EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D1
PN Junction Diode
Semiconductor
Elements
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D2
Diode Cases
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D3
Basic Diode Feature
The essential feature of a diode is that the magnitude of the current greatly
depends on the polarity of applied voltage.
Basically the diode conducts current in only one direction. It is applied in
rectifiers for converting an alternating current into a direct current.
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D4
PN Junction Diode Structure
An ideal diode can conduct current in only one direction.
Diode specific electrical properties depends on it’s structure.
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D5
Junction Diode Schematic Symbol
A diode is a nonlinear semiconductor device with two electrodes:
an anode and a cathode.
An arrow points from the anode to the cathode to remind that the
current will only flow easily in this direction.
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D6
Diode's Mode of Operation
A diode's mode of operation depends on the various conditions possible
at the pn junction. They are caused by the polarity of the applied voltage.
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D7
Unbiased Diode
Equal number
of plus and
minus charges
Electrically
neutral as
a whole
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D8
PN Junction Formation
Unlike the free carriers, the ions will never move. They remains fixed due
to their covalent bonds in the crystal structure.
Majority carriers – free electrons and holes, diffuse (spread) across the
junction because difference in concentration in both side of the crystal.
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D9
Depletion Layer
When an electron leaves the n side, it will leave behind an uncompensated
positive ion. A positive space charge is created to the right of the junction in
the n-region.
Similarly a negative space charge will be created to the left of the junction
in the p-region.
The depletion layer is almost completely depleted of mobile charge
carriers and has very high resistance.
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D10
Barrier Potential & Electric Field
The uncompensated positively- and negatively-charged ions that are within
the depletion layer generate an electric field Eo the barrier potential Uo.
At room temperature (25 oC) the barrier potential has for Si diodes a
voltage of approximately 0.7V and for Ge diodes a voltage of about 0.3V.
ID
IE
Majority carriers
Minority carriers
Diffusion current ID
Drift current IE
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D11
Forward Bias
The barrier potential will reduce to Uo - Us and the electric field to E < Eo.
A forward current is always composed of majority carriers whose energies
are sufficient to overcome the barrier potential.
Current will easily flow in forward-biased diode.
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D12
Reverse Bias
The barrier potential will increase to Uo + Us and the electric field to E > Eo.
The diffusion of majority carriers across the junction has been greatly diminished.
A very small reverse current IR, composed of thermally produced minority carriers
still cross the junction. This makes the reverse current IR independent from the
direction of the voltage polarity and of the barrier potential.
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D13
VA Characteristic
A diode is a nonlinear device. It conducts in only one direction.
An ideal diode functions like a switch – open and closed.
In forward-biased diode, the current increases very rapidly with U. The
reverse current is very small.
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D14
Ideal Diode Equation
)1( T
U
s eII
q
kTT
11600
)(KTT 0258.0T
- Thermal potential
V for T = 25 oC
Is - Saturation current
ss SJI S - Junction area
Js - Current density
1ln
S
TI
IU
T
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D15
Ideal Diode Equation
T
U
seII
sII
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D16
Saturation Current
),,( STWfIs
Is doubles with every 10oC increase.
Because the reverse current is caused by
thermally created minority carriers, it will
also be highly sensitive to temperature
changes.
There are fewer minority carriers
in Si diodes than in Ge diodes.
A Si diode has a much smaller
Is than a Ge diode.
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D17
VA Characteristics of Si and Ge Diode
IF , mA
UF , V
IR , (µA or nA)
UR , V
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D18
Real Diode – Forward Region
R
U
d
+_
p n
+_
IF , mA
UF, V
rE rB
UB
UPN
rBI
UB
UPNI.rB
U = UPN + IrB
Ohmic
region
Е
Recombination current, base ohmic resistance rB, rB = f (I) are considered.
)( T
B
m
IrU
S eII
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D19
Real Diode – Reverse Region
UR , V
I leakage
IS
IGIR
Breakdown
region
Si ~ IG
Ge ~ IS
UR, V
IR, nA
BRутGSR IIIII
Generation current, leakage current and breakdown are considered.
