Transcript PN Junction ES230 Jack Ou Review What if we introduce n-type and p-type dopants into two adjacent sections of a piece of silicon?

PN Junction
ES230
Jack Ou
Review
What if we introduce n-type and p-type dopants
into two adjacent sections of a piece of silicon?
Electric Field/Voltage
Definition of Voltage: The work done in moving a unit
Positive charge from one point to another in an electric field.
Alternative definition:
+
Vo
-
P side is suddenly
joined with the n side
Each e- that departs
from the n side leaves
behind a positive ion.
Electrons enter the
P side and create neg.
ion.
The immediate
vincinity of the junction
is depleted of free
carriers.
Electric field within the depletion region points from the left to the right.
The direction of the electric field make it difficult for more free electrons
to move from the n side to the p side.
Equilibrium does not mean that there is no movement of carriers,
but instead
Net charge =0
E depends on the net charge
included in the imaginary surface.
Extra Credit:
Derive Built in Voltage
Net charge =0
Different ways of Crossing PN
Junction
Diffusion
Diffusion
np=ni2
Drift
Majority carriers cross the pn junction via diffusion
Minority carriers cross the pn junction via drift
Drift
PN Junction under Reverse
Bias Reverse: Connect
the + terminal to the
n side.
Depletion region widens.
Therefore, stronger E.
E
Majority carrier to cross
the PN junction easily
through diffusion.
Current is composed
mostly of drift current contributed
by minority carriers.
np to the left and pn to the right.
Current from n side to p side,
the current is negative.
PN Junction as a capacitor
Smaller capacitance.
Large capacitance.
(More charge separation)
(Less charge separation)
As the reverse bias increases, the width of the depletion region increases.
Bias dependent capacitance.
Useful in cell phone applications.
c02f25
Photodiode
1. Light is applied to the pn junction
2. Electrons are dislodged from covalent
bonds.
3. Electron-hole pair is created.
4. Electron is attracted to the positive terminal
of the battery.
5. Current flows through the diode is proportional
to light intensity.
Application: Digital camera.
Forward Bias Diode
Depletion region shrinks due to charges from
the battery.
The electric field is weaker.
Majority carrier can cross via diffusion;
Greater diffusion current.
Current flows from P side to N side
Equilibrium
Forward Biased Diode
Majority carriers cross the junction via diffusion.
Minority carriers increased on both sides of the junction.
nn,f enters the p side as minority carriers (np,f). np,f will recombine
with the pp,f, which are abundant.
In the vincinity of depletion region, the current consists
mostly of minority carriers.
Away from the depletion region, the current consists mostly
Of majority carriers.
At each point along the x-axis, the two components add up
To Itot.
IS=Reverse
Saturation=leakage current
The diode current is proportional to area.
Measure Forward Biased
Diode Current
Listed R1=330 Ohms, Measured R1=327.8 Ohms, % error=-0.66 %
Measured Value (Forward
Bias)
VF
(V)
0.455
IF
(Computed)
0.509
0.10 mA
0.551
0.26 mA
0.603
0.77 mA
0.650
2.10 mA
0.70
5.74 mA
0.748
13.8 mA
30.50 uA
Measured Diode Voltage
15
Measured Data
Barrier Potential is ~ 665 mV
Diode Current (mA)
12
9
6
3
0
400
440
480
520
560
600
640
Diode Voltage
680
720
760
800
On Semilog Plot
0.01
10 log (Idiode)
Measured Data
1E-3
1E-4
0.45
0.50
0.55
0.60
0.65
Diode Voltage (V)
0.70
0.75
Dynamic Resistance
VF
(V)
0.70
0.748
IF
(Computed)
5.74 mA
13.8 mA
Dynamic Resistance from the measurement:
(0.748-0.70)/(13.8 mA-5.74 mA)= 48 mV/8.06 mA =5.95 Ohms
From the manufacture’s specification=8.33 Ohms, using data from 0.7V
and 0.725 V in Figure 4.
If VD is less than VD, On, the diode behaves like an open circuit.
The diode will behave like an open circuit for VD=VD,on
Reverse Bias
Measured R2 is 0.997 MOhms. % Error is about -0.3 %
Reverse Bias
VS
IR
(Measured) (Computed)
5
3 nA
10
15
3 nA
3 nA