Semiconductor Physics and Devices

Transcription

1 Nonideal Effect The experimental characteristics of MOSFETs deviate to some degree from the ideal relations that have been theoretically derived. Semiconductor Physics and Devices Chapter 11. MOSFET: Additional Concepts & Thin Film Transistors (TFTs) Seong Jun Kang Department of Advanced Materials Engineering for Information and Electronics Laboratory for Advanced Nano Technologies Nonideal Effect: Subthreshold Conduction The experimental characteristics of MOSFETs deviate to some degree from the ideal relations that have been theoretically derived. Nonideal Effect: Subthreshold Conduction Subthreshold Conduction Subthreshold Conduction The ideal current-voltage relationship predicts zero drain current when the gate-to-source voltage is less than or equal to the threshold voltage. Figure shows a comparison between the ideal characteristic that was derived, and the experimental results. The drain current, which exists for,is known as the subthreshold current. Current-Voltage Relationship Figure (a) shows an n-channel enhancement mode MOSFET with a gate-to-source voltage that is less than the threshold voltage and with only a very small drain-to-source voltage. Current-Voltage Relationship Figure (b) shows an n-channel enhancement mode MOSFET with an applied gate voltage higher than threshold voltage. The source and substrate terminals are held at ground potential. There is no electron inversion layer, the drain-to-substrate pn junction is reverse biased, and the drain current is zero. An electron inversion layer has been created so that when a small drain voltage is applied, the electrons in the inversion layer will flow from the source to the positive drain terminal. The conventional current enters the drain terminal and leaves source terminal.

2 Nonideal Effect: Subthreshold Conduction Subthreshold Conduction Figure shows the energy-band diagram of an MOS structure with a p-type substrate biased so that 2. At the same time, the Fermi level is closer to the conduction band than the valence band, so the semiconductor surface develops the characteristics of a lightly doped n-type material. Some conduction between the source and drain contacts would be observed through this weakly inverted channel. Nonideal Effect: Subthreshold Conduction In the case of 2, the Fermi level at the surface is far above the intrinsic Fermi level. The electron concentration at the surface is same as the hole concentration in the bulk. This condition is the threshold inversion point, and the applied gate voltage creating this condition is the threshold voltage. In the case of 2, weak inversion would be observed. The threshold voltage is defined as the applied gate voltage required to achieve the threshold inversion point 2 : Threshold inversion condition for p-type semiconductor The condition for 2 is known as weak inversion. Nonideal Effect: Subthreshold Conduction Figure shows the surface potential along the length of the channel at accumulation, weak inversion, and threshold for the case when a small drain voltage is applied. There is a potential barrier between the source and channel region which the electrons must overcome in order to generate a channel current. (figure (b) and (c)) Similar to the pn junction, the current is an exponential function of. Nonideal Effect: Channel Length Modulation We assumed the ideal current-voltage relationship with the channel length L was constant. However, when the MOSFET is biased in the saturation region, the depletion region at the drain terminal extends laterally into the channel, reducing the effective channel length. Since the drain current is inversely proportional to the channel length, In the inversion mode (figure (d)), the barrier is so small that is more like an Ohmic contact. The current is proportional to the. The I-V curves in the saturation region shows a positive slopes due to channel length modulation. Nonideal Effect: Channel Length Modulation Nonideal Effect: Channel Length Modulation

3 Nonideal Effect: Ballistic Transport Scattering events in a semiconductor limit the velocity of carriers to an average drift velocity. The average drift velocity is a function of the mean time between collisions or the mean distance between scattering events. Thin Film Transistors Gate, Source and Drain Electrodes with an active channel materials and gate insulator. No pn junction As the MOSFET channel length is reduced, the mean distance between collisions may become comparable to the channel length. If the channel length is further reduced (in submircron), then carriers could travel from the source to the drain without experiencing a scattering event. This motion of carriers is called ballistic transport. Ballistic transport means that carriers travel faster than the average drift velocity, and this effect can lead to very fast devices. Introduction Introduction A transistor is a semiconductor device that controls current between two terminals based on the current or voltage at a third terminal. The first transistor was invented at Bell Laboratories on December 16, 1947 by William Shockley (seated at Brattain's laboratory bench), John Bardeen (left) and Walter Brattain (right). Generally, a transistor is used for amplifying and switching of electrical signals. Early electronics used vacuum tube amplifier for the electrical circuits. Then a transistor used a single crystal was developed. Recently, scientist developed a nano size transistor using various semiconducting materials, such as silicon, organic materials, carbon nanotube. This was perhaps the most important electronics event of the 20th century, as it later made possible the integrated circuit and microprocessor that are the basis of modern electronics. Prior to the transistor the only alternative to its current regulation and switching functions was the vacuum tube, which could only be miniaturized to a certain extent, and wasted a lot of energy in the form of heat. Introduction Vacuum tube was used as an amplifier or switching devices. The electron from cathode to the plate can be controlled by the grid. If we apply AC to the grid, the current between the cathode and the plate is changed according to the AC signal. Thin Film Transistors Why Thin Film Transistors (TFTs) is useful? TFT is a one of Field Effect Transistors fabricated on the glass substrates using thin film semiconductors Using those TFT arrays, we can operate the LCD (TFTLCD or AMLCD) and AMOLEDs Using the combination of vacuum tube, the first computer was developed. a-si TFT Poly-Si TFT TFTs are used in Active Matrix Displays as Pixel on/off switching devices The size, energy loss, heating, etc were problem.

