Practice 3: Semiconductors Digital Electronic Circuits Semester A 2012
VLSI Fabrication Process
VLSI Very Large Scale Integration The ability to fabricate many devices on a single substrate within a given process flow/timeframe independent of the number of devices fabricated. One of the most important inventions of the 20 th Century. Similar to a printing press. 3
VLSI Transistor Fabrication P-type Substrate Well Definition Gate Oxide Polysilicon Diffusion Implants Interconnect 4
VLSI nmos Transistor nmos Source is defined as lower voltage Substrate voltage (body) is shared by all nmos transistors 5
MOS Operation
MOS Operation Terminals and Sizes 7
MOS Operation Cut Off VGS<VT The channel is not Inverted. IDS=0 V gs = 0 + - g + - V gd s d n+ n+ p-type body b 8
MOS Operation Linear Region VGS>VT The channel is Inverted. VDS>0 The channel has an equivalent resistance. The current is approximately V DS /R eq. But how is this estimated? We can integrate the charge over the channel, because: I Q polysilicon gate W t ox n+ L n+ p-type body SiO2 gate oxide (good insulator, ox = 3.9) 9
MOS Operation Linear Region Charge Density: Velocity: Current estimation: Q x C x V x dx dx ox C x Cpp _ ox W dx CoxWdx t ox V x V V V V V x V cap GS XS T GS T Q x dx CoxW VGS V x VT dx dx x x dv x dv x I x Q x x CoxW VGS V x VT dx dx C pp l A 2 m V m sec m V sec 10
MOS Operation Linear Region Integrate the total current in the channel: xd xs I V V K V V DS GT DS D I x dx C V V oxw VGS V x VT dv x S V IDS L CoxW VGS VT VDS 2 V 2 2 DS 2 DS W K Cox L 11
MOS Operation Saturation Region VGS>VT The channel is Inverted. VDS>VGS-VT The channel is pinched off. The voltage drop over the channel is constant (VGS-VT). The channel resistance is almost constant. Therefore the current is almost constant. KN I V V 2 2 DS GS Tn 12
MOS Operation Velocity Saturation VGS>VT The channel is Inverted. For a high electric field, the mobility becomes saturated. We will call the VDS at which this happens VDSAT Increasing VDS past VDSAT will not increase the current. V IDS KN VGS VTn VDSATn 2 DSATn 2 13
MOS Operation Channel Length Modulation Is the resistance truly constant as VDS increases? No, it gets smaller. The depletion region of the Drain digs in to the channel. The effective length is reduced. We call this effect Channel Length Modulation This is almost linearly dependent on VDS. We will characterize this effect with a coefficient: λ I CLM I 1V DS DS DS During linear operation, this effect is almost negligible 14
MOS Operation Summary V min V, V, V DS, eff DS DSAT GT VGT VGS VT W N KN NCox L N V 2 DS, eff DS N GT DS eff DS I K V V 1 V, 2 15
Examples
Trick Question: Moed Aleph 2009-10 17
Example: Linear vs. Saturation Consider a process with: 2 cm Lmin 0.4m tox 8nm n 450 VT 0.7V 0 ox 3.45 10 V s WN 8 For a long channel transistor with m : 0.8m Bias the transistor as a current source with of 100μA. First we will calculate the Transconductance: C k ox ox t 810 m m ox 11 3.45 10 3 F ff 4.32 10 4.32 9 2 2 2 cm ff 6 F A 2 2 ' C 450 4.32 1.94 10 194 V s m V s V n n ox ' W A 8m A k N n kn 194 1.94m L 2 2 N V 0.8m V L N 11 18
Example: Linear vs. Saturation To use our nmos as a current source, we need to bias it in the Saturation region: KN I 2 DS VGS VTn 100 A 2 VGS 1.02V But we still need to make sure we are in saturation, so: V V V 0.32V DS GS T 19
Example: Linear vs. Saturation Now use the device as a resistor with R=1kΩ. We now want to operate our transistor in the linear region with a very small VDS. R V I K V V K V V 2 2 DS DS N GT DS N GT DS eq VDS VDS 1 1000 I K V V K V DS N GT DS N GT 1 1000 V 0.7 0.52V V 1.22V 6 194 10 10 VGS 0.7 GS GS 20
The Body Effect
The Body Effect - Introduction The threshold voltage (VT) of a transistor is affected by a non-zero potential between the Source and Body: V V 0 2 V 2 T T F SB F nmos pmos Φ F - + γ + - V SB + - 22
The Body Effect - Example Moed Aleph, Semester B, 2008-9 23
The Body Effect - Example The threshold voltage of a pmos was measured at standard conditions to be V Tp (V BS =0)=-0.4V. Given a body effect coefficient of -0.4 and Fermi Potential of 0.3V, calculate the threshold voltage with a Reverse Body Bias of 2.5V. V V 0 2 V 2 T T F SB F 0.4 0.4 0.6 2.5 0.6 0.4 0.40.98 0.79V 24