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

Different ways of Crossing PN Junction np=n i 2 Diffusion 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. Majority carrier to cross the PN junction easily through diffusion. 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. 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 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

EquilibriumForward Biased Diode Majority carriers cross the junction via diffusion. Minority carriers increased on both sides of the junction.

n n,f enters the p side as minority carriers (n p,f ). n p,f will recombine with the p p,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 I tot.

I S =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) IF (Computed) uA mA mA mA mA mA mA

Measured Diode Voltage

On Semilog Plot

Dynamic Resistance VF (V) IF (Computed) mA mA Dynamic Resistance from the measurement: ( )/(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 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 MOhms. % Error is about -0.3 %

Reverse Bias VS (Measured) IR (Computed) 53 nA 103 nA 153 nA