Connecting a p-n diode in a circuit A p-n diode (Fig. 25.7) is capable of conducting current if: •
•
it is connected in forward bias (see below) the applied voltage exceeds the junction voltage.
p-type NOTE
A forward bias p-n junction allows conduction of current; a reverse bias p-n junction does not allow conduction of current.
a anode b n-type anode depletion layer conducts cathode n-type
stripe marks cathode
c
FIG. 25.7 Orientation and structure of p-n diode when connecting in a circuit
Forward-biased p-n diode
To connect a p-n diode in forward bias, connect the positive terminal to the p-type and the negative to the n-type (Fig. 25.8). The positive forces holes into the depletion layer and the negative forces electrons into the depletion layer. This eventually eliminates the layer and current flows. It takes the junction voltage to achieve this.
Reverse-biased p-n diode STS
The semiconductor industry is worth billions as every modern piece of technology requires them. Semiconductor devices can be used for: 1. Rectifying alternating current (a.c.): Since diodes only allow current to flow one way, if you put alternating current into the diode, it will eliminate the reverse- biased part and allow only the forward-biased part to flow. This gives a kind of rectified direct current (Fig. 25.11).
280 FUSION
To connect a p-n diode in reverse bias, connect the positive terminal to the n-type and negative terminal to the p-type (Fig. 25.9). The diode will not conduct and the depletion layer widens. Leakage current exists and eventually it will break.
cathode n-type
does not conduct
anode
depletion layer
p-type
FIG. 25.9 In a reverse-biased p-n diode, the positive terminal is connected to the n-type and the negative to the p-type
Continued cathode
FIG. 25.8 In a forward-biased p-n diode, the positive terminal is connected to the p-type and the negative to the n-type
