Presentation on theme: "Basic MOS Device Physics Lecture 16 MSE 515. Topics MOS Structure MOS IV Characteristics CCD."— Presentation transcript:
Basic MOS Device Physics Lecture 16 MSE 515
Topics MOS Structure MOS IV Characteristics CCD
Revolution and Evolution in Electronics
NMOS Structure L D is caused by side diffusion Source: the terminal that provides charge carriers. (electrons in NMOS) Drain: the terminal that collects charge carriers. Substrate contact--to reverse bias the pn junction Connect to most negative supply voltage in most circuits.
Although no current should ideally conduct before threshold, a small percentage of electrons with energy greater than or equal to a few kT have sufficient energy to surmount the potential barriers! Subthreshold Characteristics Short-Channel MOSFETs As a result, there is a slight amount of current conduction below V T
Potential contours in a long channel MOSFET. In a long channel MOSFET, the potential is uniform and parallel to the gate. Short-Channel MOSFETs
Narrow Width Effect If the Polysilicon gate is atop the region of a LOCOS isolation where the oxide is increasing in thickness. It is possible to form a channel under LOCOS away from the thin gate oxide! This is quite important for devices with L < 1 m. Short-Channel MOSFETs
CMOS Structure PMOS NMOS Reverse bias the pn junction Connect to most positive supply voltage in most circuits.
MOS IV Characteristics Threshold Voltage Derivation of I/V Characteristics – I-V curve – Transconductance – Resistance in the linear region Second Order Effect – Body Effect – Channel Length Modulation – Subthreshold conduction
Threshold Voltage 1. Holes are expelled from the gate area 2.Depletion region (negative ions) is created underneath the gate. 3.No current flows because no charge carriers are available.
MOSFET as a variable resistor The conductive channel between S and D can be viewed as resistor, which is voltage dependent.
Threshold Voltage (3) When the surface potential increases to a critical value, inversion occurs. 1.No further change in the width of the depletion region is observed. 2.A thin layer of electrons in the depletion region appear underneath the oxide. 3.A continuous n-type (hence the name inversion) region is formed between the source and the drain. Electrons can no be sourced from S and be collected at the drain terminal. (Current, however, flows from drain to source) 4.Further increase in VG will fruther incrase the charge density. The voltage VG required to provide an inversion layer is called the threshold voltage.
Implantation of p+ dopants to alter the threshold Threshold voltage can be adjusted by implanting Dopants into the channel area during fabrication. E.g. Implant p+ material to increase threshold voltage.
Formation of Inversion Layer in a PFET The VGS must be sufficient negative to produce an inversion layer underneath the gate.
Channel Charge A channel is formed when V G is increased to the point that the voltage difference between the gate and the channel exceeds V TH.
Application of VDS What happens when you introduce a voltage at the drain terminal?
Channel Potential Variation V X the voltage along the channel V X increases as you move from S to D. V G -V X is reduced as you move from S to D. E.g. VS=0, VG=0.6, VD=0.6 At x=0, V G -V X =0.6 (more than VTH) At x=L, V G -V X =0 (less than VTH)
Pinch Off Small VDS Large VDS No channel Electrons reaches the D via the electric field in the depletion region Saturation Region Linear Region
MOSFET as a controlled linear resistor 1.Take derivative of I D with respect to V DS 2. For small VDS, the drain resistance is
Transistor in Saturation Region I-V characteristics Transconductance Output resistance Body transconductance
Saturation of Drain Current
Transconductance Analog applications: How does I ds respond to changes in VGS?
IDS vs VGS 0.13 um NMOS VDS=0.6 V W/L=12um/0.12 um VB=VS=0 Y axis: I ds X axis: V gs
Different Expressions of Transconductance
Channel Length Modulation As VDS increases, L1 will move towards the source, since a larger V DS will increase V X. L is really L1 ID will increase as VDS increases. The modulation of L due to V DS is called channel length modulation.
Controlling channel modulation For a longer channel length, the relative change in L and Hence ID for a given change in VDS is smaller. Therefore, to minimize channel length modulation, minimum length transistors should be avoided.
Output resistance due to gds
MOS Device Layout
Detector zoology X-rayVisibleNIRMIR [ m] Silicon CCD & CMOS HgCdTe InSb STJ 0.1 Si:As In this course, we concentrate on 2-D focal plane arrays. Optical – silicon-based (CCD, CMOS) Infrared – IR material plus silicon CMOS multiplexer Will not address:APD (avalanche photodiodes) STJs (superconducting tunneling junctions)
Step 2: Charge Generation Silicon CCD Similar physics for IR materials
33 CCD Introduction A CCD is a two-dimensional array of metal-oxide- semiconductor (MOS) capacitors. The charges are stored in the depletion region of the MOS capacitors. Charges are moved in the CCD circuit by manipulating the voltages on the gates of the capacitors so as to allow the charge to spill from one capacitor to the next (thus the name charge-coupled device). An amplifier provides an output voltage that can be processed. The CCD is a serial device where charge packets are read one at a time.
38 Charge Transfer Efficiency When the wells are nearly empty, charge can be trapped by impurities in the silicon. So faint images can have tails in the vertical direction. Modern CCDs can have a charge transfer efficiency (CTE) per transfer of , so after 2000 transfers only 0.1% of the charge is lost. good CTEbad CTE
Threshold Voltage VG=0.6 V VD=1.2 V CMOS: 0.13 um W/L=12um/0.12 um NFET
I-V characteristic Equation for PMOS transistor
More on Body Effect Example Analysis gmbs
Variable S-B Voltage constant
gm as function of region saturation 0.13 um NMOS VGS=0.6 V W/L=12um/0.12 um VB=VS=0 Y axis: gm X axis: vds linear
gds saturation 0.13 um NMOS VGS=0.6 V W/L=12um/0.12 um VB=VS=0 Y axis: gm X axis: vds linear Slope due to channel length modulation
Body Effect The n-type inversion layer connects the source to the drain. The source terminal is connected to channel. Therefore, A nonzero V SB introduces charges to the C dep. The math is shown in the next slide. A nonzero VSB for NFET or VBS for PFET has the net effect Of increasing the |VTH|
Experimental Data of Body Effect
W/L=12 um/0.12um CMOS: 0.13 um process VDS=50 mV Simulator: 433 mV Alternative method: 376 mV
Subthreshold current Subtreshold region As VG increases, the surface potential will increase. There is very little majority carriers underneath the gate. There are two pn junctions. (B-S and B-D) The density of the minority carrier depends on the difference in the voltage across the two pn junction diode. A diffusion current will result the electron densities at D and S are not identical.
Conceptual Visualization of Saturation and Triode(Linear) Region NMOS PMOS
I-V Characteristic Equations for NMOS transistor (Triode Region: VDSVGS-VTH To produce a channel (VGS>VTH)
V TH as a function of V SB (V TH0 : with out body effect) Body effect coefficient VSB dependent
Sensitivity of I DS to V SB (chain rule) gmgm η=1/3 to 1/4, bias dependent
Bias dependent C GS and C GD
Complete NMOS Small Signal Model
Complete PMOS Small Signal Model
Transconductance in the triode region (Triode region) For amplifier applications, MOSFETs are biased in saturation
Small signal model of an NMOS
Small Signal Model If the bias current and voltages of a MOSFET are only disturbed slightly by signals, the nonlinear amd large signal model an be reduced to linear and small signal representation.