Introduction to Transistors Presented: October 23, 2001 Chris Green Carl Hanna Ancil Marshall Kwame Ofori

Overview Introduction & History Semiconductors Operation of Transistors Transistor Types Applications Examples Questions Conclusion

Background Invented at Bell Laboratories in John Bardeen, Walter Brattain, and William Schockly received Nobel Prize in Physics in 1956 for Inventing Transistors. First application: telephone signal amplification Replaced cumbersome and inefficient vacuum tubes Transistors can now be found on a single silicon wafer in most common electronic devices

Background Model of First Transistor

What are Transistors? Versatile three lead semiconductor devices whose applications include electronic switching and modulation (amplification) Transistors are miniature electronic switches. Configuration of circuit determines whether the transistor will serve a switch and amplifier Building blocks of the microprocessor, which is the brain of the computer. Have two operating positions- on and off. Binary functionality of transistors enables the processing of information in a computer.

Semiconductors Silicon Basic building material of most integrated circuits Has four valence electrons, which allow it to form four covalent bonds. Silicon crystal is an insulator-- no free electrons.

Semiconductors Resistance to current flow in the silicon crystal is reduced by adding small amounts of foreign impurities, which is referred to as doping. Doping transforms a silicon crystal from a good insulator into a viable conductor; hence, the name semiconductor.

Semiconductors Two Dopant Types N-type (Negative) –Free flowing electrons are added to the silicon crystal structure. Examples include Group V elements including Phosphorous, Arsenic, and Antimony. P-type(Positive)- Lack electrons and serve as potential slots for migrating electrons. Examples include Group III elements such as Boron, Aluminum, and Gallium

Comparison of Energy Bands Semiconductor resembles an insulator, but with a smaller energy band. Small energy band makes it a marginal conductor

Simple Semiconductors: Diodes Diode is the simplest semiconductor. Allows current to flow in one direction only.

Diode Sign Conventions Power dissipated by a load = (+) quantity Current flows from (+)  (-) Forward Biased Supplied Current flows with natural (hole) diffusion current Reversed Biased Supplied Current fights against natural diffusion (hole) current and diode orientation

Forward-Bias Example Charge Diffusion aided by Supply Current Current is allowed through easily “p” (positive charges Dominate) “n” (negative charges dominate) P-N Junction (Depletion Region / Offset voltage = 0.7V) Diode Electric Field Supplied Current Diffusion (hole) Current

Reverse-Bias Example Charges cannot diffuse unless supplied current flows towards “n” “p” (positive charges Dominate) “n” (negative charges dominate) (Depletion Region) Diode Electric Field Supplied Current Diffusion (hole) Cuurent

Diodes States Forward biased (on)- Current flows Real: Need about 0.7 V to initiate electron-hole combination process. Reversed biased (off)- Diode blocks current Ideal- Current flow = 0 Real : I flow = Amps

Bipolar Junction Transistors (BJT) Three Layers in a BJT Collector Base (very thin) has fewer doping atoms Emitter Two Types of BJT’s PNP (figure on left) operates with outgoing base current NPN (figure on right) operates with incoming base current p P+P+ n emitter collector base n n+n+ p emitter collector ii

BJT Schematic Representation p P+P+ n emitter collector base i n n+n+ p emitter collector i iBiB Corresponds to:

BJT Operation Characteristics I C vs. V CE graph allows us to determine operating region. Works for any I B or V CE VBE tops out around ~0.7V

BJT Operation Regions Operation Region I B or V CE Char. BC and BE Junctions Mode CutoffI B = Very small Reverse & Reverse Open Switch SaturationV CE = SmallForward & Forward Closed Switch Active Linear V CE = Moderate Reverse & Forward Linear Amplifier Break-downV CE = LargeBeyond Limits Overload

Cutoff NPN BJT n p n V2 V1 +++ C B E Emitter current Collector current Base current Reverse biased Reverse Biased

Saturated NPN BJT n p n V2 V C B E Emitter current Collector current Base current Forward biased ----

Active Linear NPN BJT n p n V2 V C B E Emitter current Collector current Base current Forward biased Reverse Biased

Possible Uses for BJT’s Can act as Signal Current Switch (Cutoff Mode) Can act as Current Amplifier (Active Region) Where: Beta = intrinsic amp property ( )

FIELD-EFFECT TRANSISTORS In 1925, the fundamental principle of FET transistors was establish by Lilienfield. In 1955, the first successful FET was made. Types of Transistors MOSFET ( metal-oxide-semiconductor field-effect transistors ) JEFT ( Junction Field-effect transistors) ( BACKGROUND )

MOSFET Four types: n-channel enhancement mode Most common since it is cheapest to manufacture p-channel enhancement mode n-channel depletion mode p-channel depletion mode (Types) Depletion type n-channelp-channel Enhancement type n-channelp-channel

MOSFET (n-channel Enhancement-Mode) Device Structure Three terminals Gate, Drain, and Source Analogous respectively to the base, collector, and emitter. Substrate electrically connected to the source.

