2-6.1 2-6.2 Learning Outcomes • I understand why a diode conducts current under forward bias but does not under Spiral 2-6 reverse bias • I understand the three modes of operation of a MOS transistor and the conditions Semiconductor Material associated with each mode MOS Theory • I can analyze circuits containing MOS transistors to find current and voltage values by first determining the mode of operation and then applying the appropriate equations 2-6.3 2-6.4 Current, Voltage, & Resistors • Kirchoff's Current Law – Sum of current _______ a node is equal to current coming _____________ a node • Kirchoff's Voltage Law – Sum of voltages around a loop is _______ • Ohm's Law (only applies to resistors or devices DIODES that "act" like a resistor) – I = ____ or ____________________________ – Note: For a resistor, current and voltage are _____________ related with R as the slope
2-6.5 2-6.6 Semiconductor Material Transistor Types p-type n-type p-type • Bipolar Junction Transistors (BJT) + + - – npn or pnp silicon structure • Semiconductor material is not a great conductor + – Small current into very thin base layer - + material in its pure form + controls large currents between emitter and + – Small amount of free charge collector emitter base collector – However the fact that it requires a current npn BJT • Can be implanted (“doped”) with other elements into the base means it burns power (P = I*V) (e.g. boron or arsenic) to be more conductive and thus limits how many we can integrate on a chip (i.e. density) Source Drain – Increases the amount of free charge Gate Input • Metal Oxide Semiconductor Field Effect + - - + - + + + + - Transistors conductive - + - - - - + polysilicon - + - - – nMOS and pMOS MOSFETS + - + - + - - - – Voltage applied to insulated gate controls + N-Type Silicon Pure Silicon P-Type Silicon + current between source and drain + (Doped with arsenic) (Doped with boron) n-type • Gate input requires no constant current…thus low p-type Electron donors Electron acceptors power! We will focus on MOSFET in this class N-type MOSFET 2-6.7 2-6.8 PN Junction Diode The PN Junction • Our understanding of how a transistor works will • When we join the two substances the free electrons at the junction will combine with the nearby free holes in a "loose" start by analyzing a simpler device: a diode bond • A diode can be formed by simply butting up some p- • This has two effects: type and n-type material together – Around the junction there are ______________ free charges (they've all combined) creating a _______________________ – Now remember the dopants in n- and p-type material are still _____________ charged (same # of protons/electrons). So this migration has actually created ions and thus an ________________ Cathode Anode + p-type n-type - (and thus voltage) in the opposite direction + - + + - p-type n-type + Schematic - + - - symbol of a diode + + - - Physical view
2-6.9 2-6.10 Depletion Region Forward Bias • Now let's place an external positive voltage source across the diode • Depletion region has __________ free or mobile charge – Holes and electrons are pushed toward each other and reduce the depletion region • A small ___________ is induced due to this recombination – If the external voltage is high enough the charges will have enough ________ – N-type material LOST an electron leaving a __________ ion to overcome the gap and start ______________ through the diode – P-Type material LOST a hole (GAINED an electron) leaving a – The positive external voltage needed to overcome the depletion region is ____________ ion known as the __________________ Voltage – The voltage is in the opposite direction V D + - - - - +++ - - - +++ + + n-type n-type p-type p-type + + - - - - + + + - + - p- n+ - - Human convention of Physical reality current as positive of electron flow charge flow Depletion Region Depletion Region 2-6.11 2-6.12 Ideal Diode Reverse Bias • A perfect diode would ideally allow current to flow in _______________________ • It would therefore be a perfect conductor in • Now let's place an external negative voltage source across the diode one direction (forward bias) and a perfect – Holes and electrons are attracted to the voltage source terminal (pulled away insulator in the other direction (reverse bias) from the depletion area making the depletion area expand) • Example: Determine the value of ID if a) VA = – ______ current is flowing across the junction because both holes and 5 volts (forward bias) and b) VD = -5 volts electrons are attracted in opposite directions (reverse bias) – Ideal model: -V D a) I D = _______ = __________ = 100 mA - + b) Diode is in reverse bias and is acting R S = 50 Ω like a perfect insulator, therefore no current can flow and I D = ______ I D - - - +++ – More realistic: + - p-type n-type + a) I D = _____________/R S = 86 mA + - V A + - b) I D = 0 _ + - Depletion Region
2-6.13 2-6.14 Carrier Concentration • Even silicon has some amount of free electrons (n) and holes (p) – We refer to this as the _________________________ concentration – Note: n = p since a free electron leaves a hole behind • When we add dopants we change the carrier concentration – N A and N D is the concentration of ____________ and _____________ respectively – Note: N A >> p and N D >> n TRANSISTORS + - - + - + + + + - - + - - - + - + - + - + - + - N-Type Silicon Pure Silicon P-Type Silicon (Doped with arsenic) (Doped with boron) Electron donors Electron acceptors 2-6.15 2-6.16 Doped Valence and Conduction Bands • Impurity atoms, i.e., donors or acceptors replace some silicon atoms in the crystal lattice – Donors: a valence of five e.g., phosphorus (P) or arsenic (As)) – Acceptors: a valence of three, e.g., boron (B)) – Remember these are electrically neutral (same # of protons/electrons), but are easily induced to donate or accept an electron under certain circumstances (i.e. under a voltage) • If the donors or acceptors get ionized, each donor delivers an A FEW QUICK NOTES electron to the conduction band. Also each acceptor will capture an electron from valence band leaving a hole behind – Normally, at room temperature all donors (density N D ) and acceptors (concentration N A ) are ionized 15
2-6.17 2-6.18 Conventions Body Terminal • Recall a PN junction acts like a diode and allows current flow when V pn > P N • Since the source is always at the lowest voltage (for NMOS) and V thresh highest voltage (for PMOS) we generally define all voltages w.r.t. • We don't want that current flow so we must always maintain appropriate voltage to keep the "intrinsic" diodes in _____________ V S – Always keep the P-type area at a voltage __________ than the N-type • Conventionally all terminal voltages are defined wrt V S • For NMOS: Keep Body = ______; For PMOS: Keep Body = ________ • We also often draw our schematic symbols w/o showing the body – We will often not show the body connection and assume it is appropriately connected terminal Gate Input Gate Input Source Input + Drain Output Drain Input - Source Input + - + n-type - p-type - + + - - + + - - + - + + + + + - - - - - + - + - - - - - + + -+ + + - + - + - + p-type n-type Body/Substrate Body/Substrate + + + + - - - - Vdd NMOS PMOS 18 2-6.19 2-6.20 Source or Drain • Since MOSFETs are symmetric, which terminal is the source and which is the drain? • It ____________________________! • For NMOS: Source is terminal connected to ________ voltage • For PMOS: Source is terminal connected to ________ voltage Gate Input Gate Input Source Input Source Input + Drain Output Drain Input - + - + n-type - p-type - + + - - + + - - + - + THE BASIC IDEA OF MOS + + + + - - - - - + - + - - - - - + + -+ + + - + - + - + OPERATION p-type n-type Body/Substrate Body/Substrate + + + + - - - - Vdd NMOS PMOS
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