Introduction to systems - Continuous and Discrete Time Signals and Systems - page 65

Fig. 2.3. Electrical circuit
consisting of three passive
components: resistor R ,
capacitor C , and inductor L . The
RLC circuit is an example of a CT
linear system.

R

node 1

i R ( t )

+
−

−

C

v ( t ) L y ( t )

i L ( t ) i C ( t )

2.1.2 Semiconductor diode

When a piece of an intrinsic semiconductor (silicon or germanium) is doped
such that half of the piece is of n type while the other half is of p type, a pn
junction is formed. Figure 2.4(a) shows a pn junction with a voltage v applied
across its terminals. The pn junction forms a basic diode, which is fundamental
to the operation of all solid state devices. The symbol for a semiconductor diode
is shown in Fig. 2.4(b). A diode operates under one of the two bias conditions.
It is said to be forward biased when the positive polarity of the voltage source
v is connected to the p region of the diode and the negative polarity of the
voltage source v is connected to the n region. Under the forward bias condition,
the diode allows a relatively strong current i to flow across the pn junction
according to the following relationship:

i = I s [exp( v / V T ) − 1] (2.6)

where I s denotes the reverse saturation current, which for a silicon doped diode
is a constant given by I s

− 15 A, and V T is the voltage equivalent of
the diode's temperature. The voltage equivalent V T is given by

= 4 . 2 10

= kT
e

V T . (2.7)

Fig. 2.4. Semiconductor diode:
(a) pn junction in the forward
bias mode; (b) diode
representing the pn junction
shown in (a); (c) current-voltage
characteristics of a
semiconductor diode.

i

+ v −

+ v −

i

v

i

p n

I s

(a) (b) (c)