PN Junction Section

Tentative Schedule #DateDayTopicSection 1 1/14TuesdayDiagnostic Test L 1/14Tuesday Lab protocol, cleaning procedure, Linus/Cadence intro 2 1/16Thursday Fundamental concepts from Electric Circuits 31/21TuesdayBasic device physics2.1 L1/21Tuesday I-V characteristics of a diode (Simulation) 4 1/23Thursday Drift/Diffusion current 5 1/28TuesdayPhysics of PN junction diode L 1/28TuesdayDiode logic circuit 6 1/30Thursday Applications of diodes: diode logic/Review 7 2/4TuesdayTest #1 L 2/4Tuesday Diode Logic 82/6ThursdayClass Canceled!

Important Dates 2/4: Test #1 2/6: Class Canceled!

Review Intrinsic Semiconductor Extrinsic Semiconductor Currents Drift Current Diffusion Current

At T=0KElectrons gain thermal energy and break away from the bonds. They begin to act as “free charge carriers”— free electron. Not A Whole Lot of Free Electrons at Room Temperature

Phosphorus has 5 valence electrons. The 5 th electron is “unattached”. This electron is free to move and serves as a charge carrier. Add Phosphorous to Silicon to Create an silicon

if we dope silicon with an atom that provides an insufficient number of electrons, then we may obtain many incomplete covalent bonds. A boron has only 3 valence electrons and can form only 3 covalent bonds. Therefore, it contains a hole and is ready to absorb a free electron. Add Boron to Silicon to Create a p-type Silicon

Two Ways to Produce Currents Mechanism: Electric FieldMechanism: Concentration Gradient

Drift Current Drift current is composed of the drift current due to holes and the drift current due to electrons. Drift current is caused by the presence of an electric field.

if charge carriers are “dropped” (injected) into a semiconductor so as to create a nonuniform density. Even in the absence of an electric field, the carriers move toward regions of low concentration, thereby carrying an electric current so long as the nonuniformity is sustained.

Diffusion current due to Holes Where does the – sign come from?

Diffusion Current Due to Electron

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

I S =Reverse Saturation=leakage current

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. Creation of Depletion Region

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. PN Junction Without Bias Voltage

Electric Field/Voltage Definition of Voltage: The work done in moving a unit positive charge in an electric field. Alternative definition: - + Vo Caution: You can’t use Vo as a battery!!!

Net charge =0 E depends on the net charge included 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=n i 2 Diffusion 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

PN Junction under Reverse Bias Reverse: Connect the + terminal to the n side. 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. 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.

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. (gradient of minority carriers)

In the vincinity of depletion region, the current consists mostly of minority carriers because you have the gradient! 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. (This is the bottom line) I D must be constant at all points along x

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) uA mA mA mA mA mA mA

Measured Diode Voltage

Reverse Biased Diode I S =Reverse Saturation=leakage current

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