Lecture4 Diode Once Again
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Transcript Lecture4 Diode Once Again
The Devices:
Diode Once Again
Si Atomic Structure
Electron Configuration:
First Energy Level:
2
Second Energy Level: 8
Third Energy Level: 4
Doping Process
Doping: The process of adding impurities to the intrinsic material
giving the material a Positive or Negative characteristic.
N
SI
P
SI
SI
SI
SI
SI
N
SI
SI
SI
SI
SI
SI
SI
SI
SI
SI
SI
SI
P
SI
SI
SI
SI
SI
SI
SI
SI
SI
SI
Covalent Bonding;
Pentavalent Doping;
Trivalent Doping;
Undoped Material
Shares its 4 electrons
w/other atoms and
forms a pure crystal.
Donor Material
Acceptor Material
Impurities that have an
excess of electrons. N
type Material, called
Electrons. - charged
Impurities that have
missing electron, called
Holes or P type Material.
+ charged.
n-type material
N
I
Donor Material w/an excess electron in the
covalent bond w/Silicon
displays a Negative charge.
Majority Carriers are Electrons.
SI
N
SI
SI
SI
N
SI
SI
N
SI
SI
SI
SI
SI
SI
SI
N
SI
SI
SI
N
SI
SI
V
p-type material
P
I
Acceptor Material has a missing electron in the
covalent bond w/Silicon,
displays a Positive charge.
Majority Carriers are Holes.
SI
SI
P
SI
SI
SI
P
SI
SI
SI
SI
SI
SI
SI
P
SI
SI
SI
P
SI
SI
SI
I
P
P
V
P
Remember…
Majority Current Carriers, Holes or Electrons.
N Type Material:
N
P Type Material:
P
2 Current Carriers:
Donor Material with an excess electron in the covalent
bond in Silicon & displays a Negative charge.
Majority Carriers are Electrons.
Acceptor Material has a missing electron in the covalent
bond in Silicon, & displays a Positive charge.
Majority Carriers are Holes.
Majority & Minority
Intrinsic impurities inherent in
silicon result in current flow in the opposite
direction to Majority flow. Becomes evident in
heat, leakage and break down of the device.
Minority Current carriers
The pn Junction in Si Material
The pn junction is made from a
single crystal with the impurities
diffused into it. The n end has a
surplus of negative electrons.
The p end has a surplus of holes.
Depletion region
At the junction, electrons fill
holes so that there are no free
holes or electrons there.
The actual junction becomes an
insulating layer. This barrier
must be overcome before
current can flow through the
pn junction.
The pn Junction in Si Material
cathode
anode
When a battery is connected as shown, the negative terminal pushes
negative electrons towards the junction. The positive terminal pushes
holes towards the junction. A high enough voltage will overcome the
barrier and a current will flow through the pn junction.
There is a voltage across the diode of 0.7V for the silicon. The
junction is said to be FORWARD BIASED. The p-type is the anode of
the diode, the n-type the cathode, as shown by the diode symbol. The
resistor limits the current to a safe level.
The pn Junction in Si Material
cathode
anode
When the battery is connected as shown, the positive terminal of the
battery attracts negative electrons away from the barrier. The
negative terminal attracts holes away from the barrier. The insulating
barrier widens and no current flows.
The junction is REVERSED BIASED. If the reverse voltage is made
high enough, then the junction will break down and electrons will flow
from anode to cathode (under normal conditions, electrons flow from
cathode to anode, when forward biased).
FORWARD
BIASED
+
-
-
REVERSED
BIASED
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Depletion
Region
+ +
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Depletion Region
Depletion Region
-
I
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+
Depletion Region
•Zero bias conditions
hole diffusion
electron diffusion
p
•p more heavily doped
than n (NA > NB)
•Electric field gives rise
to potential difference in
the junction, known as
the built-in potential
(a) Current flow.
n
hole drift
electron drift
Charge
Density
+
x
Distance
-
Electrical
Field
(b) Charge density.
x
(c) Electric field.
V
Potential
-W 1
W2
x
(d) Electrostatic
potential.
Built-in Potential
N A ND
0 T ln
2
n
i
Where T is the thermal voltage
kT
T
26mV (at 300 K )
q
ni is the intrinsic carrier concentration for
15 1
pure Si (1.5 X 1010 cm-3 at 300K), so forN A 10 3 ,
cm
10151016
0 26 ln
mV 638mV
10 2
1.5 *10
1
N B 1016 3 ,
cm
Models for Manual Analysis
+
ID = IS(eV D/T – 1)
VD
ID
+
+
VD
–
(a) Ideal diode model
•Accurate
•Strongly non-linear
•Prevents fast DC bias
calculations
–
VDon
–
(b) First-order diode model
•Conducting diode replaced
by voltage source VDon=0.7V
•Good for first order
approximation
Typical Diode Parameters
Geometry, doping and material
constants lumped in Is
Diffusion coefficient
minority carrier concentration
+
VD
ID = IS(eV D/T – 1)
–
•Dn=25 cm2/sec
•q=1.6*10-19 coulombs
•Dp=10cm2/sec
•pn0=0.3*105/cm3
•Wn=5 mm
•np0=0.6*104/cm3
•Wp=0.7 mm
•W2=0.15 mm
•W1=0.03 mm
I S qAD ( WnpWn 02 W pn Wp 01 )
D p
typical value
I S 10 17 A / mm 2
D n
Secondary Effects: Breakdown
Cannot bear too large reverse biases
» Drift field in depletion region will get extremely large
» Minority carriers caught in this large field will get very energetic
– Energetic carriers can knock atoms and create a new n-p pair
– These carriers will get energetic, too, and so on: thus large currents!
0.1
Two types
» Avalanche breakdown
ID (A)
– Above mechanism
» Zener breakdown
– More complicated
0
–0.1
–25.0
–15.0
–5.0
VD (V)
0
5.0
Can damage diode
Diode SPICE Model
Required for circuit simulations
» Must capture important characteristics but also remain efficient
» Extra parameter in the model: n (emission coefficient, 1 n 2)
– Fixes non-ideal behavior due to broken assumptions
Additional series resistance accounts for body+contact
Nonlinear capacitance includes both CD and Cj
I D I S (eVD /nT 1)
RS
+
VD
-
ID
CD