Field Effect Transistors
- Unipolar device, i.e., conduction by either electrons or holes.
- It is a voltage controlled device.
- Has high efficiency, i.e., power dissipation is less.
- Instant operation, i.e., delay is much lesser than BJT.
- Low power consumption compare to BJT.
- Input impedance is very high making them very sensitive to input signals, but can be easily damaged by static electricity.
- This has a narrow channel of N-type or P-type silicon with electrical connections at either ends commonly called the drain and the source. Within this channel there is a third connection which is called gate (can be either p-type or n-type).
- The semiconductor channel of a JFET is a resistive path through which a voltage VDS causes a current ID to flow. A voltage gradient is thus established and the pn junction (formed between gate and channel) therefore
- has a high reverse bias at the drain terminal and a lower reverse bias at the source terminal. This causes a
- depletion layer to be formed within the channel.
Operation of an N-JFET
- With no voltage applied between gate and source, electrons flow through the channel.
- If a voltage is applied between gate and source, gate channel junction becomes reverse biased and the flow of electrons between source and drain becomes limited.
- At the maximum gate source voltage the channel pinches off, forcing JFET into cutoff mode. This is due to the reason that depletion region of the pn junction occupying the whole of the channel. This can be illustrated as reducing the flow of a liquid through a flexible hose by squeezing it with enough force, the hose will be constricted enough to completely block the flow.
Modes of operation of MESFET
- MESFET’s are fabricated in two types: Enhancement type and Depletion type.
- Enhancement type is normally off type.
- Depletion type is normally on type.
- The gate voltage modulates the width of the depletion region in the semiconductor of schottky barrier diode. Hence resistance and resulting electron flow from source to drain is controlled.
- In the enhancement device the built in voltage causes the channel to be completely closed at zero gate source voltage and hence requires positive VG for normal operation
- In depletion device with VG = 0, the depletion region extends only partly across the channel and is possible to modulate the width of depletion region by applying negative gate voltage and positive drain voltage.
Advantages of MESFET over other transistors
- Unipolar device whereas BJT is a bipolar device.
- Has high gain bandwidth product compared to JFET.
- Fabrication is simpler than JFET.
- Has higher mobility of carriers in the channel as compared to MOSFET.
- High transit frequency of MESFET can be used in microwave circuits.