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Abstract— In this paper we have to study hot carrier injection phenomenon (HC degradation) in p-channel MOSFET and effect of Lightly Doped Drain Structure (LDD structure) in minimizing it. HC degradation leads to effective channel length shrinkage. It is due to the hot electrons which are there trapped in gate SiO2 accumulate holes near the channel region of drain. This reduces the effective channel width.
On the other hand, in case of p-channel LDD MOSFETs, the HC generation in LDD region is one tenth of that in p+ drain MOSFETs under the same Vgs. The lifetime of MOSFET is defined as the time it takes to deviate it’s transconductance by 10 percent. The lifetime of LDD MOSFETs are longer by two orders of magnitudes than those of the p+ drain MOSFETs. Best lifetime can be achieved when the boron dose is in between 1 & 3×1013cm-2
Index Terms— Lightly Doped Drain(LDD), Effective channel length, transconductance(gm), MOSFET lifetime.
- INTRODUCTION
A major problem in a small sized MOSFET operating at 5V is hot carrier degradation. Especially in n-channel MOSFETs, this is a serious problem as the ionization rate of electrons which is the charge carrier in N-channel MOSFET is greater than that of holes. Also, hot carrier injection is also a problem in p-channel MOSFETs, so LDD structures also have to be implemented in p-channel MOSFETs to decrement its effect.
This paper focuses on the usefulness of LDD structure by minimizing the HC degradation in p-channel LDD MOSFET and p+-drain MOSFET.
- SAMPLE PRPERATION
The boron concentration in Lightly Doped regions is approx. 1×10 13cm-2 for 3 types of LDD structures like ones with p- doses in LDD regions equating to 1013, 3×1013, 8× 1013.The region depths of LDD region is around 0.2µm. The heavily doped source-drain region did not reach the Silicon surface under gate terminal. Therefore, SiO2 layer exists over LDD region.
The effective channel length depends on the Boron dose of
LDD region. As shown in the graph, Channel length shrinkage increases as the Boron dose increases.
- GENERATION OF HOT CARRIERS IN P-CHANNEL LDD MOSFETs
A.)Substrate current in LDD structure MOSFET
In Fig.3 HC injection phenomenon is shown both in case of p-channel LDD MOSFETs and p+- Drain MOSFETs. The substrate current Isub shows the HC concentration in p-channel MOSFET. Drain voltage (Vd) varies from -13V to -5V keeping Vsub=0V. The max value of substrate current appears when |Vg| < |Vd|. The characteristics of both p+-drain and LDD structure is similar except the fact that substrate current in latter is lesser.
The variation of effective channel length with p-conc is shown in Fig.4. It can be seen that peak Isub decreases with the decrease in Leff. However the peak Isub decreases with decrease in p- conc.
B.)Gate current in LDD structure MOSFET
In Fig.5 Ig v/s Vg characteristics are shown of both p+-drain and LDD MOSFETs. Vd is varied from -13V to -6V and Ig is measured keeping the substrate Voltage Vsub=0V.
A sharp bell-shaped structure was obtained as Ig vs Vg characteristics of both p+ drain and LDD MOSFETs. This current is the electron gate current which is HC current developed by the impact ionization. The electric field strength reduction effect can only be observed in LDD structures.
The characteristics are same for both the structures however the peak gate current decreases when the p- concentration is decreased. The dependence of peak gate current on the p—concentration is shown in Fig.6 as a function of effective channel length Also, the peak Ig in the LDD MOSFET was smaller by two orders of magnitude than p+ drain MOSFET. This leads to significant electric field reduction effect in the p- region.
C.)Difficulties in p-channel MOSFET scaling
Thresh-hold voltage degradation and transconductance degradation is smaller in p-channel MOSFET than n-channel MOSFET because the ionization rate of holes is lesser than that of electrons. But the difference in the ionization rate of holes in p-channel and electrons in n-channel MOSFET become less with the increment in electric field.
From Fig.7 we can see that the ionization rate of both holes and electrons became close to each other as the drain voltage became high. With increment in Vd the gate length became shorter. Thus, it is concluded that the difference in the ionization rate is small for small MOSFETs.
IV.)HOT CARRIER INFLUENCE AND EFFECTIVE CHANNEL LENGTH SHRINKAGE IN P-CHANNEL LDD MOSFETs.
A.)Hot carrier influence in p-channel LDD mosfet
The deviation in transconductance in p-Channel MOSFET is due to the HC degradation and the curve is shown in Fig.8. After keeping the MOSFET on for some time( stress time), the transconductance is measured again. The curve shifted slightly towards the positive gate voltage. The observed characteristics were same for both p+-drain and LDD MOSFETs, except the fact that the deviation was lesser in p-channel LDD MOSFET.
The change in transconductance can be written as
Δgm=gm(t)-gmo
t=stress time, gmo=initial transconductance
And the graph of Δgmvs stress time at different Vd’s is shown in Fig.9. And from the figure it can easily be observed that time dependence on transconductance is very less.
B.)Effective channel length shrinkage logic for p-channel LDD MOSFET
There is a similar shifts in transconductance in both p-Channel LDD MOSFET and p+ - drain MOSFET. But there is a huge difference in p-Channel and n-Channel MOSFET dependency.
The hot electrons generated are injected into the SiO2 layer present over lightly doped channel region due to inter-collision and attraction due to gate electrode. Now in case of:
1.)N-Channel MOSFET- the electrons are trapped in SiO2 layer thus reducing the number of conduction carriers hence increasing the resistance of the system. Therefore gm is decreased.
2.)P-Channel MOSFET- the electron are trapped in SiO2 layer present over the lightly doped channel region. These electrons attract more holes in the inversion layer thus increasing the concentration of conduction carriers, hence reducing the resistance. Thus, gm is increased.
As the channel conductance is increased effective channel length decreases, because resistance is directly proportional to channel length.
C.)Lifetime of p-channel LDD MOSFET
The lifetime of an LDD MOSFET is defined as the time it takes to shift the transconductance by 10 percent due to hot carrier injection. As the HC injection is lesser in case of LDD MOSFET so the transconductance degradation is also small. Thus, it takes longer time to shift the transconductance by 10 percent hence increase in the lifetime of LDD MOSFETs.
Experimentally is shown that the channel region doped with 1-3×1013cm-2 has the longest lifetimes which is approximately 100 times than that of p+ drain MOSFET.
V.)REFERENCES
W. N. Grant. "Electron and hole ionization rates in epitaxial silicon at high electric fields." Solicl-SrotcElectrori..vol. 16, p. 1189. 1973.
J .J .Tzou, C. C. Yao. R. Cheung, and H. W. K. Chan, "Hot-carrierinduceddegradation in p-channel LDD MOSFET's.
F. C. Hsu and H. R. Grinolds. "Structure-enhanced MOSFET degradationdue to hot-clcctron injection”.
Effects of Lightly Doped Drain Structure withOptimum Ion Dose on p-ChannelMOSFET’s, Toru Kaga and Yoshio Sakai members of IEEE.
[]The authors are with the Department of Electrical Engineering, Indian Institute of Technology, Delhi, 110 016 India
(e-mail:; ).