Pn Junction Diode :
Construction
It is a two-terminal device consisting of a P-N junction formed either in Ge or Si crystal. Its circuit symbol is shown in Fig. The P-and N-type regions are referred to as anode and cathode respectively. In Fig arrow head 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. Standard notation consists of type numbers preceded by
‘IN’ such as IN 240 and IN 1250. Here, 240 and 1250 correspond to colour bands. Fig.shows typical
diodes having a variety of physical structures whereas Fig. illustrates terminal identifications. Also refer to the picture of two commercial diodes shown in Fig
The low-current diodes whose body is about 3 mm long can carry a forward current of about 100 mA, have saturation current of 5 µA at room temperature (25ºC) and can withstand a reverse voltage of 75 V without breaking down. The medium-current diodes can pass a forward current of about 500 mA and can withstand a reverse voltage of 250 V. The high-current diodes or power diodes can pass a forward current of many amperes and can survive several hundred volts of reverse voltage.
Working :
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.
Forward Characteristics :
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 isbfound 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.
Reverse Characteristics :
When the diode is reverse-biased, majority oarriers 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 Fig. 52.4, when reverse voltage exceeds a certain value called break-down voltage (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. When P-N junction diodes are employed primarily because of this breakdown property as voltage
regulators, they are called Zener diodes
VI Characteristics :
These characteristics can be described by the analytical equation called Boltzmann diode equation
given below :
I = I0 ( e^(eV/kT)−1) ampere
where I0 = diode reverse saturation current
V = voltage across junction − positive for forward bias 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
Hence, the above diode equation becomes
I = I0 (e^(eV/kT )− 1). – for germanium
I = I0 (e^(eV/2kT) − 1) – for silicon
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Construction
It is a two-terminal device consisting of a P-N junction formed either in Ge or Si crystal. Its circuit symbol is shown in Fig. The P-and N-type regions are referred to as anode and cathode respectively. In Fig arrow head 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. Standard notation consists of type numbers preceded by
‘IN’ such as IN 240 and IN 1250. Here, 240 and 1250 correspond to colour bands. Fig.shows typical
diodes having a variety of physical structures whereas Fig. illustrates terminal identifications. Also refer to the picture of two commercial diodes shown in Fig
Working :
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.
Forward Characteristics :
certain safe limit.
Reverse Characteristics :
When the diode is reverse-biased, majority oarriers 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 Fig. 52.4, when reverse voltage exceeds a certain value called break-down voltage (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. When P-N junction diodes are employed primarily because of this breakdown property as voltage
regulators, they are called Zener diodes
VI Characteristics :
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
These characteristics can be described by the analytical equation called Boltzmann diode equation
given below :
I = I0 ( e^(eV/kT)−1) ampere
where I0 = diode reverse saturation current
V = voltage across junction − positive for forward bias 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
Hence, the above diode equation becomes
I = I0 (e^(eV/kT )− 1). – for germanium
I = I0 (e^(eV/2kT) − 1) – for silicon
Comment Below If You Have Regarding This Topic
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