Lecture #17 MOS transistors MIDTERM coming up a week from Monday (October 18th) Next Week: Review, examples, circuits Reading: MOS: chapter 14 10/8/2004 EE 42 fall 2004 lecture 17 1 Midterm • Monday, October 18, • In class • One page, one side of notes 10/8/2004 EE 42 fall 2004 lecture 17 2 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 10/8/2004 EE 42 fall 2004 lecture 17 3 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. 10/8/2004 EE 42 fall 2004 lecture 17 4 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. Metal Oxide Semiconductor 10/8/2004 EE 42 fall 2004 lecture 17 5 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 Metal Oxide ------------------------Semiconductor 10/8/2004 EE 42 fall 2004 lecture 17 6 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) 10/8/2004 EE 42 fall 2004 lecture 17 7 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. 10/8/2004 EE 42 fall 2004 lecture 17 8 NMOS (N-Channel Metal Oxide Semiconductor) Transistor gate source metal n-type metal oxide insulator drain metal n-type p-type metal 10/8/2004 EE 42 fall 2004 lecture 17 9 NMOS Transistor in Equilibrium gate source metal _ + n-type+ _ + + _ _ metal oxide insulator h h drain metal _ + n-type + _ + _+ _ p-type h h metal 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 10/8/2004 EE 42 fall 2004 lecture 17 10 NMOS Transistor in Cutoff VGS > 0 gate - + source metal n-type + + + _ _ _ metal oxide insulator _ _ _ _ drain metal n-type + + + _ _ p-type h h h h h metal When a small, positive VGS is applied, holes “move away” from the gate. There is still no current flow between the source and the gate 10/8/2004 EE 42 fall 2004 lecture 17 11 NMOS Transistor Channel VGS > VTH(n) gate - + source metal n-type + + + _ _ metal oxide insulator ee_ e _e_ e _ _ drain metal n-type + + + _ _ p-type h h h h h h h h h h metal When VGS is larger than a threshold voltage VTH(n), the attraction to the gate is so great that free electrons collect there. The applied VGS 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. 10/8/2004 EE 42 fall 2004 lecture 17 12 NMOS Transistor Drain Current - + VGS > VTH(n) VDS > 0 gate - + source metal n-type + + + _ _ metal oxide insulator ee_ e _e_ e _ _ drain metal n-type + + + _ _ p-type h h h h h h h h h h metal When a positive VDS 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 VDS, as well as the number of electrons in the channel, channel dimensions, and material. 10/8/2004 EE 42 fall 2004 lecture 17 13 NMOS Transistor CircuitVSymbol DS - + VGS drain gate - + source ID IG metal oxide insulator metal n-type metal n-type p-type metal G IG ID S 10/8/2004 - VDS + D EE 42 fall 2004 lecture 17 14 NMOS I-V Characteristic G IG ID S - VDS + D • Since the transistor is a 3-terminal device, there is no single I-V characteristic. • Note that because of the insulator, IG = 0 A. • We typically define the MOS I-V characteristic as ID vs. VDS for a fixed VGS. • The I-V characteristic changes as VGS changes. 10/8/2004 EE 42 fall 2004 lecture 17 15 NMOS I-V Curves ID triode mode saturation mode VGS = 3 V VDS = VGS - VTH(n) VGS = 2 V VGS = 1 V cutoff mode (when VGS < VTH(N)) 10/8/2004 EE 42 fall 2004 lecture 17 VDS 16 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 10/8/2004 EE 42 fall 2004 lecture 17 17