MOSFET (n-channel Enhancement-Mode) Device Structure Substrate, source connected to ground The drain-body n + p junction is reverse-biased. The body-source pn + junction is reverse-biased. Enhancement MOSFET acts as an open circuit with no gate voltage.

n-channel Enhancement Mode Cutoff region V GS < V T. (Regions of operation) I DS V GS VTVT Characteristic Curve Cutoff region

n-channel Enhancement Mode Ohmic region V DS V T Voltage controlled resistor. (Regions of operation) I DS V GS VTVT Characteristic Curve

n-channel Enhancement Mode Saturation region V DS ≥ V GS -V T, V GS > V T Constant-current source. (Regions of operation) I DSS OhmicSaturation I DS V DS V GS V GS V TH Characteristic curves

Breakdown region V DS > V B n-channel Enhancement Mode (Regions of operation)

Comparison p-type charge carrier. Direction of drain current is opposite. V DS and V GS are negative. n-channel, p-channel behave the same way. (n-channel and p-channel)

Depletion MOSFET Addition of an n-type region between the oxide layer and p-type substrate. Thus, depletion MOSFETs are normally on. V T, threshold voltage, is negative. Unlike enhancement MOSFET, depletion MOSFET : Allows positive and negative gate voltages. Can be in the saturation region for V GS = 0

JFET n-channel p-channel D G S D G S n-channelp-channel

JFET (Physical and circuit representations)

JFET Cutoff region V GS < -V P, -VP is the threshold voltage. V DS = 0 (Regions of Operations)

JEFT Ohmic region V DS -V P. Resistance controlled by V GS VPVP I DS V DS Transfer characteristic in saturation region (| V DS |>| V P |) I DSS (Regions of Operations)

JFET Saturation region V DS ≥ V GS +V P, V GS > -V P. Constant- current source. (Regions of Operations) I DSS Ohmic region Saturation region I DS -VP-VP V DS V GS = 0V V GS V GS = V P Idealized output characteristic

JFET Breakdown regions. V DS > V B. (Regions of Operations)

JFET (Physical representation of the regions) Illustration of depletion layer growth and pinch-off voltage

Use the I-V characteristic curves of BJT and MOSFET Use the regions of operation of these transistors BJT Cutoff Region Active Linear Region Saturation Region MOSFET Cutoff Region Ohmic or Triode Region Saturation (Active Region) Transistors as Amplifiers and Switches Switch operation Amplifier operation Switch operation Amplifier operation

I-V Characteristic Curves Operating Point for BJT For each, IB there is a corresponding I-V curve. Selecting IB and VCE, we can find the operating point, or Q point. Applying KVL around the base-emitter and collector circuits, we obtain : I B = I BB V CE = Vcc – I C R C I C = Vcc V CE R C R C

I-V Characteristic Curves I C = Vcc V CE R C R C Q Load-line curve

Transistors as Amplifiers BJT – common emitter mode In Linear Active Region Significant current Gain Example let Gain,  = 80 V B = 2V V E = 1.3V Find I C and V C

V BE = V B – V E = 0.7V I B = V BB – V B R B 40,000 = 50  A I C =  x I B = 80 x 50  A = 4mA V C = Vcc – I C x R C = 12 – (4x10 -3 )(1x10 3 ) = 8 V V CE = V C – V E = 8 – 1.3 = 6.7 V Transistors as Amplifiers =

Transistors as Switches Basis of digital logic circuits Used in microprocessors Input to transistor gate can be analog or digital Common names are TTL – Transistor Transitor Logic CMOS – Complementary Metal Oxide Semiconductor

Transistors as Switches – BJT Inverter Use of the cutoff and saturation regions in the I-V curves. VCE = Vcc - (IC)(RC) Vout = VCE

Transistors as Switches – BJT Inverter Vin Low Cutoff region No current flows Vout = VCE = Vcc Vout = High Vin High Saturation region VCE small Vout = small Vout = Low

Transistors as Switches- MOSFET Advantages over BJT logic gates Normally Off. Does not require much current from input signal Easy Fabrication – Economical for large scale production CMOS – consumes very little power. Used in pocket calculators and wrist watches Disadvantages over BJT logic gates Cannot provide as much current as BJT Switching speed is not as fast

Transistors as Switches- MOSFET Inverter Vin Low Cutoff region No Voltage drop across RD Vout = VDD Vout = High Vin High Ohmic region VDS small Vout = small Vout = Low

Transistors as Switches- CMOS Inverter Employs a p-channel, Q p, and an n-channel, Q n MOSFET Vin = Low Qn = off Qp = on Vout = High Vin = High Qn = on Qp = off Vout = Low

References Rizzoni - Principles and Applications of Electrical Engineering, 2 nd Edition