Transistor is a fundamental component in electronics and logic circuits, used for switching and amplification. MOSFET is a type of field-effect transistor (FET) whose gate is electrically insulated using an oxide layer, also called IGFET (Insulated Gate FET).

1. What is a MOSFET?

MOSFET, or Metal-Oxide-Semiconductor Field-Effect Transistor, is a four-terminal FET: drain, gate, source, and body/substrate. The body terminal is shorted to the source, leaving three working terminals like any other transistor.

MOSFET conducts current between the source and drain through a channel, whose width is controlled by the gate voltage. MOSFET is a voltage-controlled device. A thin silicon dioxide layer isolates the gate from the channel, significantly increasing input impedance to the megaohm range. Thus, no gate current flows.

2. Main Types of MOSFETs

MOSFETs are mainly classified as:

   * Depletion MOSFET (D-MOSFET)

   * Enhancement MOSFET (E-MOSFET)

Both types can be N-channel or P-channel.

   * D-MOSFETs are “normally ON” with a built-in channel; applying gate voltage reduces channel width to turn OFF.

   * MOSFETs are “normally OFF”; the channel is induced by applied voltage.

Symbols:

   * D-MOSFET: continuous line between drain and source = current flows at VGS = 0

   * E-MOSFET: broken line = no current flow at VGS = 0

   * Arrow direction: inward = N-channel, outward = P-channel

3. MOSFET Operating Regions

MOSFET acts as an insulator or conductor based on small signals and works in three regions:

   * Cutoff Region: MOSFET is OFF, drain current ID = 0; used as switch OFF state

   * Saturation Region: Constant current flows; MOSFET fully ON; maximum drain current ID

   * Linear / Ohmic Region: Provides controlled resistance; drain current varies with VDS and is controlled by VGS; used for amplification

4. Types of MOSFETs

MOSFETs are divided into:

   * Depletion-mode MOSFET (D-MOSFET) – N & P channel

   * Enhancement-mode MOSFET (E-MOSFET) – N & P channel

5. Depletion MOSFET (D-MOSFET)

D-MOSFET has a built-in channel during fabrication, so it conducts even at VGS = 0 V. Reverse-bias gate depletes carriers and narrows the channel until it stops conducting, defined by threshold voltage VTH.

Forward-bias gate induces more majority carriers, widening the channel and increasing drain current ID.

Key points:

   * D-MOSFET can operate in depletion or enhancement mode

   * N or P channel affects bias, speed, and current capacity

6. N-Channel D-MOSFET

Source and drain are placed on an N-type layer; the gate electrode is placed on top of a metal oxide insulating layer. 

The channel is made of N-type material; electrons are the majority carriers.

   * Negative VGS < 0 V: holes in P-substrate deplete electrons, narrowing channel; MOSFET stops conducting at VGS = -VTH

   * Positive VGS: enhances conductivity; drain current ID increases with VDS in linear region and saturates at IDSS when VDS reaches pinch-off voltage VP

7. Operating Regions of N-Channel D-MOSFET

   * Cutoff Region:$V_{GS} \le -V_{TH}$VGS≤−VTH. Drain current$I_D = 0$ID=0, MOSFET is OFF.

   * Saturation Region:$V_{GS} > -V_{TH}, V_{DS} > V_P$VGS>−VTH,VDS>VP. Maximum drain current$I_{DSS}$IDSS flows, depending on$V_{GS}$VGS.

   * Linear / Ohmic Region:$V_{GS} > -V_{TH}, V_{DS} < V_P$VGS>−VTH,VDS<VP. MOSFET acts as an amplifier;$I_D$ID increases with$V_{DS}$VDS, gain depends on$V_{GS}$VGS.

8. P-Channel D-MOSFET

P-Channel D-MOSFET has the same structure as N-channel, except source and drain are on P-type layer. Channel is made on N-type substrate, carriers are holes.

   * Holes are heavier than electrons → lower speed

   * Gate voltage controls channel width and current

   * Positive VGS: depletes carriers, narrows channel, reduces current

   * Negative VGS: induces more holes, enhancing current

9. Operating Regions of P-Channel D-MOSFET

   * Cutoff Region:$V_{GS} = +V_{TH}$VGS=+VTH,$I_D = 0$ID=0, MOSFET OFF

   * Saturation Region:$V_{GS} < +V_{TH}, V_{DS} > V_P$VGS<+VTH,VDS>VP, max$I_{DSS}$IDSS flows

   * Linear / Ohmic Region:$V_{GS} < +V_{TH}, V_{DS} < V_P$VGS<+VTH,VDS<VP, acts as amplifier;$I_D$ID increases with$V_{DS}$VDS, gain depends on$V_{GS}$VGS

10. Enhancement MOSFET (E-MOSFET)

Enhancement MOSFET has no built-in channel; channel is induced by gate voltage, enhancing conduction.

   * No gate voltage: MOSFET is OFF → “normally OFF”

   * VGS > VTH: channel forms, current flows

   * Channel width increases with higher VGS → drain current increases

Types: N-Channel and P-Channel E-MOSFET

11. N-Channel E-MOSFET

Structure similar to D-MOSFET, but no built-in channel.

