Symbols Structure Operating principle Terminal characteristics Operating regions Quiescent point position Metal-Oxide-Semiconductor Field Effect Transistor n-channel and p-channel enhancement-type MOSFET 1 / 18
Symbols general symbols dignifying the B substrate terminal n-channel enhancement-type p-channel enhancement-type simplified symbols where the substrate is connected to the source other symbols drain G gate S source B body (substrate) 2 / 18
Physical structure L - channel length W channel width 3 / 18
Physical Operation Optional No possibility for the current to flow from drain and source; no channel I < 0 0 4 / 18
Physical Operation cont. I > 0 0 ue to the electric field created by GB >0 ( >0), the electrons in the substrate will be attracted and accumulated just under the oxide (gate region). When is increased further, the electron concentration becomes larger than the hole concentration; this process is called inversion. Inversion creates the conducting n-channel between the drain and the source. e gate voltage at which inversion produces an n concentration equal to the unbiased p concentration is called the threshold voltage. I 0 Optional because 0 5 / 18
Physical Operation cont. Optional e current flows through the channel under the action of >0. e transistor operates in the linear region. e channel will became shallower at the drain end because the electrons from the close vicinity of the drain region are cached by the positive drain region. 0 < I > > 0 < sat I I β K W 2 L [ ( ) ] 2 2 [ ( ) ] 2 2 6 / 18
Physical Operation cont. Optional I > > > 0 sat I I e voltage is greater than the sat voltage. All the electrons from the close vicinity of the drain region are cached by the more positive drain region. e channel depth becomes zero at the drain end. e transistor is now in the pinch-off region, or saturation region or active region. I is almost independent of the. Conduction from the source to drain still occurs with current passing through the depletion region next to the drain. β + β A K W 2 L + A 2 ( ) 1 ( ) ( ) 2 1 ( ) 2 K 2 W L 2 7 / 18
Operating principle For a better understanding of MOSFET we will study the terminal characteristics for the n-channel enhancement-type MOSFET v v Co Transfer characteristics i - Output characteristics i + i G i S i i G current 0 i S node 8 / 18
Transfer characteristic v treshold voltage < v T is off (no current) I v Co 0 v > T is on (in conduction) 0,58 I > 0 9 / 18
Transfer characteristic v > sat i β ( v ) 2 i depend just on the square of v sat v 0.58, β104μa/ 2 10 / 18
Transfer and output characteristics 11 / 18
Illustration 5 What are the values of, I,, G, sat for the following Co? Co1 Co2 Co3 2 2.5 2.8 What are the position of the operating points (quiescent points) Q(, I ), in the plan of output characteristics? What are the operating region of T? 12 / 18
Linear region: Active region: i β[2( v ) v v 2 ] i β ( v ) 2 β parameter of the MOSFET - beta factor - unit of measure µa/ 2, ma/ 2, A/ 2 - constructive parameter For integrated transistors: K W 2 L K W 2 L β i ( v ) 2 In the active region K transconductance parameter, µa/ 2 W - the width of the channel L - the length of the channel 13 / 18
Operating point (quiescent point): Q(I, ) Load line: v PS -R i Q(I, ) is situated at the intersection of the load line with the output characteristic corresponding to the applied 14 / 18
Operating regions I ex T (off): < T (a F ): < < 3 T (exc): > 3 Observation: the transistor shouldn t be biased very close to the origin of the axes or to one axis switching mode (off) (exc) Ways to use as an amplifier (a F ), eventually (a R ) voltage-controlled linear resistance 15 / 18
Exemplification a) What is the operating region of T for 1.v 0.8; 3.v 4 2.v 2.5; b) e minimum value of v such that T stays in (exc)? β2ma/ 2 1 PS 1. v < so T (off) 2. v > so T (exc)or(a F ) compare v with v sat v PS -R i Let s assume T in (a F ) i β (v ) 2 i 2 (2.5-1) 2 4.5mA v 20-3 4.5 6.5 v sat v 2.5-1 1.5 v >v ssat,t is in (a F ). v G v -v 2.5-6.5-4 < 16 / 18
β2ma/ 2 1 PS 3. v >, so T (exc)or (a F ) i 2 (4-1) 2 18mA v 20-3 18-34 the assumption T - (a F ) is false T is in (exc) Another method: compare the value of i in (a F ) with i ex PS v, ex PS 20 iex 6,67mA R R 3 i 18mA > i ex 6.67mA T - (exc) 17 / 18
β2ma/ 2 1 PS b) minimum value v min for that T is in (exc) e operating point of T placed on the v sat curve v sat PS -Ri v sat v min - PS -Ri v min i β(v min - ) 2 R β(v min ) 2 + (v min - ) PS 0 Two solutions for v min Select the convenient one v min > min 2.744 18 / 18