Transistors are fundamental active components that enable switching, amplification and power control functions in nearly all modern electronics. The field effect transistor (FET) and metal-oxide-semiconductor FET (MOSFET) are two important transistor families extensively used in analog and digital circuits. But which of these technologies is better and more suitable for various applications? This article provides a detailed feature comparison between basic FET and MOSFET transistors to understand their relative advantages, drawbacks and best usage scenarios.
FET Transistor Overview
The field effect transistor, or simply FET, was one of the earliest transistor technologies invented in the 1930s. It uses an electric field effect to control current flow through a semiconductor channel.
Figure 1: Symbol and structure of a JFET (junction FET)
Some key characteristics of FET transistors:
- Use junction or MOS gate constructions to modulate channel conductivity
- Mainly depletion mode devices (normally ON)
- Bidirectional drain current capability
- Low input impedance gate
- Low noise performance
Common FET types include:
- JFET – Junction gate FET
- MESFET – Schottky (metal-semiconductor) gate FET
- HEMT – High electron mobility transistor
FETs are simple, robust devices used for analog signal switching, RF power amplification, buffers, stable current sources etc. But they have limited gain and are not suitable for high frequency digital logic.
MOSFET Transistor Overview
The metal-oxide-semiconductor field effect transistor (MOSFET) augments the basic FET by using an insulated gate (oxide layer) above the channel region.
Some salient properties of MOSFET transistors:
- Inexpensive fabrication requiring fewer process steps
- Fast switching speeds
- Very high input impedance insulated gate
- Low power consumption
- Both enhancement and depletion mode types
- Good transconductance gain
MOSFETs revolutionized the electronics industry by enabling dense digital IC fabrication. They continue to be the most ubiquitous transistor in digital VLSI and analog circuits alike.
Structural Comparison
FETs and MOSFETs vary in their internal construction as highlighted below:
| Parameter | FET | MOSFET |
|---|---|---|
| Gate type | Junction (PN) or Schottky | Insulated with oxide layer |
| Gate bias | Reverse biased | Either polarity |
| Main current carriers | Majority (electrons/holes) | Majority (electrons/holes) |
| Channel type | N-type or P-type doping | N-type or P-type doping |
| Gate current | Moderate | Negligible (pA) |
| Symbols | Varies | Standardized symbols |
Table 1: Structural comparison between FET and MOSFET transistors
The FET relies on a PN junction or Schottky junction gate interacting with the channel through depletion. The MOSFET utilizes an insulated metal gate with fixed threshold enabling both enhancement and depletion type devices.
Electrical Characteristics Comparison
The different gate designs result in some key electrical performance differences between basic FETs and MOSFETs:
| Parameter | FET | MOSFET |
|---|---|---|
| Input impedance | Low (μS) | Very high (TΩ) |
| Transconductance (gain) | Low | High |
| Output impedance | High | Medium-High |
| Switching speed | Moderate (μs) | Very fast (ns) |
| Noise figure | Excellent | Moderate |
| Temperature stability | Fair | Good |
| Current rating | Medium | High |
| Voltage rating | Medium | High |
| Power rating | Low-Medium | Very high |
Table 2: Electrical characteristic comparison between FET and MOSFET transistors
In summary, MOSFETs offer superior gain, switching speed, power handling and ease of integration at the cost of slightly increased noise over FETs.
biasing Comparison
FETs and MOSFETs require different biasing arrangements:
FET biasing
- Reverse bias gate to pinch-off channel
- Drain-source bias controls output current
- Depletion mode – normally ON devices
MOSFET biasing
- Apply gate-source threshold voltage to enhance channel
- Drain current controlled by gate-source voltage
- Both enhancement and depletion types
This allows MOSFETs to act as voltage-controlled current sources, unlike FETs which are simply current-controlled resistors.
Transconductance Comparison
Transconductance (gm) indicates voltage-to-current gain. FETs have lower transconductance compared to MOSFETs as evident from typical curves:
The MOSFET demonstrates significantly higher gain leading to much greater amplification capabilities.
Switching Speed Comparison
MOSFETs can switch on and off much faster than junction FETs due to their insulated gate and low gate charge requirements.
JFET turn-on time approximately follows:
$$t_{on} = \frac{C_{gd}}{g_m}$$
Where, $$C_{gd}$$ is gate-drain capacitance
$$g_m$$ is transconductance
MOSFET turn-on time is approximately:
$$t_{on} = \frac{Q_g}{I_D}$$
Where, $$Q_g$$ is gate charge
$$I_D$$ is drain current
The smaller gate charge of MOSFETs combined with their higher drain currents allow switching in nanoseconds rather than microseconds for FETs.
