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 surface 3. The implanted ions displace semiconductors atoms.

4. A follow-up anneal (heating) of the wafer is necessary for removing damage and for placing dopants correctly in the lattice.

Electric Field/Voltage Definition of Voltage: The work done in moving a unit positive charge 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 We have the gradient to push holes to the left.

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

(P is neutral, even though it carries 5 electrons, one of them being a free electron.) Net charge =0 (B is neutral, even though it carries 3 electrons. ) Net charge =0 E depends on the net charge included in the imaginary surface.

Extra Credit: Derive Built in Voltage

Different ways of Crossing PN Junction

Diffusion

np=n i 2

Drift Diffusion Drift 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)

PN Junction under Reverse Bias

Reverse: Connect the + terminal to the n side.

E Depletion region widens.

Therefore, stronger E.

Minority carrier to cross the PN junction easily through drift.

Current is composed mostly of drift current contributed by minority carriers.

n p to the left and p n to the right.

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

PN Junction as a capacitor

Large capacitance.

(Less charge separation) Smaller capacitance.

(More 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.

(gradient of minority carriers) n n,f enters the p side as minority carriers (n p,f ). n p,f with the p p,f , which are abundant. will recombine

I D must be constant at all points along x 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 I tot .

I S =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

0.551

0.603

0.650

0.70

0.748

0.10 mA 0.26 mA 0.77 mA 2.10 mA 5.74 mA 13.8 mA

Measured Diode Voltage

15 12 Measured Data Barrier Potential is ~ 665 mV 9 6 3 0 400 440 480 520 560 600 640 Diode Voltage 680 720 760 800

0.01

On Semilog Plot

Measured Data 1E-3 1E-4 0.45

0.50

0.55

0.60

0.65

Diode Voltage (V) 0.70

0.75

Application: PN Junction Temperature Sensor

Reverse Biased Diode

I S =Reverse Saturation=leakage current

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=V D,on

Reverse Bias

Measured R2 is 0.997 MOhms. % Error is about -0.3 %

Reverse Bias

VS (Measured) IR (Computed) 5 10 15 3 nA 3 nA 3 nA

PN Junction Based Devices

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