Semiconductors and Transistors 101 – The FET

                 RF Power JFET                                        RF Power MOSFET

We saw from the earlier post on the operation of the BJT that it was a current amplifier, requiring a base current to create a collector current. The FET family of transistors are all voltage operated devices similar to the valve, requiring a change in input voltage to cause a change in device current. There are two classes in this family:

1. The Junction FET – JFET
2. The Metal Oxide FET – MOSFET

The Junction Field Effect Transistor – JFET

The junction field effect transistor has several advantages over the BJT.

1. It can be fabricated smaller.
2. Generates less electrical noise.
3. Switches faster and has a higher bandwidth.
4. Has a much higher input impedance.
5. Can be fabricated with two gates.

This device is again a sandwich of either NPN or PNP material but the controlling signal is created by an electrical field NOT a continuous current. Also the main current flow is NOT across a PN junction(s) but through a channel of semiconductor material. Its operation is much closer to that of a valve as it only needs a change in input voltage to cause a change in device channel current.

This device has three connections just like a BJT but they have different names:

1. Drain
2. Gate
3. Source

As with the BJT there are two basic types:

• N channel: -ve gate voltage, +ve drain voltage
• P channel: +ve gate voltage, -ve drain voltage

Both types are referred to as depletion mode devices as their drain-to-source current flow is based upon the width of their channel depletion layers. Both types are conductive with zero gate voltage.

JFET Symbols

Simplified JFET Construction

JFET Operation

With no voltage applied to the gate (Vgs=0), a voltage applied between the source and drain (Vds) causes a current to flow through the N channel causing both side P-N junctions to become reverse biased and creating depletion regions. Depending upon how large the source/drain voltage is these depletion regions can almost touch, severely restricting the N channel width, increasing its resistance and restricting current flow. If we now fix the drain/source voltage at some intermediate value and apply a negative voltage between the gate and source (Vgs), as shown above, the depletion layers width will change according to the value of the gate/source voltage. This will cause the drain/source current to vary as the N-channel width and hence its resistance varies. This device is therefore referred to as a DEPLETION mode device as the width of the depletion regions control the current flow. We now have a device that uses a voltage change on its gate to cause a current change through it, similar to the triode valve. The devices resulting characteristic is as shown below:

Typical N-Channel JFET Characteristic

Just like the valve control grid current, the JFET gate current is extremely small. As a result the device has a very high input resistance and draws virtually no input current. The basic differences for the P channel silicon JFET is that both battery voltages are reversed as is the current flow.

The range of JFET devices is vast being specifically designed for low/high power, AF, RF, low noise, switching, high/low voltage and all combinations in between. Very common audio devices would include: voltage amplifiers like the N-channel 2N3819, AN6602, JFE150 and power devices like the N-channel Silicon Carbide (SiC) UJ3N065025K3S. Electrically matched and complementary P and N channel pairs are also made for class AB push-pull audio amplifiers. The JFET is often used in audio voltage amplifiers, MC pre-amplifier input stages, as an electronic variable resistor and as a constant current source. They are found in many of the same items of HT equipment; pre-amps, AVR’s, power amplifiers, projectors and TV’s etc. NOTE: Power JFET’s are not very popular and most designs tend to use power MOSFET’s – see below.

Typical Individual Devices – Low To High Power

The Metal Oxide Semiconductor FET – MOSFET

The MOSFET, Insulated Gate FET (IGFET) or Metal Insulator FET (MIFET), are field effect transistor that uses the electric field of a capacitor to control its channel current flow. It is the most widely manufactured of all the transistor devices.

