Temperature Inversion in VLSI

There In a lower technology node, as temperature increases the threshold voltage decreases so overdrive voltage and drain current increase which leads decrease in cell delay. Here overdrive voltage is dominating over the mobility factor. But in higher technology nodes, overdrive voltage is not much dominating, and the delay of the cell varies as per variation in carrier mobility we have discussed as temperature increases mobility decreases and so drain current decreases which lead to increase in cell delay.

What is temperature inversion?

In general, as temperature increases, the delay of standard cells increases because of mobility degradation at higher temperatures. . In Lower nodes, the delay of the cell decreases with an increase in temperature. So in the lower technology node, the effect of temperature on the delay of the cell is inverted and this effect is called the temperature inversion. The main reason behind this inversion is in the lower technology node, the effect of the threshold voltage is dominating over the mobility.

Temperature Inversion effect in higher technology

In Higher technologies (>65nm) the operating voltage is higher than 1.2 V. So, the Overdrive voltage factor (Vgs-Vt) doesn’t play many roles in deciding the cell delay when the temperature increases from -40c to 125c. There will be a direct depending on the mobility factor alone.

The temperature inversion effect in lower technology

In Lower technologies(>65nm), the voltage threshold predominantly affects the cell delay as the voltage the chip is operated on is scaled down. Between FF-40c and FF125c, the overdrive voltage depends on Vgs as the second subtractor Vt is not that significant because of the 10% increase in the nominal voltage. But in the case of ss-40c and ss125c, the voltage itself is less than 10% of the nominal voltage. So, Vt plays a major role along with mobility.

Threshold voltage effect on cell delay

Threshold voltage decreases when the temperature increases as more opposite carriers are available to form a conducting channel. For example, in the case of NMOS, the substrate is P-type. If the temperature raises, that leads to an increase in the minority carriers which is an electron in this case. So, vt less than the usual value is required to form the channels. Thereby, cell delay decreases.

Mobility effect on cell delay

An increase in temperature causes electron scattering to an extent in which it reduces the conductivity. So, if there is an occurrence of a scattering event, the electrons have deviated from their path, thus decreasing its mobility factor. This in turn reduces the drain current. Thereby, cell delay increases.

In short, we can say that, with  increases in temperature,

• Decreases the voltage threshold and decreases the cell delay.
• Decreases mobility and increases cell delay.

Factors affecting cell delay

A cell delay depends on the charging and discharging curve of the load capacitor that is sitting at the output node of it. These curves are modified by the drain current that flows from the source to the output node for charging and ground to the output node for discharging.

Various Process Corners:

FF –> Fast Fast corner, SS –> Slow Slow corner, TT –> Typical corner

Let’s assume that the nominal voltage is 1.2 V.

• FF –> fast P-mos and fast N-mos — Voltage (1.32 V) which is 10% greater than the nominal voltage.
• SS –> slow P-mos and slow N-mos — Voltage (1.08 V) which is 10% lesser than the nominal voltage.
• TT –> P-mos and N-mos have typical characteristics — Voltage (1.2 V)

Usually, the timing closure is done in a temperature range from -40c to 125c.

What is temperature inversion mobility?

Temperature inversion is a phenomenon where, in certain voltage regions, transistors accelerate and operate faster at higher temperatures. When the temperature rises, transistor performance is influenced by two primary factors: a decrease in carrier mobility and a reduction in threshold voltage.

Why is it important to consider how the reference current changes with temperature in electronic circuits?

It is important to consider how the reference current changes with temperature because temperature variations can impact the behavior of electronic circuits. Knowing how the reference current varies with temperature helps ensure that circuits remain stable and reliable across different temperature conditions.

How does the temperature affect the output current in a basic current mirror circuit?

In a basic current mirror circuit, if the reference current (IREF) remains constant and independent of temperature, the output current will also be independent of temperature. This means that even though the MOSFET characteristics change with temperature, the relationship between the mirrored currents remains valid as long as the reference current is temperature-independent.