Dr. Hari Kishore Kakarla Professor @ ECE CO2 CMOS VLSI Design (13EC206) Course Coordinator: Dr. Hari Kishore Kakarla Professor @ ECE

Drain-to-source current Ids versus Vds relationships Ids = -Isd = Charge induced in channel(Qc ) Electron Transit time

Aspects of MOS Transistor Threshold Voltage (Vt)

ALTERNATIVE FORMS OF PULL UP 1. LOAD RESISTANCE (RL) This arrangement is not often used because of the large space requirements of resistors produced in a silicon substrate.

MOS Transistor Circuit Model

Comparative Aspects of Key Parameters of CMOS and Bipolar Transistors

Bi-CMOS Inverter MOS Switches for Logic; Bipolar to drive the output loads Fig (b) An Alternative BiCMOS Inverter with no static current flow Fig (a) Simple BiCMOS Inverter

Fig (c ) An Improved BICMOS Inverter with better output logic level Fig (d) An improved BICMOS inverter using MOS transistors for base current discharge

Latch-up in CMOS Circuits Latch-up effect in p-well structure

Cont.. Latch-up effect in n-well structure

Fig: Latch-up CKT model Fig: Latch-up Current Vs Voltage SCR Gradual Raise of Current Fig: Latch-up CKT model Fig: Latch-up Current Vs Voltage When Rs decreases that makes T1 to ON Making T1  ON Makes T2 to ON (i.e Rp decreases)

Scaling Models and Scaling Factors

Device Characteristic Constant Electric Field Scaling: Device Characteristic Scaling β W / (L tox) S Current, Ids β (VDD – Vt ) 2 1/S Resistance, R VDD / Ids 1 Gate capacitance, C W L / tox Gate delay, τ RC Clock frequency, f 1/ τ Dynamic power per gate, P CV 2 f 1/S 2 Chip area, A Power density P/A Current density Ids /A

Short Channel Effect

The effective channel length Leff will be reduced due to channel-length shortening. (1) Since the channel-end voltage is equal to VDSAT, the saturation current can be found as follows: (2) The dependence of the surface electron mobility on the vertical electric field can be expressed by the following empirical formula: (3)

where is the low-field surface electron mobility and is an empirical factor. For a simple estimation of field-related mobility reduction, (3) can be approximated by (4) where is also an empirical coefficient Following the modification of the bulk charge term, the threshold voltage of the short-channel MOSFET can be written as where VT0 is the zero-bias threshold voltage calculated using the conventional long channel formula (3.23) and is the threshold voltage shift (reduction) due to the short-channel effect. Then, the bulk depletion region charge contained within the trapezoidal region is

Narrow Channel Effects MOS transistors that have channel widths W on the same order of magnitude as the maximum depletion region thickness Xdm are defined as narrow-channel devices. Similar to the short-channel effects examined earlier, the narrow-channel MOSFETs also exhibit typical characteristics which are not accounted for by the conventional General Component Analysis (GCA). For MOSFETs with small channel widths, however, the actual threshold voltage increases as a result of this extra depletion charge.

Threshold Voltage Figure: Formation of an inversion layer (channel) in an n-channel enhancement-type MOSFET In the following, physical parameters affecting the threshold voltage of a MOS structure will be examined by considering the various components of VTO. For all practical purposes, we can identify four physical components of the threshold voltage: (i) the work function difference between the gate and the channel (ii) the gate voltage component to change the surface potential (iii) the gate voltage component to offset the depletion region charge, and (iv) the voltage component to offset the fixed charges in the gate oxide and in the silicon-oxide interface.

We can calculate the depletion region charge density at surface inversion using (3.12) Note that if the substrate (body) is biased at a different voltage level than the source, which is at ground potential (reference), then the depletion region charge density can be expressed as a function of the source-to-substrate voltage VSB.

Now, we can combine all of these voltage components to find the threshold voltage. For zero substrate bias, the threshold voltage VTo is expressed as follows: For nonzero substrate bias, on the other hand, the depletion charge density term must be modified to reflect the influence of VSB upon that charge, resulting in the following generalized threshold voltage expression. The generalized form of the threshold voltage can also be written as Note that in this case, the threshold voltage differs from VTo only by an additive term. This substrate-bias term is a simple function of the material constants and of the source-to substrate voltage VSB. Thus, the most general expression of the threshold voltage VT can be found as follows: where the parameter is the substrate-bias (or body-effect) coefficient