Week 11b Lecture Materials

Diodes and some of their uses: Review of pn-diode structure Diode I-V characteristics: Actual characteristic – exponential Ideal diode characteristic – switches at V = 0 Large-signal diode model – a switch and a voltage source to represent required turn-on voltage Zener diode, varactor diode, light-emitting diode (LED), solar cell PN-diodes used for isolation in integrated circuits Diodes and MOSFET operation – need for a gate

The band picture of semiconductors (if there’s time)

Answering one of your questions: A quick picture of superconductivity – EECS Prof. Ted Van Duzer

The pn Junction Diode

Schematic diagram

p-type n-typeID

+ VD –

Circuit symbol

Physical structure:(an example)

p-type Si

n-type Si

SiO2SiO2

metal

metal

ID+

VD

–

net donorconcentration ND

net acceptorconcentration NA

For simplicity, assume thatthe doping profile changes abruptly at the junction.

cross-sectional area AD

Figure 0.1 Schematic symbol and water model of a pn-diode

 

Water Model of Diode Rectifier

Simplistic

Simplistic view of why a pn-diode conducts differently in forward and reverse bias: When the p side is made positive with respect to the n side (forward bias), the positively charged holes move toward the negatively charged electrons, and they recombine. Then more carriers flow in from the contacts. In reverse bias, the holes and the electrons move away from each other, leaving no mobile carriers in the middle – hence, the diode has an insulator in its middle region and no current flows through.

Summary: pn-Junction Diode I-V

• Under forward bias, current increases exponentially with increasing forward bias

• Under reverse bias, a potential barrier in the middle of the junction is increased, so that negligible carriers flow across the junction

ID (A)

VD (V)

The net result is an I-V curvethat looks like this, with typically nA currents in thereverse direction (VD < 0), andmA or more in the forwarddirection (VD > 0)

0.7 V

for Si

|

reverse bias

forward bias

• An ideal diode passes current only in one direction.

• An ideal diode has the following properties:

• when ID > 0, VD = 0

• when VD < 0, ID = 0

Ideal Diode Model of pn Diode

ID (A)

VD (V)

ID +

VD

–

+VD –

ID

Circuit symbol I-V characteristic

Diode behaves like a switch: • closed in forward bias mode • open in reverse bias mode

Switch model

Large-Signal Diode Model

reverse bias

forward bias

ID (A)

VD (V)

ID +

VD

–

+

VD

–

ID

Circuit symbol I-V characteristic Switch model

Vturn-on

+ Vturn-on

RULE 1: When ID > 0, VD = Vturn-on

RULE 2: When VD < Vturn-on, ID = 0Diode behaves like a voltage source in series with a switch: • closed in forward bias mode • open in reverse bias mode

For a Si pn diode, Vturn-on 0.7 V

Application Example: Rectification using the ideal

diode model

vs(t)+

+

vR(t)

–

vs(t)

t

RC

vR(t)

t

20 : 1

120V

Figure 0.7 Regulated power supply

MC7805

2500F 1F

regulator

To get a really steady voltage out we can add an integrated circuit regulator to the circuit.

ba

0.7 V

Figure 0.1 Battery and resistor circuits with (a) forward- and (b) reverse-biased diodes.

Potential

Potential plots for forward- and reverse-biased diodes in series with a voltage source and a resistor

Figure 0.1 Diode circuit with alternating supply voltage

Vi n

-

+

Time (s)

Vi n ,Vo ut

Vi n

Vi n ,Vo ut (volts)

Vo ut

+

-

43210-1-2-3-4

Clipping circuit using a pn-diode

Figure 0.1 The Zener diode

ID

VD

Diode Symbol

Idealized diodecharacteristic

Diode turn-onvoltage

Zener diode I–V characteristic

VD

ID

Zener

Zener breakdown

voltage

One uses the reverse-bias breakdown voltage as a voltage reference in some circuits

Varactor diode

Light-emitting diode (LED)

Screen Printedcontact finger

Single-layerantireflection

coating

n+

P

finger

Contact

Double-layerantireflectioncoating

P+P+ Rear

n+ doping

Oxide

Solar cells – two designs

Short-circuitcurrent

Ope

n-ci

rcu

itvo

ltage

light

-

curr

ent

MaximumPower

IV

0 0.5

light

-

curr

ent

VoltagePhotovoltaic

junction

1-0.5-1

Dark characteristic

gene

rate

d

gene

rate

dV

I

Solar cell with load and its I-V characteristic

• The basic building block in digital ICs is the MOS transistor, whose structure contains reverse-biased diodes.

– pn junctions are important for electrical isolation of transistors located next to each other at the surface of a Si wafer.

– The junction capacitance of these diodes can limit the performance (operating speed) of digital circuits

Why are pn Junctions Important for ICs?

p-type Si

n n n n n

regions of n-type Si

No current flows if voltages are applied between n-type regions, because two pn junctions are “back-to-back”

n-regionn-region

p-region

=> n-type regions isolated in p-type substrate and vice versa

Device Isolation using pn Junctions

a

ba

b

p-type Si

n n

Transistor A

n n

Transistor B

We can build large circuits consisting of many transistors without worrying about current flow between devices. The p-n junctions isolate the transistors because there is always at least one reverse-biased p-n junction in every potential current path.