Standby and Active Leakage Current Control

IC power dissipation consists of standby and active leakage components. The active component is dominated by the switching or dynamic power component. In addition, the standby component can be made significantly smaller than the active component by changing the body bias condition or by power gating.

Reducing supply voltage is one way to help optimizing the power consumption. However, it will lead to slower current drive of the transistor.

Reducing supply voltage means reducing the VG or the gate voltage. Thus, due to the square component the current drive is substantially lowered too. However, we can also lower down the threshold voltage of the transistor. This is because the ON-state of the transistor is achieved when:

VGS > Vth, where Vth is the threshold voltage of the transistor. 

The threshold voltage, in turn, can be controlled by body bias voltage as shown in below equation. VSB is the source-to-body substrate bias. Normally, it is a negative value, so the more negative the back bias voltage, the higher the threshold voltage will be.

That leads to harder criteria for transistor to turn-on. Another way of reducing the threshold voltage is by thinning the oxide (related to the gamma sign in the equation).

One penalty, though, in reducing the threshold voltage is the increase in sub-threshold current as shown in the figure below.

The ISUB is thus determined by an equation that looks like the IDS current equation itself, except for several exponential numbers.


Out of several possible reasons in the high IOFF. The sub-threshold current dominates as the technology node shrinks.

In addition, the sub-threshold current increases with temperature. This is the reason why testing IDDQ at high temperature can aggravate the soft-defects especially in catching non-uniform oxide thickness issue.


Cheers,
Pungky

Electron Mobility versus Temperature

Many engineers are having misconception on the relationship between temperature and electron mobility. 

Well, if you awake only during the first 15 minutes of semiconductor fundamental lecture, then you may think that high temperature results in higher electron mobility. The answer is it depends. It is just one piece of story. 

In the first place, high temperature will help electron to excite from valence to conduction band as mentioned in my earlier post. This is related to ion impurity. As the temperature gets increased further, the lattice (phonon) start to move. As more and more electrons get excited, there is a congestion of them leading to higher probability of collisions. This slows down the mobility. The higher the doping concentration, the slower the electron mobility.

 As shown in the chart on the left, the higher the doping concentration results in higher probability of collisions. This causes the electron mobility to drop.

Then, at what temperature does the higher electron mobility be at? You may for a while think that it must have been at lower temperature. But that is not true.

 As the figure on the right shows, there is an optimum temperature whereby it yields in higher electron mobility. 

The figure illustrates clearly what I have mentioned above. There are two types of phenomena affecting the electron mobility. First being the impurity scattering and second being the lattice scattering.

The optimum temperature also depends on the doping concentration. As you see on the 5 curve lines illustrating different doping concentration. At 10^19, the optimum temperature is higher than the one for 10^18 and so on. Clearly at 10^14, the curve line is not yet shown sign of going down at ~150K. 

Well hope this helps to clear your doubts. Nothing heavy, but good enough to get us to a proper foot-ground.

Cheers,
Pungky

Basic p-n junction and MOSFET transistor

Let me share very basic concepts of semiconductor as simple as possible.

Theoretically, semiconductor materials need energy to excite its electrons from valence to conduction band. You can think of valence band as the parking lots by the side of the road. Each cars needs to be activated in order to drive its way to the main road. The main road in this case is the conduction band. The activation energy can be coming from either voltage or heat. That is also the basic of p-n junction.

Now, let's talk about the transistor itself. It is actually a combination of several p-n junction. A MOSFET transistor is analogous to a tap water. It is able to block the water from flowing and able to open the flow. So, in MOSFET, the switch lies in the gate. We apply a voltage to either turn on or off the transistor. If the absolute voltage is greater than the threshold voltage, then the transistor will be on and vice versa. The higher the gate voltage, the higher the drain-source current.

There are two possible states of transistor when gate voltage is higher than threshold voltage. First, drain-source voltage is lower than the delta between gate and threshold voltage. In this case, the transistor is at linear or triode region. The transistor is acting like a resistor. Second, drain-source voltage is greater than the delta between gate and threshold voltage. In this case, the transistor is at saturation or active mode.

That's the basic on how the p-n junction and MOSFET transistor work. Next post we will go a little bit deeper.