Explain the construction and V/L characteristics of diode.

Explain the construction and V/L characteristics of diode.

Ans.  Diode is the most basic of semiconductor devices. It is created when a metal and a semiconductor or two semiconductors form a junction. When two diffusions of opposite polarity form a junction, a junction diode is formed. When a metal and semiconductor merge either an ohmic contact is made or a Schottky diode is generated. Only ohmic contacts are formed in most CMOS processes, where metal contacts diffusions. In an nMOS ( or pMOS) transistor, the source and drain terminals form np (or pn) junction diodes to the substrate (or well). Fig 2.18(a) shows the schematic symbol for a junction diode. The two terminals are designated the anode and cathode. Fig. 2.18(b) shows the V/I characteristics of a diode. In a diode, the current is expressed as –

I = AdIS   M          …….(i)

Where,    K = Boltzmann’s constant

M= A constant between 1 and 2 to account for various nonlinearities

T = Temperature,     q = Electronic charge

Ad = Area of the diode,   Is = Saturation current/unit area.

2.18

In a diode, a number of characteristics are of interest. If a positive voltage is given to the cathode with respect to the anode, then electrons are attracted to the supply and holes are repelled, leading to a “reverse-biased” condition in which a very small reverse current flows, which results in a depletion region similar to that in the MOS transistor if it is in the depletion region before inversion. In the equation (i), the exponential term is reduced in importance, and the current is approximated as (Ad = 1) –

Ireverse = -Is ( ~1 x 10-15A)                                        ………..(ii)

This condition applies until voltage exceeds the reverse breakdown voltage of the junction, at which point the current increases quickly owing to avalanche multiplication. This takes place when electrons accelerated by the high field across the junction impact silicon atoms, thereby generating electron hole pairs. If a negative voltage is given to the cathode, then diode becomes forward biased. The current is approximated as (Ad = 1 ) –

Iforward = IseM

The current rapidly increases when the cathode-anode voltage is less than -0.6V as shown in fig. 2.18(b). The x-axis is reflected.

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