4 Thin Film Transistors: Materials MOSFET vs. TFTs Substrate: Glass Gate electrode: Metal film (Cr, Al, MoTa films) Source electrode: Metal film (Cr, Mo, Ta films) Drain electrode: Metal film (Cr, Mo, Ta films) Gate insulator: SiNx, SiONx, SiOx films Active layer (Semiconductor) : Crystalline Si Amorphous Si Oxide semiconductor (InGaZnO) Organic semiconductor (Pentacene) Passivation layer: SiNx Thin Film Transistors: use in Display Thin Film Transistors: several structures Where are the TFTs in the Pixel? Gate Bus Line/Data Bus Line/Pixel electrode Using On/OFF switching by TFT, we can make Image and characters Thin Film Transistors: Silicon active materials

5 Superior flexibility Low cost and simple process Solution based process However, poor performance. Output curve of Organic TFTs (a) Linear Region : (b) Saturation Region : Top contact Pentacene OTFT 5000 SiO2 gate insulator W/L = 1mm/15.4μm (L. Kosbar et.al. MRS Proc. 2001) Transfer curve of Organic TFTs Solution based organic TFTs Threshold voltage Top contact Pentacene OTFT 5000 SiO2 gate insulator W/L = 1mm/15.4μm (L. Kosbar et.al. MRS Proc. 2001)

6 Future AMLCD TV Large size High resolution (FHD UD) Motion picture (>120Hz) Key issue : TFT 120Hz TFT turn on time decrease : 5.2 us (FHD) 0.4 us (UD) Not enough charging 1. Increase of the number of gate line 2. RC delay due to increase of cell # Line # : us 5.2 us 3.4 us 0.4 us Line # : 2160 UD~1/2 FHD FHD UD Why high mobility TFTs? Why high mobility TFTs? Silicon TFTs do not give the solution!!! until now. a-si : fundamental issue of weak bond poly-si : manufacturing issues (yield & investment) Reliable TFT under constant current stress is needed Requirement of TFT performance for AMOLEDs E- paper Low-T, Low-cost fabrication On-current (μ) is not critical (<<1cm 2 /Vs is OK) Off-current is not critical : E-Ink inself has a memory function Low performance TFT (slow responses, I on/off ~10 3 are OK) Moderately low-t (~300ºC), low-cost fabrication On-current (μ) is not critical : a-si:h and organic TFTs (μ FET <1cm 2 /Vs) are OK Low off-current is favorable : I off <10-12 A is required. LCD High On-current (μ FET >3cm 2 /Vs) Low temperature High performance Natural 3D LCD High On-current (μ FE >4cm 2 /Vs) Very High stability ( V th <<1V) Prof. H. Hosono OLED display Low-T process(<<200ºc) Flexibility (Brittleness) Flexible display

7 Why Transparent Oxide Semiconductors? 60Hz RT Hz Hz Hz Semiconductor Polycrystalline1.8 Si us Amorphous 0.9 us Si Amorphous IGZO Several ten TFT uniformity Poor GoodTimes higher Good Poly-Si Pixel circuit Complex (5T+2C) 3.7 us Complex 1.8 (4T+2C) us 0.9 Simple us (2T+1C) Several TFT Times Limit A-Si:Hof a- Channel mobility ~100 cm higher Si based Furnace annealing at TFT 2 /Vs 1 cm 2 /Vs 10~40 cm 2 /Vs TFTs 300 o C of higher is Circuit integration YES No YES effective in obtaining 10.4 us 7.4 us good transfer 3.7 us 1.8 us TFT mask steps 5~11 4~5 4~5 characteristics Refresh Rate Poly-Si TFT a-si TFT Oxide TFT Cost/Yield High/Low Low/High Low/High Organic A wide variety of TFT20.7 us 14.8 us 7.4 us 3.7 us applications flexible SD HD FHD displays, OLED, UD Post UD Temperature stability (4FHD) (16FHD) High uniformity 1 High reliability Resolution Mobility (cm 2 /Vs) Process Temperature 450~550 o C 150 ~ 350 o C ~350 o C Device merits No hot carrier effect Excellent I on /l off ratio Oxide TFT based UD 3D TV (LCD): Samsung The first atom-thick material Graphite is a mineral of the carbon with a layered and planar structure. The same structure of graphite was constructed artificially, called graphene, but in a monolayer structure. Graphene has remarkable chemical, mechanical and physical properties. 70 inch UD LCD TV (240 Hz 구동 ): Oxide TFT New 2D materials MoS 2 transistor technology

8 Atomic thick electronics Ultimate heterostructure technology by layer-by-layer stacking of atomic films