Announcements EE0 - Fall 00 Microelectronic evices and Circuits ecture 7 Homework, due today Homework due net week ab this week Reading: Chapter MO Transistor ecture Material ast lecture iode currents MO capacitor This lecture MO transistor Models MO C Curve F Q Tn Q,ma ( ( mall-signal capacitance is slope of Q- curve Capacitance is linear in accumulation and inversion Capacitance is depletion region is smallest Capacitance is non-linear in depletion Co F C Tn Co C- Curve Equivalent Circuits Co C o C dep Co ε s CdepCo Cdep = Co Co Ctot = = = dep Cdep + C Cdep ε s t o o + + C ε o o dep n accumulation mode the capacitance is ust due to the voltage drop across t o n inversion the incremental charge comes from the inversion layer (depletion region stops growing. n depletion region, the voltage drop is across the oide and the depletion region MOFET Cross ection gate body source drain diffusion regions n+ n+ Add two unctions around MO capacitor The regions forms PN unctions with substrate MOFET is a four terminal device The body is usually grounded (or at a C potential For Cs, the body contact is at surface 6
MOFET ayout PMO & NMO contact poly gate n+ n-type substrate PMO Planar process: complete structure can be specified by a layout esign engineer can control the transistor width and Process engineer controls t o, N a,, etc. 7 A MOFET by any other name is still a MOFET: NMO, PMO, nmo, pmo NFET, PFET FET Other flavors: JFET, MEFET CMO technology: The ability to fabricated NMO and PMO devices simultaneously 8 CMO Circuit ymbols n+ n-type substrate PMO Complementary MO: oth P and N type devices Create a n-type body in a through compensation. This new region is called a well. To isolate the PMO from the NMO, the well must be reverse biased (pn unction 9 The symbols with the arrows are typically used in analog applications The body contact is often not shown The source/drain can switch depending on how the device is biased (the device has inherent symmetry 0 Observed ehavior: - Observed ehavior: - / k = non-linear resistor region constant current resistor region = Current zero for negative gate voltage Current in transistor is very low until the gate voltage crosses the threshold voltage of device (same threshold voltage as MO capacitor Current increases rapidly at first and then it finally reaches a point where it simply increases linearly T = For low values of drain voltage, the device is like a resistor As the voltage is increases, the resistance behaves non-linearly and the rate of increase of current slows Eventually the current stops growing and remains essentially constant (current source
inear Region Current NMO > Tn p-type nversion layer channel 00m y f the gate is biased above threshold, the surface is inverted This inverted region forms a channel that connects the drain and gate f a drain voltage is applied positive, electrons will flow from source to drain MOFET inear Region The current in this channel is given by = v Q y N The charge proportional to the voltage applied across the oide over threshold Q = C ( N o Tn = v C ( y o Tn f the channel is uniform density, only drift current flows vy = µ ne y Ey = = nco( Tn > Tn µ 00m MOFET: ariable Resistor Notice that in the linear region, the current is proportional to the voltage = nco( Tn µ Can define a voltage-dependent resistor Req = = = R ( µ C ( n o Tn MOFET: ariable Resistor Notice that in the linear region, the current is proportional to the voltage = C ( µ n o Tn Can define a voltage-dependent resistor Req = = = R ( µ C ( n o Tn This is a nice variable resistor, electronically tunable! This is a nice variable resistor, electronically tunable! 6 Finding = f (, Approimate inversion charge Q N (y: drain is higher than the source less charge at drain end of channel nversion Charge at ource/rain Q ( y Q ( y = 0 + Q ( y N N N = ( y = 0 = Co ( Q N ( y = = Co ( = 7 8
Average nversion Charge ource End rain End Co( T + Co( T ( y Co ( T + Co ( T ( y Co ( T Co ( y = Co( T Charge at drain end is lower since field is lower imple approimation: n reality we should integrate the total charge minus the bulk depletion charge across the channel 9 rift elocity and rain Current ong-channel assumption: use mobility to find v µ n vy ( = µ ney ( µ n( / y = ubstituting: = v µ Co( T C ( µ o T nverted Parabolas 0 quare-aw Characteristics The aturation Region TROE REON oundary: what is,at? hen > Tn, there isn t any inversion charge at the drain according to our simplistic model ATURATON REON hy do curves flatten out? quare-aw Current in aturation Current stays at maimum (where = Tn =,AT = C ( µ o T T, sat Co( T ( T µ = µ Co, sat = ( T Measurement: increases slightly with increasing model with linear fudge factor µ C o, sat= ( T ( +λ Pinching the MO Transistors epletion Region NMO > Tn p-type Pinch-Off Point hen >,sat, the channel is pinched off at drain end (hence the name pinch-off region rain mobile charge goes to zero (region is depleted, the remaining elecric field is dropped across this high-field depletion region As the drain voltage is increases further, the pinch off point moves back towards source Channel ength Modulation: The effective channel length is thus reduced higher
hort-channel MOFET Model inear - Characteristics: short-channel MOFET Channel (inversion charge: neglect reduction at drain elocity saturation defines,at =E sat = constant rain current: -v sat / µ n = vq = ( v [ C (, AT N sat o Tn E sat = 0 /cm, = 0. µm,at = 0.! = v C ( ( + λ, AT sat o Tn n ], 6 versus 6 0- Resistive =. aturation =.0 -. 0. =. =.0 (A = - T =. (A =. =.0 0. =.0 0 0 0... ( ong Channel 0 0 0... ( hort Channel 7