Temperature Stability Comparison
FETs rely on maintaining a fixed junction voltage to pinch-off the channel. As temperature rises, the gate threshold voltage reduces. This leads to increased channel leakage and reduces gate control effectiveness.
MOSFETs have an insulated metal gate and fixed threshold voltage independent of temperature. This gives them better temperature stability and consistent operation over a wide range.
Noise Figure Comparison
FETs have extremely low noise figures owing to their high input impedance gate. Typical room temperature noise figures are 0.1 to 0.5 dB for FETs, compared to 0.5 to 3 dB for MOSFETs.
This makes FETs advantageous for low-noise pre-amplifier and amplifier applications. MOSFETs have greater noisecontribution from their insulating gate oxide layer.
Voltage and Current Ratings
Both transistors cover a wide range of voltage/current specifications. In general, modern high voltage MOSFETs can match or exceed voltage blocking capabilities of most FETs. High transconductance also allows greater current in smaller MOSFET die sizes.
Discrete FETs are available with voltage ratings up to 1000V and currents up to 50A. LDMOSFET devices support over 1000V blocking with 100A+ continuous currents for power applications.
Failure Rate Comparison
Analog FETs are simple junction devices with very robust construction. They exhibit lower failure rates around 20 to 70 FITs (failures in time).
MOSFETs are more complex with insulated gates susceptible to ESD damage or oxide breakdowns. But modern FET manufacturing achieves good reliability with failure rates of 100 to 200 FITs.
For both transistors, over-voltage or excessive temperature are the main causes of premature failures.
Application Comparison
FET applications
- Low noise preamplifiers
- Stable current sources
- Analog switches and signal routing
- RF power amplifiers
- Buffer and gain stages
MOSFET applications
- Digital logic gates and IC
- Microprocessors and CPUs
- High frequency amplifiers
- DC-DC converters and motor drives
- RF power amplifiers
While some application domains like RF amplification overlap, FETs are optimized for linear analog applications whereas MOSFETs excel in digital switching and power conversion applications.
Cost Comparison
Small signal FETs can be in the sub $1 range based on volume, comparable to small signal MOSFETs. Power FETs range from $2 to $10 for discrete devices with the higher currents and voltages being more expensive.
MOSFET prices vary similarly with small signal devices starting around $0.10 and high current power MOSFETs in the $3 to $20 range. High voltage, high power MOSFETs with advanced packaging tend to be costlier than equivalently rated FETs.
Summary of Key Differences
| Parameter | FET | MOSFET |
|---|---|---|
| Gate type | Junction or Schottky | Insulated Metal |
| Transconductance | Low | High |
| Switching Speed | Moderate | Very Fast |
| Noise Figure | Excellent | Moderate |
| Temperature Stability | Fair | Good |
| Key Applications | Low noise analog amplification, RF power | Digital logic, power conversion, high frequency amplification |
| Cost | Medium | Medium-High |
Table 3: Summary of key differences between FET and MOSFET transistors
In essence, FETs are optimized for low noise analog applications whereas MOSFETs are better suited for high gain, speed and integration digital/power applications.
Frequently Asked Questions
Q: Why are FETs mainly depletion type devices?
A: The junction gate can only pinch-off the channel and not enhance it. This limits FETs to normally ON depletion behavior.
Q: Can FETs be used for digital logic?
A: Yes, but their slower switching and lower gain makes FET logic far less practical than MOSFET based logic.
Q: What are the key materials used to fabricate FETs and MOSFETs?
A: FETs use silicon, GaAs, GaN etc. MOSFETs additionally require insulators like silicon dioxide, sapphire, silicon nitride.
Q: How does MOSFET gate oxide breakdown occur?
A: Dielectric breakdown of the thin insulating oxide layer leads to gate contact with the channel destroying gate control.
Q: Which provides higher gain – JFET or MOSFET?
A: MOSFETs have far higher transconductance compared to JFETs enabling much greater amplification.
Conclusion
In conclusion, both FET and MOSFET transistors have solidified their importance in modern electronics – FETs for analog applications and MOSFETs for digital designs. While FETs offer certain advantages like lower noise and robustness, MOSFETs are superior in gain, speed, integration density and controllability. The insulating gate MOSFET construction enabled their dominance in VLSI ICs but discrete and RF FETs continue to flourish in their linear amplification niche roles. Engineers must weigh the trade-offs when choosing between FETs and MOSFETs for optimum performance.
