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PN Junction

Forward & Reverse Biased

Engr. Fazal Ur Rehman

Contents

1. PN Junction Formation
2. Space Charge Distribution
3. Barrier Potential
4. Forward Biased and Reverse Biased Diode
5. Diode Equation
6. Diode Models
7. Schottky Diode

PN Junction Formation

P-Type semiconductor having excessive holes

N-Type semiconductor having excessive free electrons

Junction; 𝑁𝐴= 𝑁𝐷are both zero. Thin area where the material is neutral

Acceptor atoms

Donor atoms

Space Charge Distribution

P N

Positive and negative ions are created when holes and free electrons cross the junction and move to the other side this charge is called “space charge”

Diffusion current flows whenever P and N materials are joined together

Barrier Potential

P N

Charges of opposite polarities establish “Electric Field” also known as “Barrier Potential”. This field prevents any further diffusion current

E

Depletion Region

Barrier potential is also known as “Junction Potential” or “Diffusion Potential”. It is denoted by Vo .

P-N Junction

• A potential barrier Vo is created near the junction, because of this potential barrier an Electric field will develop & its direction will be from N to P region.
• The formation of this electric field will try to stop further crossing over of the majority carriers.
• Electric field will support the movement of minority carriers (holes) in N- region to cross over the junction & go into the P-region
• Movement of minority carriers from N to P and P to N is the drift current.
• Diffusion & drift current in an unbiased PN-junction are equal and opposite in direction. 𝑰𝒅𝒊𝒇𝒇𝒖𝒔𝒊𝒐𝒏 = - 𝑰𝒅𝒓𝒊𝒇𝒕
• When diffusion current tries to increase, it will result in expansion of the space charge layer which will immediately curb this tendency of diffusion current to increase.
• Potential barrier formed across the PN-junction of the semiconductor is typically 0.6-0.7 V for silicon & 0.2-0.3 V for germanium.

Length of depletion Layer

• Derivation
• Cases

Forward Biased Junction

E

P^ N

• (^) -

Opposing field due to battery

Depletion region shrinks and drift current begins to flow

Reverse Biased Junction

E

P^ N

-^ +

Supporting field due to battery

Depletion region expands and opposes flow of current

Reverse Biased Junction

• When P side of the battery is connected with – ive terminal of battery & N side of battery is connected with +ive terminal of the battery is it said to be reverse biased
• The – ive terminal of the battery attract the hole & +ive terminal of the battery attract the electrons due to this hole & electron flow away from the junction & no current flow due to the majority carrier
• Depletion layer gets wider during reverse bias as majority carriers in each region are drawn away from the junction, they leave behind more uncovered charges (or immobile ions)
• Depletion layer stop growing when its difference of potential equals to the applied reverse voltage 𝑽𝟎,𝒆𝒏𝒍𝒂𝒓𝒈𝒆𝒅 = - 𝑽𝑩𝑩
• Sufficiently large reverse bias the space charge layer can expand so much that it touched the boundaries of the P & N regions, this will cause the PN junction to rupture or break down& will result to the permanent damage to the junction. This is called Punch-through
• Due to the thermal energy minority carrier exits on both side of the junction

Assignment

 Merits & Demerits of Si & Ge

 Usage of semiconductor in Electronics devices

Peak Inverse voltage (PIV)

Maximum value of reverse voltage that can be applied across diode before it

enter into the break down region

Diode Equation

 T 

V V

I Is e

I = Diode current; A V = Applied Voltage; Volts Is = Saturation Current; A η = emission coefficient; a function of V; also depends on material; 1 ≤ η ≤ 2 VT = Thermal Voltage (0.026V at room temp)

I =? V = 0.55 V Is = 0.1 pA η = 1 T1 = 20ºC T2 = 100ºC

Saturation Current: “The magnitude of the current that flows for negative input voltage in excess of a few VT , typically 10-^12 A ”

This current is directly proportional to temperature and diode area and decreases with better material used

Diode Models

• Diode model is the mathematical representation

of the behavior of the diode

• There are three diode models used:

1. Ideal Diode Model

• Used for heavily doped diodes; for industrial semiconductor devices

2. Practical Diode Model

• For laboratory experiments where you do not care of sensitive information

3. Actual Diode Model

• For designing and manufacturing of diodes; for research purposes