(a) Construction
It is a two-terminal device consisting of a P-N junction formed either in Ge or Si crystal. The Pand N-type regions are referred to as anode and cathode respectively. In Fig. 4.1.1.15(b), arrowhead indicates the conventional direction of current flow when forward-biased. It is the same direction in which hole flow takes place.
Commercially available diodes usually have some means to indicate which lead is P and which lead is N.
Figure 4.1.1.15
A P-N junction diode is one-way device offering low resistance when forward-biased and behaving almost as an insulator when reverse-biased. Hence, such diodes are mostly used as rectifiers i.e. for converting alternating current into direct current.
4.1.1.5.1 V/I Characteristic
1. Forward Characteristic: when the diode is forward-biased and the applied voltage is increased
from zero, hardly any current flows through the device in the beginning. It is so because the
external voltage is being opposed by the internal barrier voltage VB whose value is 0.7 V for
Si and 0.3 V for Ge. As soon as VB is neutralized, current through the diode increases rapidly
with increasing applied battery voltage. It is found that as little a voltage as 1.0 V produces a
forward current of about 50 mA. A burnout is likely to occur if forward voltage is increased
beyond a certain safe limit.
2. Reverse Characteristic: When the diode is reverse-biased, majority carriers are blocked and
only a small current (due to minority carriers) flows through the diode. As the reverse voltage
is increased from zero, the reverse current very quickly reaches its maximum or saturation
value I0 which is also known as leakage current. It is of the order of nanoamperes (nA) for Si
and microamperes (µA) for Ge. The value of I0 (or Is ) is independent of the applied reverse
voltage but depends on (a) temperature, (b) degree of doping and (c) physical size of the
junction.
As seen from Figure 4.1.1.18, when reverse voltage exceeds a certain value called break-down voltage VBR (or Zener voltage Vz ), the leakage current suddenly and sharply increases, the curve indicating zero resistance at this point. Any further increase in voltage is likely to produce burnout unless protected by a current-limiting resistor.
4.1.1.5.2 Equation of the Static Characteristic
The volt-ampere characteristics described above are called static characteristics because they describe the d.c. behaviour of the diode. The forward and reverse characteristics have been combined into a single diagram of Fig. 4.1.1.18.
Figure 4.1.1.18
These characteristics can be described by the analytical equation called Boltzmann diode equation given below :
where I0 = diode reverse saturation current
V = voltage across junction (positive for forward and negative for reverse bias). k = Boltzmann constant = 1.38 × 10-23 J/ºK
T = crystal temperature in ºK
η = 1 – for germanium = 2 – for silicon
