Experimental result and discussion

The threshold voltage independence on Lg is excellent Vth characteristics. Fig1

and Fig2 show Vth vs. drawn gate length (Lg) and Ion vs. Ioff respectively for diverse

pocket implant doping concentration. The heavy pocket doping could lead to high

31

could compress off current while keeping Ion current fixed

In analog circuit applications, the voltage gain is directly proportional to Rout .In [8], it’s shown existing analytical models for MOSFET Rout are not adequate, because only

channel length modulation effect is included. Here, we notice, The Rout curve can be

clearly divided into four regions with each region dominated by a mechanism in Fig3 be

shown .It is worth noting that the second region is the near-saturation region dominated by

CLM. The third region is dominated by DIBL and the fourth is the high field region in

which Rout is greatly reduced by the substrate current induced body effect (SCBE).[8] In

this case, Rout is continual raise so that SCBE could be neglected.

In view of foregoing concept, let us then consider that the variation of Rout with

different pocket and LDD process devices to Lg at the same Vgst (Vgs-Vt=0.1 V) and

Vd bias which Vd=0.7 V for IO and Vd=0.6v for Core. It is shown in Fig4 and Fig5. It

clearly shows that cross phenomenon. The different devices structure and process

could move the cross points to gate length. Hence, the Rout at least should be

differentiated into two physic effect. We believe that different channel length could

lead to change of physic mechanism boundary of Fig3. The fore and back cross due to

CLM and DIBL respectively. The concept of Rout to variable Lg should be changed as

well. The most important characteristics of Rout in analog circuit designs are the

maximum Rout which determines the maximum available gain from the device. Fig6

shows the gain of wide part dependence on the Rout. To obtain the high gain devices

have different notions in distinct Lg. The lighter pocket implant concentration could

induce high Rout and gain at after of cross. Adversely, that could induce poorer Rout

and gain at before of cross.

VA has three components, i.e., VACLM, VADIBL and VASCBE, corresponding to CLM, DIBL

and SCBE, respectively. Each component can be evaluated separately.[8]

]

The individual component of VA together with the resultant VA is shown in Fig7. The

dominant mechanism is the one with the smallest Early voltage in each region.

In this case of VA which obtaining by intersection of X axis with tangent of Vd

bias at 0.7V and 0.6V for IO and Core could exclude VASCBE because of this Vd bias is

not enough to induce substrate current. For experimental data in Fig8 and Fig9 these

are similar to the Rout that VA appears cross points around 3 time technology node to

33

The concept of V^A of fore cross due to CLM dominated. The length ∆L of the pinch-off region increases by an expansion in the direction of the source end with

lighter pocket doping concentration and effective channel length has been reduced by

∆L at the same bias condition. In other word, the ∆L increase with decreasing pocket

dose concentration could reduce VA. CLM is not a special short-channel phenomenon,

but its relative importance becomes distinctly more pronounced at short gate lengths.

This is traditional notion for design VA , but the concept of back cross is in conflict with

pocket implant concentration. For long channel devices of back cross, despite CLM

and DIBL should slowly be negligible. However, large residual DIBL exists at back

cross for the pocket devices in Fig8, dominating the VA values which function (1). In

these dimension, higher pocket doping concentration lead to Vt,sat shift even more.

The conventional explanation of channel length modulation is not entirely valid in

pocket devices. Hence, these new observations have a significant impact on design of

VA by cross phenomenon of the pocket device. Anyway, the experimental results from

pocket device are shown in Fig8 and Fig9. It can be seen that these devices have lower

DIBL leading to higher VA at back cross. Adversely, leading to poorer VA at fore cross.

3.4 Summary

We have presented experimental data showing that certain pocket implant

concentration for digital CMOS technology and then found out the interesting cross

points, which imply pocket implants affect at least have two physic mechanisms CLM

and DIBL. The conventional explanation of channel length modulation is not entirely

valid in pocket devices. Hence, these new observations have a significant impact on

design of VA and Rout. The concept of back cross differs from CLM so that the

analog device design for pocket implant must be notice. The lighter pocket implant

concentration could induce higher Rout, gain and VA at back of cross. On the other

hand, the heavier pocket implant concentration could induce higher Rout, gain and

VA at fore of cross.

35

0.1 1 10

300 350 400 450 500

550 NMOS IO Tox=32A

for different pocket implant concentraion

Vt(mv)

Vd

heavy In and BF light In and BF light BF

Fig1. Vt vs. Lg for different pocket implant concentration

1E-6 1E-5 1E-4 1E-10

1E-9 1E-8

1E-7 NMOS IO Tox=32A

for different pocket implant concentration

Ioff

Ion heavy In and BF light In and BF light BF

Fig. 2 Ion vs. Ioff for different pocket implant concentration

37

Fig. 3 Typical drain current and output resistance.

1 0

5 10 15 20 25

30 NMOS IO Tox=32A Vd=0.7v Vgt=0.1v for diffent pocket devices

Rout (kohm)

Lg heavy In and BF light In and BF light BF

Fig4. Rout vs. Lg for NMOS IO

39 light In and medium As heavy In and light As

NMOS Core Tox=20A

for different PKT and LDD concentration

Rout (kohm)

Lg

light In and heavy As light In and medium As heavy In and light As

Fig5.Rout vs. Lg for NMOS Core

0.1 1

Fig6. Gain and Rout vs. Lg for different pocket implant devices.

41

Fig7 Early voltage and its components versus Vds

0

43

light In medium As heavy In light As

Chapter 4

An Investigation on Hot Carrier Effect at Elevated