--- title: "N-channel depletion MOSFET construction, working" subject: "Basic Electronics Engineering" chapter: 3 key: "semester_2/basic_electronics_engineering:mosfet-n-channel-depletion" source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/mosfet-n-channel-depletion site: BCT Engineering --- # N-channel depletion MOSFET construction, working Basic Electronics Engineering · Chapter 3: Transistor - Asked in: 2075 Baisakh, 6 marks; 2071 Bhadra, 6 marks; 2073 Magh, 8 marks (asked 3 times) - Video: [MOSFET- Depletion Type MOSFET Explained (Construction, working and Characteristics Explained)](https://www.youtube.com/watch?v=XqGBNyhImV4) — ALL ABOUT ELECTRONICS - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/mosfet-n-channel-depletion - Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-3 ## Question Describe the construction and working principle of n-channel depletion type MOSFET with necessary diagrams. ## Short answer Has an n-channel diffused in p-substrate under a SiO₂ layer and metal gate. At VGS = 0, ID = IDSS. Negative VGS depletes channel (pinch-off at VP), positive VGS enhances it. Used in depletion and enhancement modes, follows ID = IDSS (1 − VGS/VP)² in saturation. ## Answer The **n-channel depletion MOSFET (D-MOSFET)** has a physical n-channel built between source and drain, so current flows even at VGS = 0. A gate voltage can either deplete the channel (negative VGS) or enhance it (positive VGS). So it can work in both depletion and enhancement modes. #### Construction - A p-type substrate. - Two heavily doped n⁺ regions (source and drain) diffused in the substrate. - A thin lightly doped n-type **channel** diffused between the source and drain, below the oxide. - A thin SiO₂ layer over the channel, with a metal gate on top (insulated from the channel). - Substrate (body) usually joined to the source. ```text S G D | +-----+ | ---+ |metal| +--- | n+| +-----+ | n+| | |~~ n-channel ~~| | | p substrate | +----------B------------+ ``` #### Working principle 1. **VGS = 0:** with VDS applied, electrons flow through the n-channel from source to drain. Drain current is IDSS. 2. **Negative VGS (depletion mode):** the negative gate repels electrons from the channel (holes are induced), so the channel becomes narrower and ID decreases. At VGS = VP (pinch-off, a negative value), the channel is fully depleted and ID ≈ 0. 3. **Positive VGS (enhancement mode):** the positive gate attracts more electrons into the channel, so the channel widens and ID rises above IDSS. #### Characteristics Shockley equation (saturation region): ID = IDSS (1 − VGS/VP)² ```text ID ID | VGS=+1 ___ | / | VGS= 0 ___ IDSS | IDSS _.--' | VGS=-1 ___ | .-' | VGS=-2 ___ |/ +----------------- VDS +---+--------- VGS Drain characteristic VP 0 ``` - VGS < VP: cut-off. - Ohmic region at low VDS, saturation at higher VDS. **Comparison with E-MOSFET:** D-MOSFET has a built-in channel, conducts at VGS = 0 (normally-ON), and works with both polarities of VGS; E-MOSFET has no channel at VGS = 0 and needs VGS > VT. **Applications:** RF amplifiers, input stages of high-impedance amplifiers, and constant current sources. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What distinguishes the D-MOSFET? A. No channel B. A physical n-channel, so it conducts at VGS = 0 C. p-type source D. No oxide Answer: B. It has a diffused channel, with ID = IDSS at VGS = 0. (Key point: Built-in channel) 2. What does negative VGS do? A. Widens the channel B. Turns off the substrate C. Repels electrons, narrowing the channel and reducing ID D. Does nothing Answer: C. The channel is depleted of electrons. (Key point: Negative VGS) 3. What happens at VGS = VP? A. ID = IDSS B. The channel is fully depleted and ID ≈ 0 C. ID doubles D. VDS=0 Answer: B. VP is negative cut-off. (Key point: Pinch-off) 4. Which equation gives ID in saturation? A. ID = k(VGS - VT)^2 B. ID = β IB C. ID = IDSS(1 - VGS/VP)^2 D. ID = VDS/R Answer: C. Shockley equation. (Key point: Shockley) ## Close variants set in other exams - [N-channel enhancement MOSFET construction, working](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Amosfet-n-channel-enhancement) — Basic Electronics Engineering, chapter 3, asked 9× ## Related in Basic Electronics Engineering - [MOSFET construction and working principle](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Amosfet-construction-working) — Basic Electronics Engineering, chapter 3, asked 2× ## Related in other subjects - [n-channel E-MOSFET construction and working](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3An-emosfet-construction-working) — Electronics Devices and Circuit, chapter 3, asked 9× - [n-channel depletion MOSFET operation](https://www.bctengineer.com/question.md?key=semester_1%2Ffundamental_of_electrical_and_electronics_engineering%3Adepletion-mosfet-operation) — Fundamental of Electrical and Electronics Engineering, chapter 5 - [D-MOSFET construction and working](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Admosfet-construction-working) — Electronics Devices and Circuit, chapter 3, asked 6× - [N-channel JFET construction and working](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Ajfet-construction-working) — Electronics Devices and Circuit, chapter 3, asked 9×