pn junction when p – and n – type semiconductors of same material either Si or both Ge are joined to form a homojunction such a pn junction is formed.
Depletion layers close to the junction a layer exists devious of carriers due to the migration of carriers to the opposite sides the electrons migrate from n – side taken up by acceptor impurity atoms near the junction and become negatively ionized. Similarly donor impurity atoms close to the pn junction on n – side get positively ionized and hence create a potential barrier or factious battery.
Forward biasing if positive terminal of an external battery is connected to p – type and negative terminal to n- type width decreases.
Reverse biasing if positive terminal of the external battery is connected to n – type and negative terminal to p – type. The potential barrier increases. The deletion layer width increases and current is due to minority carrier.
Note that when forward biased by a voltage greater then, barrier potential, a large current flows and when reverse biased a very small current flows. Hence pn junction acts very closely like a value
Equation of current in pn junction
I = Is [ev/vt-]
Where IS reverse starvation current, V = applied potential VT = thermal voltage
VT = KT/e = 0.026V at 300 K.
Where k is Boltzmann’s const T is temperature of an electron and e is the charge.
Dynamic or incremental resistance
r = ?V/?I = dV/dI
is called dynamic resistance
From the equation of the diode
dI/dV = Is/VT e v/NT = I/VT
in case diode is forward biased
or r = dV/dI = VT/I if I = I mA then rf = 26 O (low).
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