1. SMV ELECTRIC TUTORIALS Aditya Kuroodi 2016 Relevant Course(s): EE121B, EE115A

2. INTRODUCTION TO TRANSISTORS

3. Transistors: BJTs & MOSFETs  There are 2 main types of transistors: Bi-Polar Junction Transistors and Metal- Oxide Semiconductor Field Effect Transistor  For each transistor type, there are 2 variations  Combined effect of transistors revolutionzed electronics  We will focus less on the semiconductor theory behind transistors and more on their functionality BJTsMOSFETs

4. The PN Junction: Forward and Reverse Bias  If you hook up + terminal of battery to P-Type, and – terminal to N-Type you will forward bias the junction  Forward bias repels majority carriers back into depletion zone, causing depletion zone to shrink (due to recombination) and that lets current flow easily  If you hook up – terminal of battery to P-Type, and + terminal to N-Type, you will reverse bias the junction  Reverse bias attracts majority carriers to terminals, expanding the depletion zone and impeding current flow

5. (Junction) Diodes  A diode is a device that only allows current to flow in one direction, it is achieved through a PN junction  The blue arrow represents conventional current flow (opposite of electron flow) + -

6. Bi-Polar Junction Transistor (BJT)  Add an extra semiconductor layer to a junction diode and you get BJT  BJT is a 3 layered (doped) semiconductor sandwich, can either be PNP or NPN variety  A BJT is a current-controlled current regulator  The main current flows from Emitter to Collector (PNP) or Collector to Emitter (NPN)  The controlling current flows from Emitter to Base (PNP) or Base to Emitter (NPN)  You control the main current by varying how much base current you supply to the BJT The above arrows represent electron flow

7. Bi-Polar Junction Transistor (BJT)  The little arrow on Emitter always points in direction of conventional current flow  Emitter current = Base current + Collecter current by KCL  BJTs are “bi-polar” because they use both carrier types (electrons + holes)  When base current is 0 (or less than threshold current), transistor is in cutoff (fully nonconducting)  When base current at max, transistor is saturated (fully conducting)  To conduct NPN: have to pull Base high relative to Emitter  To conduct PNP: have to pull Base low relative to Emitter  Since electron mobility > hole mobility, NPN is more common  Controlled current flows through the 2 outer layers, not base layer  How to differentiate the two BJTs: N ot- P ointing-i N = NPN PNP NPN

8. BJT as a Switch  Note: BJTs actually have 5 operating modes (not just cutoff or saturation)  For our purposes we will deal mostly with cutoff and saturation regions, enabling us to use the BJT as a progressive switch  The tiny signal picked up from microphone (imagine a clap), once rectified, can be used to bias the base of the transistor on and turn on the lamp  Now we can use tiny current to control a much larger current (amplification)  NOTE: The battery provides the larger current, not the transistor (no magic)  The louder the clap, the brighter the bulb (active mode). That is, until we reach saturation

9. The Field Effect Transistor (JFET, FET)  FETs are voltage-controlled current regulators  3 terminals: Gate, Drain, Source  2 varieties: N-Channel (NMOS) and P-Channel (PMOS)  FETs, unlike BJTs, are unipolar devices (one major carrier for main current)  As you vary Gate voltage, current through Drain and Source will vary

10. JFET (vs BJT) & MOSFETs  JFETs have high input impedance, meaning little current flows through base in operation (minimum impact on rest of circuit, unlike BJT)  JFETs have less amplification abilities than BJTs  JFETs, unlike BJTs, are restrictive devices  When left untouched, transistor will be normally closed  As you throttle “base” voltage, main controlled current will decrease  MOSFETs fall under the larger branch of Junction FETs  MOSFETs are JFETs with even higher input impedance  Come in either depletion mode or enhancement mode  Enhancement mode MOSFETs act like BJTs (normally open, amplify current when throttled)

11. The (Enhancement Mode) MOSFET N-Channel MOSFET  Arrow points inwards  ON when gate bias voltage (gate to source) > threshold voltage  When gate votlage equal to 0, transistor is ON  Source often connected to GND, load attached to Drain (low-side switching)  Lower ON resistance than P-Channel P-Channel MOSFET  Arrow points outwards  ON when gate bias voltage < threshold voltage  When gate voltage equal to 0, transistor is OFF  Source often connected to load, Drain connected to GND (high-side switching)

12. MOSFET as a Switch  Suppose we want to turn a lamp (or LED) on and off with a MOSFET  Using N-Channel, we connect Source to GND  Load placed between voltage rail and the MOSFET  Input voltage pulses, either biases gate- source to saturation or leaves transistor open  When gate voltage high, lamp is on  Special safety precautions must be taken if load is NOT purely resistive (for power switching)  Inductive load requires voltage spike protection  Capacitive load requires inrush current limitaion NOTE: VDD >> Vin

13. MOSFET Power Switching Considerations  Inductors, when quickly powered off, will generate huge voltage spike in opposition to decreasing current  V = L *di/dt  Use FlyBack Diode (Snubber, Supression, Flywheel, etc.) to protect circuitry (including the MOSFET!)  Now current flows through diode, back through inductor and slowly dies down from resistive losses  Capacitive loads will draw in large current when first connected to voltage rail (capacitors act like shorts initially)  Simply place resistor in series with whatever you want to protect to limit inrush current (an NTC thermistor better than static resistor)  NTC thermistors start with high resistance, then lower resistance as they heat up  NOTE: by definition, motors are inductive loads!

14. BJTs & MOSFETs BJT  Superior for use in amplifiers  Requires both voltage and current to drive  Since current-controled, sometimes the additional current affects rest of circuit  Bottom Line: BJTs usable (cheaper, stable) for lower power applications MOSFET  Superior for power supply regulators  Higher switching frequency, so better for high power applications  Requires only voltage to drive  Higher gate impedance (so draws in little current), little impact on rest of circuit  Easier to use with MCUs that have digital outputs  Bottom Line: MOSFETs preferrable for higher performance (aside from amplifiers) or high power applications

15. OTHER TYPES OF DIODES

16. Other Diodes + Applications  So far we’ve discussed the general diode (a.k.a. junction diode)  As you start to use transistors in circuits, you will start to see many more types and applications of diodes  Each diode type has it’s own symbol and functionality, we will briefly cover a few of the common types now  Zener Diode  Schottky Diode  Light Emitting Diode Schottky Diode + -

17. Zener/Avalanche Diode  Avalance breakdown: A semiconductor phenomona where large current flows in an otherwise insulating material (when you reverse bias A LOT)  Zener diodes are built to safely operate in breakdown region, in addition to acting like a normal diode during forward biasing  Zener votlage: a reduced breakdown voltage for Zener diodes  The Zener diode, when in breakdown, will maintain its Zener voltage over a wide range of currents  Ex: A 3V Zener will output 3V when in breakdown, (almost) regardless of current  Applications: Voltage Regulation, Waveform Clipper, TVS

18. Schottky Diode  Schottky diodes charecterized by low forward voltage drop and fast switching  Instead of ~.7V drop for a normal Silicon based diode, Schottkys have ~.15V to.45V drop  However, their low voltage ratings make them unsuitable for high power applciations  Small voltage drop means they are more power efficient  Fast switching makes them suitable for high frequency applications (RF devices and SMPS)

19. Light Emitting Diode (LED)  LEDs are diodes that light up when forward biased  Specially construct PN junction (not just Si) with materials that glow when current passed through them  The long lead of an LED is the anode (+)