Goa University > Electronics Engineering > Sem 6 > VLSI

Marks: 5M

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Capacitance are of 2 types:

1. Parasitic
2. Junction

Parasitic Capacitance:

These are unwanted capacitance, but still are part of the transistor. Together with the resistances in the circuit, they put an upper limit to the speed of the transistor. First, let us see what kind of capacitance a transistor has.

Junction Capacitance:

We define 3 normalized capacitance.

$C_{ja}$ - Bottom plate capacitance per unit area $(fF/μm^2)$ $C_{jp}$ - Sidewalls away from channel capacitance per unit length $(fF/μm)$. The height is a process parameter and is included in the capacitance $C_{jpg}$ - Sidewall in front of the channel capacitance per unit length $(fF/μm)$.

The height is a process parameter and is included in the capacitance In order to calculate CSB or CDB, we need to sum all these capacitances.

$CSB=CDB=WEC_{ja}+(W+2E)C_{jp}+WC_{jpg}$

Overlap and gate-channel capacitances

$C_4$ and $C_5$ are overlap capacitances and are only proportional to the width of the transistor. $C_6$ is the gate-channel capacitance and its total value is split between drain and source in a way that depends on the region of operation of the transistor.

First, we define the normalized capacitances:

$C_{ov}$ - Overlap capacitance per unit length $(fF/μm)$

$C_{ox}$ - Gate to channel capacitance per unit area $(fF/μm^2)$

CGS and CGD have a base value of the overlap capacitance $WC_{ov}$. To that we add the gate to channel capacitance $WLC_{ox}$

In subthreshold region, there is no gate-channel capacitance because there is no channel. In saturation, the channel is pinched-off and there is no gate-channel capacitance at the drain and only two-thirds go to the source. In triode, the channel is not pinched-off and the gate-channel capacitance is split equally between drain and source.

Channel-bulk depletion capacitance

This capacitance is only relevant in subthreshold regime. In strong inversion, the channel is being driven and shields the transistor from this capacitance. However, in subthreshold there is no channel and this capacitance is in series with the oxide capacitance. Furthermore, CGS and CDS are made of only the overlap capacitance and they are small.