Is has the following advantages over the BJT and JFET:

1. Easier to manufacture
2. Can be manufactured on a much smaller scale.
3. Very high input impedance
4. Very high switching speeds and bandwidths
5. Very high power
6. Operates in either depletion or enhancement mode
7. Low power consumption

This device normally has three connections just like a JFET but there are dual gate versions:

1. Drain
2. Gate 1
3. (Gate 2)
4. Source

There are four basic types of construction:

• N – Channel Depletion MOSFET: -ve gate voltage, +ve drain voltage
• P – Channel Depletion MOSFET: +ve gate voltage, +ve drain voltage
• N – Channel Enhancement MOSFET: +ve gate voltage, +ve drain voltage
• P – Channel Enhancement MOSFET: -ve gate voltage, -ve drain voltage

Depletion mode devices are conductive with zero gate voltage. Enhancement mode devices are non-conductive with zero gate voltage

MOSFET Symbols

Simplified MOSFET Construction

In all cases their control is by the application of a voltage on the gate that forms one plate of a capacitor that is separated from the main device by an insulating layer of Silicon Oxide. The devices main substrate forming the other capacitor plate. The electrostatic gate charge causes either electrons or holes to be liberated in the channel or substrate material causing an electron flow between the devices source and drain to either increase or decrease. So again its operation is much closer to that of a valve as it only needs a change in input voltage and hence gate charge level, to cause a change in device channel current flow.

N – Channel Depletion MOSFET Operation

With Vgs set to 0volts the free electrons in the N-channel allow a current to flow between the source and drain. As Vgs is increased the -ve charge on the gate repels (depletes) electrons from the N channel upper area reducing the conductive cross-sectional area of the N-channel and restricting the flow of electrons. Eventually a gate voltage is reached where the N-channel is completely depleted of electrons and no current will flow.

N – Channel Enhancement MOSFET Operation

With Vgs set to 0volts no current can flow between the source and drain as the p-type substrate has no free electrons to allow conduction. As Vgs is increased the +ve charge on the gate attracts electrons from the substrates upper areas and N regions creating a channel of free-electrons between the source and drain. This N-channel will now allow electrons to flow between the source and drain. Continued increase in Vgs promotes a deeper N-channel, dropping its resistance and increasing the current flow.

Typical MOSFET Characteristics

The equivalent P type MOSFET’s operate in similar manner but with different gate and drain-to-source voltage polarities .

As the gate(s) in this device are insulated they draw virtually no current and again provides no loading on devices that feed a signal to it. The lack of input current reduces device noise still further and these devices can operate at very high RF frequencies and very high powers. The addition of a second gate further improves the devices high frequency characteristics with dual gate MOSFET’s being mainly used in RF devices such as RF amplifiers, oscillators and mixers.

The range of MOSFET devices is vast being specifically designed for high/low power, AF, RF, low noise, switching, high/low voltage and all combinations in between. Very common audio devices would include: voltage amplifiers 2N7000 and VP2106N3 and power devices like the NTE2967 or IRLB8721. MOSFET’s are widely used for high speed switching, power control, high power audio and RF power amplifiers and switched mode power supplies. Low power dual gate devices are used extensively in RF circuits. Again electrically matched and complementary N-P type single gate MOSFET pairs are available for class AB push-pull audio power amplifiers.

It should be noted that the GaN MOSFET is now finding its way into high end CLASS D power amplifiers due to its very high switching speeds, high power and heat tolerance and high switching efficiency, thereby creating very low heat generation. This same GaN technology is also used to manufacture Blu-ray diodes.

Typical Individual Devices – Low To High Power

These semiconductor and transistor technologies are the basis of almost all modern electronics and can be created on a microscopic scale with 100’s of thousands being fabricated and interconnected within areas as small as 0.25″ square. Which is impressive if you look at the physical size of the devices that I grew up with and used in many of my audio and RF designs like the OC44/45, OC71/72, BC107/108/109, 40673, 2N3819, BFY51/BFX88, BDY77/2N3055.

Typical Device Sizes

Well that just about wraps up this series of posts. Here are the other two parts:

• Semiconductors and Transistors 101
• Semiconductors and Transistors 101 – The BJT (Bipolar Junction Transistor)

Related Posts:

• Amplifier Classes A Quick Primer Course.
• Valves Or Tubes To You – An Introduction