IG ≈√U
IG >> IS
IS >> IG
Si diode IR = IS + Ilekage (IG >> IS ),
Ge diode IR = IG + Ileakage (IS >> IG)
IR, nA
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D20
Forward Voltage and Temperature
constIT
U
dT
dUTKU F
TKUF ≈ - 2 mV/ oC
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D21
Diode Ratings
Max junction temperature Tjmax < Ti, where n=p=ni
Max power Pjmax
th
ai
R
TTP
max
max
th
aj
R
TTP
P = UI Power dissipated in diode
Power conducted
into the ambient
surroundings
th
aj
R
TTUI
maxmax
11j
th
a
th
TR
TR
P
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D
Maximum Power & Temperature
maxmax
11j
th
a
th
TR
TR
P
Ta,oC
Pjmax, W
Tjmax25oC
Ta,oC
Pjmax, W
Tjmax25oC
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D
Maх Power & Maх Current
th
aj
FFR
TTPIU
max
maxmax
IFmax, A
Ta,oC
Tjmax
Ta,oC
Pjmax, W
Tjmax25oC
maxmax
11j
th
a
th
TR
TR
P
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D24
Current-Limiting Resistor
The current-limiting resistor has the function to keep the diode current
smaller than the maximum rating.
maxFF I
R
UEI
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D25
Heat Removal
jcca thth RR cajc ththth RRR
hachjc thththth RRRR
Thermal energy can be easily reduced through conduction and radiation from
the device's case.
Heat sink
Rth indicates efficiency in removing
heat from the transistor in units oK/W.
The less thermal resistance the higher power rating.
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D26
Reverse Breakdown
At a reverse breakdown voltage UBR, the current will rapidly increase with
only small changes in the voltage.
Electrical breakdown
Thermal breakdownUBR
Avalanche breakdown
Zener breakdown
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D27
Avalanche Breakdown
n
BR
RR
BR
U
UI
IM
1
1
Minority carriers can be accelerated by the field and
generate another two electron-hole pairs on collisions
with crystal atoms. The process may continue, causing
the increase of the current.
When the temperature increases, the crystal atoms
start moving randomly in the lattice, preventing in this
way acceleration of carriers, so they can not reach
enough energy when to generate new electron-hole
pairs by hitting atoms. For this reason the breakdown
voltage increases.
UBR >7V
Avalanche breakdown occurs in wide depletion
layers with a reverse bias higher than 7V.
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D28
Zener Breakdown
When an electric field of the barrier potential is large enough to break the
covalent bonds additional free electrons and holes are created. Such effect is
called Zener effect or Zener breakdown. This requires high electric fields on
the order of 300 000 V/cm.
When the temperature increases the process of breaking up covalent bonds is
facilitated and the breakdown voltage decreases.
Zener breakdown occurs in very narrow depletion layers with a reverse
bias less than 5V.
Covalent
bond
UBR < 5V
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D29
Thermal Breakdown
th
aj
RRR
TTIU
UR,V
IR, mA
UBR1 UBR2
T2T1
r < 0
T2 > T1
UR,V
IR, mA
Avalanche Thermal
Zener
Surface
This breakdown occurs if
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D30
Load Line and Operating Point
E = U + IR
R
EU
RI
1
E1
E1
R
Е2E
E
R1
E2
R
E2 > E1E
R2
R2 > R1
Q
Q
QI
UR R
E
E
EQ
Kirchhoff law
UF,VUF,V
IF, mAIF, mA
If I = 0, U = ELoaf line
End points
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D31
Equivalent Circuits
Threshold model
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D
Examples
1k
12mA
12V
11.3 mA
The diode is forward connected with Uo = 0.7V
The voltage over resistor UR се is calculated from Kirchhoff's law
UR = V1 – Uo = 12 – 0.7 = 11.3V
Calculation current and voltages in the circuit by using diode threshold model.
Diode
Resistor
U, V I, mA
0.7
11.3
11.3
11.3
The current is the same through all
elements in serial circuits
Current through R1 from Ohm law is:
Operating
point12V
1k
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D
Examples
Diode
Resistor
U, V I, mA
3
0
0
0
The diode is revers connected, I=0.
From Ohm law UR = I.R1 = 0
Voltage oved diode is calculated from Kirchhoff's law
UD = V1 – I.R1 = 3-0 = 3V
=13 mA
Operating
point
Calculation current and voltages in the circuit by using diode threshold model.
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D
What will be the reading of the voltmeter (UD) if the
diode is made of Si? What current flows through the
diode? Build the load line and mark the operating
point. Use a threshold model of the diode.
What will the voltmeter read if the diode is made of
Ge? What current flows through the diode?
Build the load line and mark the operating point. Use
a threshold model of the diode.
Calculate the power dissipated on the Si diode in the
circuit.
Examples – self learning
PD = I.UD I = ? Use the diode threshold voltage.
Ge
EE141© 2009, Associate Professor PhD. T.VasilevaProf. Tania Vasileva, Ph.D35
Diode Testing