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1、Additional Electrical CharacteristicsThreshold Adjustment Ion ImplantationVoltage breakdown mechanismsOxide NOT a perfect insulator:large electric field leads to breakdown,then a catastrophic failure.Ebreakdownfor SiO2:6 MV/cm.A VG30V would produce breakdown in an oxide with a thickness of 500.Howev
2、er,a safety margin of a factor of 3 is common,so that the maximum safe gate voltage with tox=500 would be 10 V.A safety margin is necessary since there may be defects in the oxide that lower the breakdown field.Oxide breakdown is normally not a serious problem except in power devices and ultrathin o
3、xide devices.Oxide Breakdown：Avalanche Breakdown：May occur by impact ionization in space charge region near the drain terminal.In an ideal planar one-sided pn junction,breakdown is a function primarily of the doping concentration in the low-doped region of the junction.For the MOSFET,the low-doped r
4、egion corresponds to the semiconductor substrate.If a p-type substrate doping is Na=3 1016cm-3,for example,the p-n junction breakdown voltage would be approximately 25 V for a planar junction.Curvature effect on the electric field in the drain junction.However,the n+drain contact may be a fairly sha
5、llow diffused region with a large curvature.The electric field in the depletion region tends to be concentrated at the curvature,which lowers the breakdown voltage.Parasitic Transistor Breakdown：n-type source and drain contacts along with the p-type substrate.The source and body are at ground potent
6、ial.The n(source)-p(substrate)-n(drain)structure also forms a parasitic bipolar transistor.Equivalent circuit including the parasitic bipolar transistorCross section of the n-channel enhancement-mode MOSFET Another breakdown mechanism results in the S-shaped breakdown.This breakdown process is due t
7、o second order effects.Currentvoltage characteristicshowing the snapback breakdown effect.Parasitic Transistor Breakdown：In the figure:Device when avalanche breakdown is just beginning in the space charge region near the drain.Think the avalanche breakdown suddenly occurring at a particular voltage.
8、Avalanche breakdown is a gradual process that starts at low current levels and for electric fields somewhat below the breakdown field.The electrons generated by the avalanche process flow into the drain and contribute to the drain current.Substrate currentinduced voltage drop caused by avalanche mul
9、tiplication at the drain.Parasitic Transistor Breakdown：The avalanche-generated holes generally flow through the substrate to the body terminal.Since the substrate has a non-zero resistance,a voltage drop is produced as shown.This potential difference drives the source-to-substrate pn junction into
10、forward bias near the source terminal.The source is heavily doped n-type;thus,a large number of electrons can be injected from the source contact into the substrate under forward bias.This process becomessevere as the voltage drop in the substrate approaches 0.6 to 0.7V.A fraction of the injected el
11、ectrons diffuses across the parasitic base region into the reverse-biased drain space charge region where they also add to the drain current.Currents in the parasitic bipolar transistor.Parasitic Transistor Breakdown：GD The avalanche breakdown process is a function of not only the electric field but
12、 the number of carriers involved.The rate of avalanche breakdown increases as the number of carriers in the drain space charge region increases.Regenerative or positive feedback mechanism：Produces the substrate currentThe forward-biased source-substrate pn junction voltageThe forward-biased junction
13、 injects carriers:can diffuse back to the drain;increase the avalanche process.Positive feedbackParasitic Transistor Breakdown：The snapback or negative resistance portion of the curve can now be explained by using the parasitic bipolar transistor.The potential of the base of the bipolar transistor n
14、ear the emitter(source)is almost floating,since this voltage is determined primarily by the avalanche-generated substrate current rather than an externally applied voltage.GDParasitic Transistor Breakdown：where is the common base current gain and ICE0is the base-collector leakage current.For an open
15、 base,IE=IC,so Equation(12.34)becomesAt breakdown,the current in the BC junction is multiplied by the multiplication factor M,so we haveSolving for IC:Breakdown is defined as the condition that produces Ic.For a single reverse biased pn junction,M at breakdown.However,from Equation(12.37),breakdown
16、is defined to be the condition when M1,breakdown occurs when M1/,which is a much lower multiplication factor than for the simple pn junction.CECB0III(12.34)(12.35)(12.37)(12.36)Parasitic Transistor Breakdown：For the open-base bipolar transistor:An empirical relation for the multiplication factor is
17、usually written asIc M VCEWhere m is an empirical constant in the range of 3 to 6 and VBDis the junction breakdown voltage.The common base current gain factor is a strong function of collector current for small values of collector current.GD(12.38)At low currents,the recombination current in the B-E
18、 junction is a significant fraction of the total current so that the common base current gain is small.Parasitic Transistor Breakdown：（1）At low currents,the Ic in the B-E junction is a significant fraction of the total current so that the common base current gain is small.Particular values of M and
19、VCEare required to produce the condition of M=1.（2）As the collector current increases,increases;smaller values of M and VCEare required to produce the avalanche breakdown condition.The snapback,or negative resistance,breakdown characteristic is then produced.Ic M VCEParasitic Transistor Breakdown：On
20、ly a fraction of the injected electrons from the forward-biased source-substrate junction are collected by the drain terminal.A more exact calculation of the snapback characteristic would necessarily involve taking into account this fraction;thus,the simple model would need to be modified.However,th
21、e above discussion qualitatively describes the snapback effect.The snapback effect can be minimized by using a heavily doped substrate that will prevent any signifi cant voltage drop from being developed.A thin epitaxial p-type layer with the proper doping concentration to produce the required thres
22、hold voltage can be grown on a heavily doped substrate.Parasitic Transistor Breakdown：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.The barrier
23、between source and drain is completely eliminated and a very large drain current would exist.Drain-Induced Barrier Lowering：drain current will begin to increase rapidly before the actual punch-through condition is reached:the near punch-through condition.Ideal energy-band diagram from source to drai
24、n for a long n-channel MOSFETFor the case when VGSVTand when the drain-to-source voltage is relatively small.The large potential barriers prevent significant current between drain and source.The energy-band diagram when a relatively large drain voltage VDS2is applied.The space charge region near the
25、 drain terminal is beginning to interact with the source space charge region and the potential barrier is being lowered.Equipotential plot along the surface of a short-channel MOSFET before and after punch-through.Near Punch-Through Effects：Since the current is an exponential function of barrier hei
26、ght,the current will increase very rapidly with drain voltage once this near punch-through condition has been reached.For a doping of 106cm-3,the two space charge regions will begin to interact when the abrupt depletion layers are approximately 0.25m apart.The drain voltage at which this near punch-through condition,occurs is significantly less than the ideal punch-through voltage.Typical IV characteristics of a MOSFET exhibiting punch-through effects.Near Punch-Through Effects：