1. MOSFETs Monday 19 th September

2. MOSFETs Monday 19 th September In this presentation we will look at the following: State the main differences between BJTs and MOSFETs Describe the structure of an n-channel enhancement MOSFET Explain the ON OFF states of a MOSFET State uses of MOSFETs

3. BJTs - basics Bipolar junction transistors (BJTs) consist of three separate doped semiconductor regions - bipolar refers to the use of electrons and holes Monday 19 th September The regions in this npn BJT are: n type emitter (e) p type base (b) n type collector (c) b c e Note the direction the arrow points in the diagram! A pnp BJT is similar but the ntype replaces the ptype and vice versa - also the arrow is in the opposite direction!

4. BJTs - biasing A common emitter BJT is of the type shown below, where the emitter is connected to -V, the base to +V and the load is placed between the collector and a larger +V Monday 19 th September 1 2 1 The emitter base p-n junction is forward biased so majority charge carriers in n type can easily move emitter to base 2 The base collector p-n junction is reverse biased so a small back (leakage) current is present - a large collector voltage allows the minority carriers in p type become majority in the n type creating a larger current base collector current emitter base current

5. BJTs - MOSFETs Monday 19 th September So a small emitter - base current (or voltage) controls a larger base-collector current (or voltage) by controlling the flow of electrons and holes However, MOSFETs use either electrons or holes to conduct by creating a channel between two metal terminals

6. MOSFETs - basics Metal Oxide Field Effect Transistors (MOSFETs) also consists of three separate doped semiconductor regions… Monday 19 th September …However, FETs have four terminals for example this n channel enhancement MOSFET: Silicon dioxide layer nnp s g d p-type substrate -+

7. MOSFETs - basics Monday 19 th September The terminals in this n channel enhancement MOSFET are: drain (d) gate (g) source (s) The additional connection at the drain indicates its connection to the fourth terminal - the substrate g s d

8. MOSFETs - basics Monday 19 th September A MOSFET is of the type shown below, where the source is connected to -V, the gate to +V and the load is placed between the drain and a larger +V A small source gate current and hence voltage V GS controls a larger drain gate voltage V DS This allows electron flow between source and drain producing a drain current I D

9. MOSFETs - basics Let’s look at how it works… Monday 19 th September nnp s g d -+ V GS is applied creating an electric field which is transmitted through the silicon layer As V GS is increased the electric field repels holes in the p-type substrate and draws the minority carriers (electrons) in towards the oxide layer Combined with electrons from the n-type this creates an n-channel

10. MOSFETs - basics Monday 19 th September As V GS exceeds the threshold voltage (V TH ) (approximately 1.8-2.0V) the number of mobile electrons in the inversion layer increases By increasing V DS the MOSFET conducts producing I D nnp s g d -+ n-channel inversion layer This increases until V GS – V DS = V TH at this point I D saturates (pinch off point)

11. Monday 19 th September