ELECTRONIC EVICES Assst. prof. Laura-Ncoleta IVANCIU, Ph.. C13 MOSFET operaton
Contents Symbols Structure and physcal operaton Operatng prncple Transfer and output characterstcs Quescent pont Operatng regons Examples 2
Prevously on E (C11): Transstors = actve semconductor devces, wth three termnals - used to amplfy or swtch sgnals - essental components of electronc crcuts - dscrete or ntegrated Operatng prncple: e voltage appled between two termnals (command) controls the current through the thrd termnal 3
Symbols Metal-oxde-semconductor feld effect transstors (MOSFETS) n-channel enhancement-type p-channel enhancement-type general symbols (w/ B substrate termnal) smplfed symbols (B nternally connected to S) other symbols 4
Symbols Metal-oxde-semconductor feld effect transstors (MOSFETS) n-channel enhancement-type p-channel enhancement-type dran (smlar to C) G gate (smlar to B) S source (smlar to E) 5
Structure and physcal operaton In order to have a dran-to-source current, two condtons must be fulflled: creaton of an n- type channel between the dran and source termnals exstence of a postve potental dfference between the dran and source to move the carrers 6
Structure and physcal operaton OPTIONAL V threshold voltage V V I S < V = 0 = 0 No possblty for the current to flow from dran to source No channel 7
Structure and physcal operaton OPTIONAL ue to the electrc feld created by V GB > 0 (V > 0), the electrons n the substrate wll be attracted and accumulated just under the oxde (gate regon). When V s ncreased further, the electron concentraton becomes larger than the hole concentraton; ths process s called populaton nverson. V V I S > V = 0 = 0 Inverson creates a conductng n-channel between the dran and the source. e gate voltage V at whch nverson produces an n concentraton equal to the unbased p concentraton s called the threshold voltage V. 8
Structure and physcal operaton V 0 I < V > > V S 0 < V Ssat OPTIONAL e current flows through the channel under the acton of V S >0. e transstor operates n the lnear regon. e channel wll became shallower at the dran end, because the electrons from the close vcnty of the dran regon are attracted by the postve dran regon. I I = β = [ ] 2 2 V V K W 2 L ( V V ) [ ] 2 2 V V ( V V ) e current depends lnearly on V, but also depends on V S. S S S S β constructve parameter [μa/v 2 ] K transconductance parameter [μa/v 2 ] W, L physcal dmensons of channel [μm] 9
Structure and physcal operaton OPTIONAL e V S voltage s greater than the V Ssat voltage. All the electrons from the close vcnty of the dran regon are attracted by the more postve dran regon. e channel depth becomes zero at the dran end. e transstor s now n the pnch-off regon, or saturaton regon or actve regon. I s almost ndependent of the V S. Conducton from the source to dran stll occurs wth current passng through the depleton regon next to the dran. V V I S > V > V > 0 Ssat I I V = β + V β A 2 S ( V V ) 1 ( V V ) K W = 2 L K W 2 L ( ) 2 S V V 1 ( V V ) 2 V + V e current depends non-lnearly (quadratc) on V A 2 10
Structure and physcal operaton e current through MOSFET I = f ( V, VS ) e current depends on V - conductng channel buldng (enhancement) V S controlled movement of the carrers (free electrons between and S termnals) 11
Operatng prncple n-channel enhancement type MOSFET Famly of transfer characterstcs ( v ), v S as parameter Famly of output characterstcs ( v ), S v as parameter v v S = v = V Co PS + G G = = 0 = S S 12
