5.2 Sspd_chapter 3_sec3.2.4. reverse biaseddiode,3.2.5.real diodes  (Page 3/3)

If we assume that diode is on then ‘1’ can be neglected and equation is expressed as:

This Equation (3.2.5.3) tells us that for every 60mV increase in Diode Voltage increases the Diode Current by 10 times that is by a decade under moderate current where η=1.

For low currents and for high currents where η=2, current increase by a decade for 129mV increase in Diode Voltage.

For Ideal Diode the semi-log plot of Ideal Diode Equation gives 60mV/decade but for Real Diode we get 120mV per decade slope at currents less than 0.5mA, 60mV/decade slope for intermediate currents 0.5mA to 5mA and again 120mV/decade slope for currents in the range of 5mA and above. Semi-log plot of a Real-Diode is shown in Figure 3.15.

3.2.6. Avalanche Breakdown .

Under reverse bias condition when E _max exceeds E _crit (constant at 3×10 ⁵ V/cm) also known as Break-down field of Silicon Diode under low doping condition then Avalanche Breakdown takes place and the diode can be irreversibly damaged if there is no limiting resistance in the circuit.

As Reverse Bias Voltage starts increasing the reverse leakage current starts multiplying by an Avalanche Mutiplication Factor ‘M’ where M is given as follows:

The reverse leakage current is given by the following expression:

As the reverse voltage approaches BV _A (Avalanche Break-Down Voltage), the drifting minority carriers get accelerated to a point where they break the co-valent bond and EHPs are produced by impact collision and ionization.Soon this becomes a runaway process and current rapidly builds up only to be limitesd by the circuit loop resistance.

For one sided step junction (P ⁺ N Junxction Diode), depletion layer extends into the lightly doped side and electric field is given by:

Therefore Avalanche Breakdown Voltage:

The corresponding depletion layer is: d _n = 1.9μm.

This picture holds true only for lightly doped diodes. At higher doping Quantum Mechanical Tunneling comes into picture.

3.2.7.Hyperabrupt Junction Diodes to synthesize Varactor Diode.

We saw in the fabrication section that doping profile can be complimentary error function from a short period constant source diffusion and Gaussian Distribution for a long period limited source –diffusion.In general the doping profile is the following:

For this generalized doping we get a generalized power function dependence of Junction Capacitance. The junction Capacitance of a diode is given by the same formula as the parallel plate Capacitor:

By detailed analysis it has been shown that :

Using Equation (3.2.7.1) and (3.2.7.2), abrupt junction diode, linealrly graded junction diode and hyper-abrupt-junction diode can be designed. Table 3.2.7.1 gives the Junction Capacitance Power Law.

Table 3.2.7.1. Junction Capacitance Power Law for generalized doping.

The doping profile of the three kinds of the junctions are given in Figure 3.16.