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EE 331 Devices and circuits ISolid-State Electronics I

Lecture #15

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BIAS EFFECT ON THE ELECTROSTATICSREGION

Under reverse bias:

• When a reverse bias, , is applied to the pn junction:

• The effective potential across the junction increases, from the

equation of potential across the junction

 =

2   +

2  

That means the values of  and  increase, which results

larger magnitude of electric field and total amount of space

charges.

  =

  =

 

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P-N JUNCTION UNDER BIAS

Under reverse bias :

• Drift force dominates diffusion force.

• A small reverse current caused by drift arising in the SCL

where EHPs are thermally generated.

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SUMMARY

The electrostatics region (depletion region) of a pn junction:

Without biasForward

biasReverse bias

Space charge   =    Reduce Increase

Electric field   =

  =

  Reduce Increase

Potential  =

2  +

2  Reduce Increase

Depletion width   Reduce Increase

Net current ZeroDiffusion

current

Drift current +

diffusion current

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Diodes under reverse bias (3.6)

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REVERSE BIAS/BREAKDOWN

Increased reverse bias

eventually results in the

diode entering the

breakdown region, resulting

in a sharp increase in the

diode current. The voltage at

which this occurs is the

breakdown voltage, .

2 V ≤  ≤ 2000 V 

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REVERSE BIAS/BREAKDOWN

External reverse bias adds to the built-in potential of the pn

 junction. The shaded regions below illustrate the increase in the

characteristics of the space charge region due to an externally

applied reverse bias, .

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REVERSE BIAS/BREAKDOWN

While increasing reverse bias, both  and  also increase, and

induces higher electric field as well as the voltage across pn-

 junction . The higher electric field would accelerate minority

charged carriers to high velocity in depletion layer

velocity =  

The energy of the charge particles described as

energy =1

2 velocity    

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REVERSE BIAS/BREAKDOWN

Avalanche Breakdown

Si diodes with high reverse bias would breakdown according to

an avalanche mechanism. As the electric field increases, some

carriers gain sufficient energy to ionize atoms upon impact and

release more free charged carriers. The new carriers also

accelerate and ionize other atoms. This process feeds on itself

and leads to avalanche breakdown.

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REVERSE BIAS/BREAKDOWN

Zener Breakdown 

Zener breakdown occurs in

heavily doped diodes. The heavy

doping results a very narrow

depletion region at the diode

 junction. Reverse bias leads to

carriers with sufficient energy to

tunnel directly betweenconduction and valence bands

moving across the junction. Once

the tunneling threshold is

reached, additional reverse bias

leads to a rapidly increasing

reverse current.