Transcript Week 6: Transistors

SMV ELECTRIC TUTORIALS
Aditya Kuroodi
2016
Relevant Course(s): EE121B, EE115A
INTRODUCTION TO
TRANSISTORS
Transistors: BJTs & MOSFETs

There are 2 main types of transistors: Bi-Polar Junction Transistors and MetalOxide 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
BJTs
MOSFETs
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
(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)
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
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:
Not-Pointing-iN = NPN
PNP
NPN
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
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
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)
The (Enhancement Mode) MOSFET
N-Channel MOSFET
P-Channel MOSFET

Arrow points inwards

Arrow points outwards

ON when gate bias voltage (gate to
source) > threshold voltage

ON when gate bias voltage <
threshold voltage

When gate votlage equal to 0,
transistor is ON


Source often connected to GND, load
attached to Drain (low-side
switching)
When gate voltage equal to 0,
transistor is OFF


Lower ON resistance than P-Channel
Source often connected to load,
Drain connected to GND (high-side
switching)
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 gatesource 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
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!
BJTs & MOSFETs
BJT
MOSFET

Superior for use in amplifiers


Requires both voltage and current
to drive
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


Since current-controled,
sometimes the additional current
affects rest of circuit
Bottom Line: BJTs usable (cheaper,
stable) for lower power
applications
OTHER TYPES OF DIODES
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
-
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
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)
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 (+)