${}_{}$   * $V_{GS} = 0$VGS=0 → non-conductive

   * Positive VGS creates electric field, attracts electrons from P-substrate, repels holes, forming induced channel

   * Channel forms at VGS ≥ VTH, width increases with VGS

12. Operating Regions of N-Channel E-MOSFET

   * Cutoff Region:$V_{GS} \le 0V$VGS≤0V,$I_D = 0$ID=0, works as switch OFF

   * Saturation Region:$V_{GS} > 0V, V_{DS} > V_{GS}$VGS>0V,VDS>VGS, maximum drain current$I_{DSS}$IDSS flows

   * Linear / Ohmic Region:$V_{GS} > 0V, V_{DS} < V_{GS}$VGS>0V,VDS<VGS, acts as amplifier;$I_D$ID rises with$V_{DS}$VDS depending on VGS

13. P-Channel E-MOSFET

P-Channel E-MOSFET is similar to P-Channel D-MOSFET but no channel at fabrication.

   * Negative VGS induces holes under the insulating layer, forming a channel

   * Voltage applied between source and drain enables current

   * Channel width depends on VGS; current increases as VGS decreases below VTH

14. Operating Regions of P-Channel E-MOSFET

  * Cutoff Region:$V_{GS} \ge 0V$VGS≥0V, no drain current$I_D = 0$ID=0, MOSFET OFF

  * Saturation Region:$V_{GS} < 0V, V_{DS} > V_{GS}$VGS<0V,VDS>VGS, maximum drain current$I_{DSS}$IDSS flows, depends on VGS level

  * Linear / Ohmic Region:$V_{GS} < 0V, V_{DS} < V_{GS}$VGS<0V,VDS<VGS, MOSFET acts as amplifier; ID rises with VDS, gain depends on VGS

Explanation: P-Channel E-MOSFET behaves similarly to N-Channel E-MOSFET but with reversed voltage polarities. At VGS = 0V, it is non-conductive; applying negative VGS induces a channel allowing current flow.

15. Working of MOSFET

MOSFET can operate as a switch or amplifier. Its operation depends on type and bias; it can work in depletion or enhancement mode.

   * An insulating layer between gate and channel increases input impedance → no gate current flows

   * Acts on voltage applied to gate terminal

   * Insulating layer forms a flat capacitor → very high input impedance, very low power consumption, but vulnerable to static damage

Depletion Mode:

   * Built-in channel exists; applying VDS allows drain current ID

   * Reverse-bias VGS depletes carriers, narrows channel, reducing or stopping current

Enhancement Mode:

   * Forward-bias VGS attracts carriers, increasing channel width

   * Higher VGS → more charge accumulation → wider channel → higher ID

Table: Four MOSFET types, VGS levels, threshold voltage VTH, pinch-off voltage VP

   * Depletion MOSFET: works in both depletion and enhancement mode

   * Enhancement MOSFET: only works in enhancement mode

   * Drain and source can be interchangeable; drain is the higher voltage end

16. Characteristics or V-I Curve of MOSFETs

   * Transfer Characteristics: Shows relationship between input gate voltage VGS and output drain current ID

   * Drain Characteristics: Shows relationship between drain-source voltage VDS and drain current ID

17. N-Channel D-MOSFET

Transfer Curve: When VGS exceeds VTH, MOSFET conducts ID; threshold voltage < 0V → conducts at 0V-GS
Drain Characteristics: Shows three regions – cutoff, ohmic, saturation; includes depletion and enhancement modes

   * Ohmic and saturation regions separated by pinch-off line; pinch-off voltage = minimum voltage for saturation

   * Ohmic: ID increases with VDS

   * Saturation: ID constant, varies only with VGS

   * Cutoff: ID = 0, requires VGS below -VTH

   * VGS = 0V → depletion mode; channel width and conductivity decrease with voltage

   * VGS > 0V → enhancement mode; conductivity increases

18. P-Channel D-MOSFET

Transfer Curve: Negative slope; MOSFET conducts when VGS < +VTH
Drain Characteristics: Shows VDS vs ID at different VGS; as VGS decreases, ID increases

   * Voltage polarity reversed compared to N-channel; otherwise similar

19. N-Channel E-MOSFET

   * No conduction at VGS = 0V due to absence of channel

   * Once VGS > VTH → conducts, same operation as D-MOSFET in enhancement mode

   * Cutoff: VGS < VTH → ID = 0

   * Ohmic: VGS > VTH, ID increases with VDS

   * Saturation: VDS crosses pinch-off voltage VP → ID saturates

20. P-Channel E-MOSFET

   * Same characteristics as N-Channel E-MOSFET, voltage polarities reversed

   * VGS < 0 → channel forms → current flows

   * VGS = 0 → non-conductive

21. Advantages & Disadvantages of MOSFET

Advantages:

   * No gate current, very high input impedance

   * Extremely low leakage → negligible power consumption

   * High switching speed → suitable for high-frequency applications

   * Very low output resistance, small size

   * Can operate in depletion or enhancement mode

   * High efficiency at low voltage, voltage-controlled device, low power

   * Unipolar → noiseless operation

Disadvantages:

   * Gate-channel capacitance can be damaged by static

   * Cannot withstand high voltage

   * More expensive than BJT

22. Applications of MOSFETs

MOSFETs are widely used in switching and amplification:

   * Fast switching and amplification of very small signals, e.g., high-frequency amplifiers

   * Power MOSFETs for DC motor control

   * Suitable for chopper circuits due to high switching speed

   * Digital ICs like microcontrollers and processors for superior switching speed and low power

   * Switch-Mode Power Supplies (SMPS)

   * CMOS logic circuits (combination of P-MOS and N-MOS layers)

   * H-bridge circuits

   * Buck and boost converters