch ki sheet hai jis se tum paani push nahi kar sakte, aur uske upar ek metal ka dhakkan hai. Jab tum dhakkan par ek strong positive charge rakhte ho, toh woh jaadui taur par jelly ke andar tiny electrons ki ek wire ko saath kheench leta hai — even though kuch bhi kaaanch se nahi guzarata. Woh electron-wire paani (current) ko ek end se doosre end tak jaane deti hai. Charge hata lo aur wire gayab ho jaati hai, toh koi flow nahi. Tum sab kuch dhakkan se control karte ho, bina kabhi paani ko touch kiye. Woh dhakkan gate hai, kaaanch oxide hai, aur electron-wire channel hai (aur kyunki yeh electrons se bani hai, iska charge negative count hota hai).

Connections

BJT vs MOSFET comparison — BJT current-controlled hai; MOSFET voltage/field-controlled hai.
MOSFET operating regions (cutoff, triode, saturation) — yahaan se $V_{O V} = V_{GS} - V_{t h}$ use karta hai.
Threshold voltage and body effect — $V_{t h}$ aur $V_{S B}$ par deep dive.
MOS capacitor and inversion — $C_{o x}$ aur inversion layer ki physics.
CMOS logic — is structure se NMOS + PMOS pair karta hai.
Channel-length modulation — saturation mein refinement.