Transfer and output characterstcs Transfer characterstcs ( v ), v as parameter S V Ssat = v V dran-source saturaton voltage Saturaton regon (a F ) v S > V Ssat Lnear regon (exc) 2 v < = β[2( v V ) v v ] S V Ssat = β ( v V ) quadratc dependence on v 2 S S actve (saturaton) regon v S < v Ssat lnear regon lnear dependence on v - also nfluenced by the output voltage v S V = 0.58 V, β = 104 μa/v 2 13
Transfer and output characterstcs Output characterstcs ( vs ), v as parameter Saturaton regon (a F ) v S > V Ssat = β ( v V ) 2 quadratc dependence on v Lnear regon (exc) 2 v < = β[2( v V ) v v ] S V Ssat S S lnear dependence on v - also nfluenced by the output voltage v S 14
Quescent pont Quescent pont Q = a pont on the output characterstc (v S ) of MOSFET Q s defned by V S and I Q(V S, I ) s at the ntersecton between the load lne and the output characterstc correspondng to v I ex Load lne: v S = V PS - R 15
Operatng regons T (off): V < V = 0 I ex T (a F ): V < V < V 3 = β ( v V ) 2 Boundary (a F ) - (exc): v S,sat = v - V T (exc): V > V 3 ] = β[2( v V ) v S v 2 S swtchng mode: (off) / (exc) Modes of use as an amplfer: (a F ) voltage-controlled lnear resstance (small v S ) 16
Operatng regons Lnear regon (exc): Actve regon (a F ): = β[2( v V ) v S v 2 S ] = β v V 2 ( ) β parameter of the MOSFET - beta factor - measured n µa/v 2, ma/v 2, A/V 2 - constructve parameter For ntegrated transstors: = K W 2 L K W 2 L β = ( v V ) 2 In the actve regon K transconductance parameter [µa/v 2 ] W - the wdth of the channel [µm] L - the length of the channel [µm] 17
Examples Example - 1 Compute the values of V, I, V S, V G, V Ssat for: ) v Co = 0.5 V; ) v Co = 2.5 V; ) v Co = 2.75 V. Place the quescent ponts Q(V S, I ) n the plan of output characterstcs. Fnd the operatng regons of T. 5V 18
Examples Example - 2 a) What s the operatng regon of T for: ) v = 0.8 V; ) v = 2.5 V; ) v = 4 V. b) What s the mnmum value of v so that T stays n (exc)? Β = 2 ma/v 2 V = 1 V V PS 19
Examples Example - 2 Soluton: V PS a) ) Because v < V => T - (off) Β = 2 ma/v 2 V = 1 V ) v > V T s on, n (a F ) or n (exc) Assume T n (a F ), so that = β(v V ) 2 Compute v S Compare v S wth v Ssat : If v S > v Ssat, the assumpton was true, and T s n (a F ) If v S < v Ssat, the assumpton was false, and T s n (exc) 20
Examples Example - 2 Soluton: V PS a) ) = 2(2.5-1) 2 = 4.5mA Β = 2 ma/v 2 V = 1 V v S = V PS - R v S = 20-3 4.5 = 6.5 V v S sat = v V = 2.5-1= 1.5 V v S > v ssat, T s n (a F ) v G = v v S = 2.5-6.5 = -4V < V 21
Examples Example - 2 Soluton: V PS a) ) v > V T s on, n (a F ) or n (exc) Assume T n (a F ), so that = β(v V ) 2 = 2(4-1) 2 = 18 ma Compute v S v S = 20-3 18 = -34 V Compare v S wth v Ssat : v S sat = v V = 4-1= 3 V v S < v ssat, T s n (exc) Β = 2 ma/v 2 V = 1 V Alternatve method: compare the value of n (a F ) wth ex ex V = PS v R S, ex V R PS 20 = = 6.67mA 3 = 18 ma > ex = 6.67 ma T - (exc) What s the actual value of? 22
Examples Example - 2 Soluton: V PS a) Mnmum value of v so that T stays n (exc) e operatng pont of T s placed on the v Ssat curve: Β = 2 ma/v 2 V = 1 V v Ssat = V PS R v Ssat = v mn - V V PS R = v mn V = β(v mn - V ) 2 R β(v mn V ) 2 + (v mn - V ) - V PS = 0 Two solutons for v mn, choose the convenent one: v mn >V V mn = 2.744 V 23
Examples Example - 3 a) etermne the operatng regon of the transstor for: ) R 1 = 4.5 MΩ; R 2 = 0.5 MΩ; ) R 1 = 12 MΩ; R 2 = 6 MΩ; b) What can be the range for R 1 wth R 2 = 0.5 MΩ so that the transstor s (on)? 24
Summary e MOSFET s no longer a random acronym, after dggng nto: Symbols Structure and physcal operaton Operatng prncple Transfer and output characterstcs Quescent pont Operatng regons Examples Next week: Recap. Preparaton for exam. To do: Homework 10 25