Metal Oxide Semiconductor Field Effect Transistor: Additional Concepts Describe and analyze subthreshold conduction. Analyze channel length modulation. Consider the effects of a decrease in carrier mobility due to increasing gate voltage. Analyze the effects of carrier saturation velocity. Discuss MOSFET scaling. Consider the deviations in threshold voltage due to small geometry devices. Describe and analyze the technique of threshold voltage adjustment by ion implantation. Consider the introduction of trapped oxide charges by ionizing radiation and hot electron effects.
11.1 NONIDEAL EFFECTS The drain current, which exists for VGS VT, is known as the subthreshold current. The semiconductor surface develops the characteristics of a lightly doped n type material. The condition for φfp < φs < 2φfp is known as weak inversion.
11.1.1 Subthreshold Conduction There is a potential barrier between the n + source and channel region which the electrons must overcome in order to generate a channel current.
11.1.2 Channel Length Modulation The circuit design must include the subthreshold current or ensure that the MOSFET is biased sufficiently below the threshold voltage in the off state.
11.1.2 Channel Length Modulation The space charge width of the drain substrate junction is approximately
11.1.2 Channel Length Modulation φ(x=0) = VDS(sat) and φ(x= L) = VDS Esat is in the range 10 4 <Esat <2*10 5 V /cm
11.1.2 Channel Length Modulation Since the drain current is inversely proportional to the channel length, we may write In the saturation region, The output resistance is given by
11.1.3 Mobility Variation Reasons for a mobility variation: Gate voltage. The effective carrier mobility decreases as the carrier approaches the velocity saturation limit.
11.1.3 Mobility Variation An effective transverse electric field can be defined as The effective mobility may be represented by
11.1.4 Velocity Saturation In the ideal I V relationship, Velocity saturation will occur when the horizontal electric field is approximately 10 4 V /cm. Then, velocity saturation is very likely to occur in short-channel devices. Modified ID(sat) : Mobility: Transconductance: Cutoff frequency:
11.1.5 Ballistic Transport As the MOSFET channel length is reduced, the mean distance between collisions l may become comparable to L so that the previous analysis may not be valid. If the channel length is further reduced so that L < l, then a large fraction of carriers could travel from the source to the drain without experiencing a scattering event. This motion of carriers is called ballistic transport.
11.2 MOSFET SCALING The principle of constant-field scaling is that device dimensions and device voltages be scaled such that electric fields (both horizontal and vertical) remain essentially constant. I D W n OX 2t L OX ( V G V T ) 2 n OX ( kv 2( kt )( kl) OX G V T ) 2 constant A WL P IV
11.2.2 Threshold Voltage First Approximation The first two terms are functions of material parameters that do not scale and are only very slight functions of doping concentration. The last term is approximately proportional to k, so the threshold voltage does not scale directly with the scaling factor k.
11.3 THRESHOLD VOLTAGE MODIFICATIONS
11.3.1 Short-Channel Effects The average bulk charge per unit area QB in the trapezoid is
11.3.1 Short-Channel Effects
11.3.2 Narrow-Channel Effects The change in threshold voltage due to the additional space charge is is a fitting parameter that accounts for the lateral space charge width The shift in threshold voltage due to a narrow channel is in the positive direction for the n-channel MOSFET. As the width W becomes smaller, the shift in threshold voltage becomes larger.
11.3.2 Narrow-Channel Effects The threshold voltage shift begins to become apparent for channel widths that are large compared to the induced space charge width.
11.3.2 Narrow-Channel Effects The narrow-channel device produces a larger threshold voltage; the short-channel device produces a smaller threshold voltage.
11.4 ADDITIONAL ELECTRICAL CHARACTERISTICS Breakdown Voltage Oxide Breakdown: In silicon dioxide, the electric field at breakdown is on the order of 6 x 10 6 V/cm. Avalanche Breakdown: May occur by impact ionization in the space charge region near the drain terminal.
11.4.1 Breakdown Voltage Near Avalanche and Snapback Breakdown is due to second order effects.
11.4.1 Breakdown Voltage Near Avalanche and Snapback Breakdown is due to second order effects. At breakdown, the current in the B C junction is multiplied by the multiplication factor M, so we have Breakdown is defined as the condition that produces IC. breakdown occurs when M 1 /α.
11.4.1 Breakdown Voltage Near Punch-Through Effects: Punch-through is the condition at which the drain-to-substrate space charge region extends completely across the channel region to the source-to-substrate space charge region. In this situation, the barrier between the source and drain is completely eliminated and a very large drain current would exist. Drain Induced Barrier Lowering (DIBL)
*11.4.2 The Lightly Doped Drain Transistor Lightly Doped Drain (LDD)
11.4.3 Threshold Adjustment by Ion Implantation The shift in threshold voltage due to the implant is If donor atoms were implanted into the p-type substrate, the space charge density would be reduced; thus, the threshold voltage would shift in the negative voltage direction.
*11.5 RADIATION AND HOT-ELECTRON EFFECTS Radiation-Induced Oxide Charge Generated electrons in the oxide are fairly mobile, with a mobility value on the order of 20 cm 2 /V s.
*11.5 RADIATION AND HOT-ELECTRON EFFECTS Radiation-Induced Oxide Charge At high electric fields, the electron velocity in the oxide also saturates at approximately 10 7 cm/s. The generated holes undergo a stochastic hopping transport process through the oxide. Holes are relatively immobile
*11.5 RADIATION AND HOT-ELECTRON EFFECTS Radiation Induced Interface States Hydrogen appears to be important in the radiation induced interface state buildup.
*11.5 RADIATION AND HOT-ELECTRON EFFECTS Hot Electron Charging Effects If the electrons have energies on the order of 1.5 ev, they may be able to tunnel into the oxide.
* Problems on Shrinking
* Trends on sub-100nm MOSFET
* Enhancing Gate Control 虽然遇到了上面的两个挑战, 人类不会就此罢休, 所以发明了 Double-Gate MOSFET, 因为我们上面讲了, 主要的漏电来自于沟道下面的 Body, 可是我又不能降低 Body 厚度, 所以发明了不降低 Body 厚度, 我在 body 两边各加一个 Gate 夹击 Body 区总可以了吧, 类似 JFET 的原理 作者 : 张竞扬链接 :https://zhuanlan.zhihu.com/p/21097675 来源 : 知乎著作权归作者所有 商业转载请联系作者获得授权, 非商业转载请注明出处
* Double Gates Approach
* Double Gates Approach
* FinFET Process
* Multi-Gates FinFET
* FinFET Patterning
* FD-SOI vs FinFET Performance Comparison
* FD-SOI vs FinFET Performance Comparison
* Future Transistor Path
* GAA Approach
*Below 10nm
*Below 10nm
* Comparison of ITRS Roadmap on 1998 and 2010
* Comparison of ITRS Roadmap on 1998 and 2010