1. 10/8/2004EE 42 fall 2004 lecture 171 Lecture #17 MOS transistors MIDTERM coming up a week from Monday (October 18 th ) Next Week: Review, examples, circuits Reading: MOS: chapter 14

2. 10/8/2004EE 42 fall 2004 lecture 172 Midterm Monday, October 18, In class One page, one side of notes

3. 10/8/2004EE 42 fall 2004 lecture 173 Topics Today: Metal Oxide Semiconductor (MOS) Transistors –Physical structure –Physical operation –Circuit symbol and current/voltage designations –Modes of operation –I-V Relationship –Solution of MOS circuits

4. 10/8/2004EE 42 fall 2004 lecture 174 MOS transistor The bipolar transistor controls current trough a reverse biased diode, so it is intrinsically a current controlled device, and it produces a current output. A MOS transistor uses a voltage to pinch off a conductive channel, and therefore is like a voltage controlled switch.

5. 10/8/2004EE 42 fall 2004 lecture 175 MOS In a MOS device, a voltage is applied to a metal layer, which pushes away mobile carriers in a semiconductor layer. The metal is separated from the semiconductor by an insulating layer, usually an oxide. Semiconductor Oxide Metal

6. 10/8/2004EE 42 fall 2004 lecture 176 MOS If the voltage on the metal is negative, it attracts holes into the semiconductor If the voltage on the metal is positive, it attracts electrons into the semiconductor Semiconductor Oxide Metal - - - - - - - - - - - - - - - - - - - - - - - - -

7. 10/8/2004EE 42 fall 2004 lecture 177 The MOS as a switch If there are mobile carriers in the semiconductor, they can conduct a current. (switch closed) A device which uses electrons needs a positive voltage on the metal to conduct –called an NMOS device A device which uses holes needs a negative voltage on the metal to conduct. –called a PMOS device If there are no mobile carriers under the oxide, then no current can flow (switch open)

8. 10/8/2004EE 42 fall 2004 lecture 178 NMOS and PMOS What determines if the mobile carriers that a device uses are holes or electrons? The contacts on either end. If the contacts on the ends are N type, they will allow electrons to flow under the oxide, but holes would be blocked by a reverse biased diode If the contacts on the ends are P type, they will allow holes to flow under the oxide, but electrons are blocked from flowing by a reverse biased diode.

9. 10/8/2004EE 42 fall 2004 lecture 179 n-type metal oxide insulator metal p-type metal gate source drain n-type NMOS (N-Channel Metal Oxide Semiconductor) Transistor

10. 10/8/2004EE 42 fall 2004 lecture 1710 n-type metal oxide insulator metal p-type metal gate source drain n-type + _ _ _ _ _ _ A PN junction separates the N regions from the P regions with a depletion region. No current flows between the source and the drain because of these depletion regions, even if there is a voltage difference between the drain and the source h h h h NMOS Transistor in Equilibrium _ _ + + + + + + +

11. 10/8/2004EE 42 fall 2004 lecture 1711 n-type metal oxide insulator metal p-type metal gate source drain n-type ++++++ _ _ _ _ _ _ When a small, positive V GS is applied, holes “move away” from the gate. There is still no current flow between the source and the gate h h hh - + ___ h V GS > 0 NMOS Transistor in Cutoff

12. 10/8/2004EE 42 fall 2004 lecture 1712 n-type metal oxide insulator metal p-type metal gate source drain n-type ++++++ _ _ _ _ _ _ When V GS is larger than a threshold voltage V TH(n), the attraction to the gate is so great that free electrons collect there. The applied V GS creates a channel under the gate (an area with free electrons). Now current can flow if there is a voltage from the source to the drain. h h hh - + ___ h V GS > V TH(n) h h hh h e e e e e NMOS Transistor Channel

13. 10/8/2004EE 42 fall 2004 lecture 1713 n-type metal oxide insulator metal p-type metal gate source drain n-type ++++++ _ _ _ _ _ When a positive V DS is applied, the free electrons flow from the source to the drain. (Positive current flows from drain to source). The amount of current depends on V DS, as well as the number of electrons in the channel, channel dimensions, and material. h h hh - + h V GS > V TH(n) h h hh h - + V DS > 0 _ ___ e e e e e NMOS Transistor Drain Current

14. 10/8/2004EE 42 fall 2004 lecture 1714 n-type metal oxide insulator metal p-type metal gate source drain n-type - + V GS - + V DS IDID IGIG G D S IDID IGIG - V DS + + V GS _ NMOS Transistor Circuit Symbol

15. 10/8/2004EE 42 fall 2004 lecture 1715 G D S IDID IGIG - V DS + + V GS _ NMOS I-V Characteristic Since the transistor is a 3-terminal device, there is no single I-V characteristic. Note that because of the insulator, I G = 0 A. We typically define the MOS I-V characteristic as I D vs. V DS for a fixed V GS. The I-V characteristic changes as V GS changes.

16. 10/8/2004EE 42 fall 2004 lecture 1716 triode mode cutoff mode (when V GS < V TH(N) ) saturation mode V DS IDID V GS = 3 V V GS = 2 V V GS = 1 V V DS = V GS - V TH(n) NMOS I-V Curves

17. 10/8/2004EE 42 fall 2004 lecture 1717 Saturation in a MOS transistor At low Source to drain voltages, a MOS transistor looks like a resistor which is “turned on” by the gate voltage If a more voltage is applied to the drain to pull more current through, the amount of current which flows stops increasing→ an effect called pinch-off. Think of water being sucked through a flexible wall tube. Dropping the pressure at the end in order to try to get more water to come through just collapses the tube. The current flow then just depends on the flow at the input: VGS This is often the desired operating range for a MOS transistor, as it gives a current source at the drain as a function of the voltage from the gate to the source. Note the different use of the word saturation for MOS and Bipolars