1 Chapter 9 Frequency Response PART B: High Frequency Model ECE 3120 Microelectronics II Dr. Suketu Naik 9.2.1 High-Frequency Model of MOSFET MOSFET has internal capacitances 2 βͺ The gate capacitive effect: The gate electrode forms a parallel plate capacitor with the channel. βͺ The source-body and drain-body depletion layer capacitances: These are the capacitances of the reverse-biased pn-junctions. Internal Capacitances: Cgs, Cgd, Csb, Cdb ECE 3120 Microelectronics II Dr. Suketu Naik Internal Capacitances: Cgs, Cgd, Csb, Cdb πͺππ = πΎπ³πππͺππ π πͺππ = πΎπ³πͺππ + πͺππ π πͺππ = πͺππ πͺππ = πͺπ π = 3 π³ππ = π% − ππ% π¨π π πππ π₯ππ§π ππ‘ π³ πΊππΊπ πͺππ = πππ Vo=Junction built-in-voltage~0.6-0.8V πͺπππ π + π½πΊπ©/π½π πͺπ ππ π + π½π«π©/π½π ECE 3120 Microelectronics II Dr. Suketu Naik 4 High-Frequency Model of MOSFET Body and Source not connected Body and Source connected Note: In IC, source to body 'ties' are made by drawing contacts ECE 3120 Microelectronics II Dr. Suketu Naik 5 High-Frequency Model of MOSFET Simplified Model What are Cgs and Cgd? What are their effects? ECE 3120 Microelectronics II Dr. Suketu Naik 6 Unity Gain Frequency fT Amplifier Current Gain=1 at fT ππ ππ» = ππ (πͺππ + πͺππ ) π. π πππ°π« ππ» = π π³ ππͺπΆπΏπΎπ³ Dependence on internal caps π/π Dependence on ID and L 0.25 μm technology L=0.25μm, Vov=0.2, μn=450cm2/Vs; Cox=8.6e-3F/m2 Faster response if L is small and ID is large Unity Gain Frequency=Transition Frequency=Bandwidth of the transistor=Speed of the transistor ECE 3120 Microelectronics II Dr. Suketu Naik Problem 9.17 ECE 3120 Microelectronics II 7 Dr. Suketu Naik High-Frequency Model of BJT 8 BJT has internal capacitances βͺ Steady-state small-signal model neglects frequencydependence βͺ Actual bipolar transistors exhibit charge-storage ECE 3120 Microelectronics II Dr. Suketu Naik 9 Internal Capacitances in BJTs: Cπ, Cμ πͺπ = πͺπ π + πͺππ πͺπ π = ππππ = πππ°πͺ/π½π» πͺππ = ππͺπππ πͺππ πͺπ = π½πͺπ© π+ π½ππ Base-emitter junction cap Small-signal diffusion cap Depletion capacitance π Look them up in the datasheet ECE 3120 Microelectronics II Dr. Suketu Naik High-Frequency Model of BJT 10 What are rx , Cπ , and Cμ? What are their effects? ECE 3120 Microelectronics II Dr. Suketu Naik 11 Unity Gain Frequency fT Amplifier Current Gain=hFE=1 at fT π·π ππ» = ππ (ππ (πͺπ + πͺπ ) Dependence on internal caps Unity gain frequency = transition frequency = frequency at which current gain=1 =Gain Bandwidth Product (GBP) ECE 3120 Microelectronics II Dr. Suketu Naik Problem 9.23 ECE 3120 Microelectronics II 12 Dr. Suketu Naik
