MOSFET

Hamidreza Movahhed

Alireza Ramezany

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:واحیه ی ترایود

.دَذرخ می VGS = Vthدر ( پیذایش کاوال)فرض می کىیم آستاوٍ ی حالت يارين .متىاسب است (VGS - Vth)با ( بار مىفی کاوال NMOSدر )پس چگالی بار يارين

.را داریم Coxخازن کاوال ي Gateاز طرفی بیه

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در ایه حالت کٍ َىًز مىطقٍ تخلیٍ داریم

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:واحیه ی اشباعشذن کاوال، يلتاش کاوال pinch-offي VDS,sat = (VGS - Vth)از VDSبا بسرگتر شذن

VDS,sat می ماوذ ي بقیٍ ی يلتاشVDS ريی واحیٍ ی بیه کاوال يDrain قرار می .گیرد

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)

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:= ΔL Xdبرای تقریبی یافته رابطه ای يلتاشبا (Cdep)ي کاوال خازن تخلیٍ ای Drainواحیٍ ی بیه شکل، با تًجٍ بٍ

(VDS - VDS,sat) بٍ اوذازٌ ی ( بر ياحذ سطح)ي بارالکتریکیQdep است.

:با تًجٍ بٍ شکل َىذسی خازن تخلیٍ داریم

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: (gm)ترارساوایی

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:با میدان عمودی µکاهش * میذان عمًدی شذیذ می شًد ي حامل َا را در یک واحیٍ ی ابعاد، یا کًچک شذن VG افسایش با

(Surface Scattering)باریک وسدیک مرز اکسیذ محذيد می کىذ ي پراکىذگی حامل َا را از سطح .می شًد( تحرّک) µزیاد می کىذ ي مىجر بٍ کاَش

:اشباع سرعت* در میذان َای عرضی شذیذ کٍ با کاَش ابعاد ممکه می شًوذ،

.سرعت حامل َا بٍ مقذار اشباع می رسذ

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:با اشباع کامل سرعت حامل َا در کاوال داریم

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*Impact ionization

Another undesirable short-channel effect, especially in NMOS, occurs due to the high velocity of electrons in presence of high longitudinal fields that can generate electron-hole (e-h) pairs by impact ionization, that is, by impacting on silicon atoms and ionizing them.

It happens as follow: normally, most of the electrons are attracted by the drain, while the holes enter the substrate to form part of the parasitic substrate current. Moreover, the region between the source and the drain can act like the base of an npn transistor, with the source playing the role of the emitter and the drain that of the collector (Parasitic BJT).

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If the aforementioned holes are collected by the source, and the corresponding hole current creates a voltage drop in the substrate material of the order of 0.6V, the normally reversed-biased substrate-source pn junction will conduct appreciably. Then electrons can be injected from the source to the substrate, similar to the injection of electrons from the emitter to the base. They can gain enough energy as they travel toward the drain to create new e-h pairs. The situation can worsen if some electrons generated due to high fields escape the drain field to travel into the substrate, thereby affecting other devices on a chip.

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*Hot electrons

Another problem, related to high electric fields, is caused by so-called hot electrons. These high energy electrons can enter the oxide, where they can be trapped, giving rise to oxide charging that can accumulate with time and degrade the device performance by increasing Vth and affect adversely the gate’s control on the drain current.

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را بٍ طًر شماتیک وشان می دَذ hot electronsشکل زیر اثر

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A brief explanation of other important effects that we have neglected:

• Bulk Charge ( )

• Body Effect (decrease of Vth due to accumulation of extra charge in channel which is caused by zero Source-Bulk voltage)

• Sub-threshold Current (when VGS < Vth , we already have a weak inversion)

• Drain Induced Barrier Lowering (DIBL): When we have a short channel, where Drain and Source are relatively close to each other, VD lower the potential barrier that is seen by majority carriers in Source. Thus, those carriers can diffuse through the depletion region between Source and Drain and finally reach the Drain and increase the current specially sub-threshold current.

• Punch-through (when the depletion region around Drain and Source join each other. As a result, the current increases dramatically)

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