SEMICONDUCTORS

1. SEMICONDUCTORS FET, JFET and MOSFET Transistors

2. SEMICONDUCTORS Transistors are categorized by: • Semiconductor material : germanium, silicon, gallium arsenide, silicon carbide, etc. • Structure: BJT, JFET, IGFET (MOSFET), IGBT, "other types" • Polarity: NPN, PNP (BJTs); N-channel, P-channel (FETs)

3. SEMICONDUCTORS • Maximum power rating: low, medium, high • Maximum operating frequency: low, medium, high, radio frequency (RF), microwave (The maximum effective frequency of a transistor is denoted by the term fT, an abbreviation for "frequency of transition". • The frequency of transition is the frequency at which the transistor yields unity gain.

4. SEMICONDUCTORS • The field-effect transistor (FET), sometimes called a unipolar transistor, uses either electrons (in n-channel FET) or holes (in p-channel FET) for conduction. • The terminals of the FET are named source, gateand drain.

5. SEMICONDUCTORS • On most FETs, the body is connected to the source inside the package, and this will be assumed for the following description. • To turn on a transistor it has to be charged like a capacitor. • One polarity of charge is responsible for conduction, the other serves for charge neutrality.

6. SEMICONDUCTORS • In the BJT, both types of charge carriers come close together and so the capacitance is high, therefore only low voltages are needed to produce a given amount of charge. • In a FET both types of charges are separated by the dielectric.

7. SEMICONDUCTORS • For low noise at narrow bandwidth the higher input resistance of the FET is advantageous. • FETs are divided into two families: • Junction FET (JFET) • Insulated gate FET (IGFET)

8. SEMICONDUCTORS • Like a BJT transistor, a JFET (field effect transistor) has three different terminals—but they have the names source (emitter), drain (collector), and gate (base).

9. SEMICONDUCTORS • JFET is the simplest type of field-effect transistor. • It can be used as an electronically-controlled switch or as a voltage-controlled resistance. • Electric charge flows through a semiconducting channel between "source" and "drain" terminals.

10. SEMICONDUCTORS • By applying a bias voltage to a "gate" terminal, the channel is "pinched", so that the electric current is impeded or switched off completely.

11. SEMICONDUCTORS • The IGFET is more commonly known as metal–oxide–semiconductor FET (MOSFET), from their original construction as a layer of metal (the gate), a layer of oxide (the insulation), and a layer of semiconductor.

12. SEMICONDUCTORS • All transistors work by controlling the movement of electrons, but not all of them do it the same way. • In a FET, the layers of N-type and P-type silicon are arranged in a slightly different way and coated with layers of metal and oxide. • That gives us a device called a MOSFET (Metal Oxide Semiconductor Field Effect Transistor).

13. SEMICONDUCTORS • The MOSFET cut away showing the substrate or body, which is typically connected to the source.

14. SEMICONDUCTORS • Although there are extra electrons in the N-type source and drain, they cannot flow from one to the other because of the holes in the P-type gate in between them.

15. SEMICONDUCTORS • However, if we attach a positive voltage to the gate, an electric field is created there that allows electrons to flow in a thin channel from the source to the drain. • This "field effect" allows a current to flow and switches the transistor on:

16. SEMICONDUCTORS • A big advantage of MOSFETs for digital switching is that the oxide layer between the gate and the channel prevents DC current from flowing through the gate, further reducing power consumption and giving a very large input impedance. • The insulating oxide between the gate and channel effectively isolates a MOSFET in one logic stage from earlier and later stages, which allows a single MOSFET output to drive a considerable number of MOSFET inputs.