PN Junction

Section 2.2-2.3

References

• Section 2.2-2.3. (PN junction without energy band-diagram.)

• Supplemental Reference: “Modern Semiconductor Devices for Integrated Circuits”, Chenming Calvin Hu (PN junction without energy band-diagram.)

Applications

• Diodes• CdS cell• Zener diode• PN junction temperature sensor

Review

What do we get by introducing n-type and p-type dopants into two adjacent sections of a piece of silicon?

How Do You Make a PN Junction Diode

Ion Implantation

1. Accelerate the ions to high energy 2. Shoot the ions onto the semiconductor surface3. The implanted ions displace semiconductors atoms.4. A follow-up anneal (heating) of the wafer is necessaryfor removing damage and for placing dopants correctly in the lattice.

Electric Field/Voltage

Definition of Voltage: The work done in moving a unitpositive charge in an electric field.Alternative definition:

-+ Vo

P side is suddenlyjoined with the n side

Each e- that departsfrom the n side leavesbehind a positive ion.

Electrons enter the P side and create neg.ion.

The immediatevincinity of the junctionis depleted of freecarriers.

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 electronsto move from the n side to the p side.

Equilibrium does not mean that there is no movement of carriers, but instead We have the gradient to push holes to the left.

E is there to push the drift current to the right.

Net charge =0 Net charge =0

E depends on the net chargeincluded in the imaginary surface.

Extra Credit:Derive Built in Voltage

(P is neutral, even though it carries 5 electrons, one of them being a free electron.)

(B is neutral, even though it carries 3 electrons. )

Different ways of Crossing PN Junction

np=ni2

DiffusionDiffusion

Majority carriers cross the pn junction via diffusion (because you have the gradient)Minority carriers cross the pn junction via drift( because you have the E, not the gradient)

Drift Drift

PN Junction under Reverse Bias

Reverse: Connect the + terminal to then side.

Depletion region widens.Therefore, stronger E.

Minority carrier to cross the PN junction easilythrough drift.

Current is composed mostly of drift current contributedby minority carriers.

np to the left and pn to the right.

Current from n side to p side,the current is negative.

E

PN Junction as a capacitor

As the reverse bias increases, the width of the depletion region increases.

Smaller capacitance.(More charge separation)

Large capacitance.(Less charge separation)

c02f25

Bias dependent capacitance.Useful in cell phone applications.

Photodiode

1. Light is applied to the pn junction2. Electrons are dislodged from covalentbonds.3. Electron-hole pair is created.4. Electron is attracted to the positive terminalof the battery.5. Current flows through the diode is proportionalto 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 recombinewith the pp,f, which are abundant.

(gradient of minority carriers)

In the vincinity of depletion region, the current consists mostly of minority carriers.

Away from the depletion region, the current consists mostlyOf majority carriers.At each point along the x-axis, the two components add upTo Itot.

ID must be constant at all points along x

IS=Reverse Saturation=leakage current

Measure Forward Biased Diode Current

Listed R1=330 Ohms, Measured R1=327.8 Ohms, % error=-0.66 %

Measured Value (Forward Bias)

VF

(V)

IF (Computed)

0.455 30.50 uA

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

Measured Diode Voltage

400 440 480 520 560 600 640 680 720 760 8000

3

6

9

12

15

D

iod

e C

urr

en

t (m

A)

Diode Voltage

Measured DataBarrier Potential is ~ 665 mV

On Semilog Plot

0.45 0.50 0.55 0.60 0.65 0.70 0.75

1E-4

1E-3

0.01

1

0 lo

g (

I diod

e)

Diode Voltage (V)

Measured Data

Application: PN Junction Temperature Sensor

Reverse Biased Diode

IS=Reverse Saturation=leakage current

Dynamic Resistance

VF

(V)

IF (Computed)

0.70 5.74 mA

0.748 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

(Measured)

IR

(Computed)

5 3 nA

10 3 nA

15 3 nA

PN Junction Based Devices

• PN Junction as a Temperature Sensor • Solar Cell (117)• Zenor Diode• LED