This article will discuss one of the two main types of semiconductor transistor, the field effect Transistor or FET. The other type is the bipolar junction transistor or BJT.
Active devices, vacuum tubes and transistors will change how they pass current depending on how the active device is biased. For simple diodes, made up of an anode and a cathode, (same for both vacuum tube and semiconductor diodes), a forward biased device has a negative voltage applied to the cathode and a positive on the anode and will pass current with only a small voltage drop across itself. A reverse biased diode has negative voltage on the anode, positive voltage on the cathode and will block current flow as if it were a bias-controlled switch.
The more complex active devices, vacuum tube triodes (anode, cathode, grid) pentodes (anode, cathode and three grid layers) and so on, and semiconductor transistors will act generally like diodes when reversed biased. When forward biased, they will conduct current, but the amount of current can be modified by grid voltage (vacuum tube) or control currents (base current for BJTs and gate currents for FETs). This effect allows small signals applied to a vacuum tube or transistor's control segment to be amplified.
Discussion of the actual manufacture of these devices will be left for another article. Here we will look at the general types of FET and how they contrast with each other, other types of semiconductors and vacuum tubes.
History and Operation
The theory for creating the field effect transistor (FET) precedes that for the bipolar junction transistor (BJT), but the technology to create the FET came after the BJT. The first FET theory patents were issued in 1925 and 1934, but practical fabrication of FETs started in the early 1960s.
In essence the FET operates in solid state to control current in much the way vacuum tubes operate.
Current flow from the cathode to the anode may be controlled by applying a bias voltage to the gate (or grid). Semiconductor construction provides "choke points" in current path where charged fields may restrict current flow in much the same way charged grid layers in vacuum tubes work for triodes or may enhance current flow depending on substrate used and mode. Designers used to working with vacuum tubes found FETs easier to work with than BJTs.
Classifications of FETs
The two broadest classifications for FETs are the Channel-type and mode. Channel type is determined by the doping of the main channel for current flow. N-Channel types have a negatively doped channel and P-Channel use positive doping.
The modes are Enhancement mode where the channel is off, (there is no current flow) when a zero volt bias is applied to the gate and Depletion mode, where the channel is on (current flows) with a zero bias.
For both modes, a greater gate bias voltage will deliver more current through an N-Channel device and less current through a P-Channel device.
Fabrication types of FETs
The earliest FETs were Junction FETs or JFETs and were constructed in much the same way as BJTs. A subset of JFETs used a Schottky junction (to give a sharper transition from Off to On) in place of the PN junction (the physical transition point between positive-doped and negative-doped semiconductor material) and are known as MEtal-semiconductor FETs or MESFETs.
The next construction type was the Insulated Gate FET or IGFET, where the gate was built with an insulating layer measured in microns or less at the PN junction. The most common IGFETs are the Metal-Oxide or MOSFET and the Complementary MOSFET or CMOS. MOS and CMOS are the most common type of FET in use today.
Other, specialty FETs are seen such as the HFET, for high speed applications.
Parts of the FET
In the FET are the Source is analogous to the BJT emitter and the vacuum tube cathode. The FET Gate is analogous to the BJT base and the vacuum tube grid. The FET Drain is analogous to the BJT collector and the vacuum tube anode. The FET Substrate is also shown in most standard electronic schematics.
Substrate materials
The most common substrate material used for making FETs remains pure Silicon (Si). It is relatively cheap, durable and easy to work. Most digital devices are made using FET transistors on silicon.
Also used are alloys such as Gallium Arsenide (GaAs) and Silicon Germanium (SiGe). Each offers a faster switching speed than pure Si, but at greater material cost. GaAs also offers special properties such as a natural radiation hardness which with Si requires a diamond coating to match, as well as being invisible to IR frequencies.
Other materials and alloys such as Indium Phosphide (InP) and sapphire are used in laboratory situations.
Advantages and disadvantages of FETs compared to BJTs and Vacuum Tubes
Compared to BJTs, FETs have faster switching speeds (time switching On to Off) and generate less heat per switch, than the BJT. FETs can be designed to smaller geometries than BJTs, allowing greater number of individual transistors per square micron of semiconductor space. However, FETs, particularly the early MOSFETs are more liable to damage from Electro-Static Discharge (ESD) than were the BJT devices.
Compared to Vacuum Tubes, FETs are orders of magnitude smaller, more power efficient and cost effective. They require no cathode heater.
Vacuum Tubes are almost immune to ESD (and Electro-Magnetic Pulses such as a side effect of nuclear detonations) and are more effective at high power (KV and above) particularly at high frequencies such as in radio and television transmitters.
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