EE 214 Electronic Circuits Laboratory
Spring 2015
BIPOLAR JUNCTION TRANSISTORS (BJTs)
A transistor is a three terminal semiconductor device that is used to amplify and switch electronic
signals and electrical power. The third terminal serves as a control point to manipulate the current flow though
the device. Detailed physics of internal structure and how transistors work internally is out of scope and will
be covered mainly in Semiconductor Devices and Modelling course. This document will explain how to use
these devices from a circuit standpoint, which is not hard at all.
Transistors may be grouped into two main categories according to their structure: Field-Effect
Transistors (FETs) and Bipolar Junction Transistors (BJTs). Since both the eighth and ninth experiments will be
related to them, operating characteristics of BJTs will be covered here.
BJTs, which were the first type of transistor to be widely commercialized, is a combination of two
junction diodes. They are formed of either a thin layer of n-type semiconductor placed between two p-type
semiconductors, which are called as PNP BJTs, or a thin layer of p-type semiconductor placed between two ntype semiconductors, which are called as NPN BJTs. (n-type semiconductors are the ones in which the majority
charge carriers are electrons whereas the majority charge carriers are holes in p-type semiconductors). Figure
1 shows the schematic representation for both NPN and PNP BJTs, where the arrow in the symbol s indicates
the direction of current flow when the devices are biased in the forward active region. Note that the three
layers of semiconductor introduces three terminals: an emitter, a base and a collector. Two junctions emerging
from the construction are base-emitter and base-collector junctions, and the base is the common region
where the minority carriers can move through.
Collector
Base
Collector
Base
Emitter
Emitter
(a)
(b)
Figure 1: Circuit symbols for (a) PNP and (b) NPN BJTs
The control terminal of BJTs is the base and the collector and emitter currents, 𝐼𝐶 and 𝐼𝐸 respectively,
are well-modeled by the Ebers-Moll model, which is illustrated in Figure 2b for an NPN BJT.
IE
E
VBE
+
IC
IB
B
C
-
VBC
+
(a)
(b)
Figure 2: (a) An NPN transistor and corresponding (b) Ebers-Moll model
EE 214 Electronic Circuits Laboratory
Spring 2015
𝑉 𝐵𝐸
𝑉 𝐵𝐶
𝑖𝐶 = 𝐼𝑆 (𝑒 𝑉𝑇 − 𝑒 𝑉𝑇 ) −
𝑖𝐵 =
𝐼𝑆
𝛽𝐹
(𝑒
𝑉 𝐵𝐸
𝑉𝑇
𝑉 𝐵𝐸
− 1) +
𝑉 𝐵𝐶
𝐼𝑆
𝛽𝑅
𝑖𝐸 = 𝐼𝑆 (𝑒 𝑉𝑇 − 𝑒 𝑉𝑇 ) −
𝐼𝑆
𝛽𝑅
𝑉 𝐵𝐶
(𝑒 𝑉 𝑇 − 1)
(1)
𝑉 𝐵𝐶
(𝑒 𝑉𝑇 − 1)
𝐼𝑆
𝛽𝐹
(2)
𝑉 𝐵𝐸
(𝑒 𝑉 𝑇 − 1)
(3)
Equations 1-3 describe the collector current 𝑖𝐶, base current 𝑖𝐵 and emitter current 𝑖𝐸 in any operating
region where
-
𝑉𝐵𝐸 : base-emitter voltage
𝑉𝐵𝐶 : base-collector voltage
𝑉𝑇 : thermal voltage (≈25 mV at room temperature)
𝛽𝐹 : forward common emitter current gain (usually between 100-200)
𝛽𝑅 : reverse common emitter current gain (0-20)
Bipolar transistors have distinct regions of operation defined by the junction biases, which is
summarized in Table 1 for a NPN transistor.
Table 1: Modes of a NPN BJT according to junction biases
Base-Emitter Junction Base-Collector Junction
Mode
Forward
Reverse
Forward-active
Forward
Forward
Saturation
Reverse
Reverse
Cut-off
Reverse
Forward
Reverse-active
In forward active region, the collector-emitter current is approximately proportional and many times
larger than the base current for small variations. The equivalent circuit can be simplified for forward active
region for DC calculation as represented in Figure 3 and simplified relations for DC emitter and collector
currents are given in Equations 4-6, where 𝛼𝐹 stands for the common base forward short circuit gain which is
very close to unity and 𝐼𝐸𝑆 is the reverse saturation current of the base-emitter diode. 𝐼𝐸𝐷 and 𝐼𝐶𝐷 in Figures
2-3 are emitter and collector diode currents, respectively, and they are expressed in terms of saturation
currents via diode 𝐼 − 𝑉 relation, which is given in Equation 4 for base-emitter junction.
Figure 3: Simplified Ebers-Moll model for an NPN transistor in the forward active mode
EE 214 Electronic Circuits Laboratory
Spring 2015
𝑉 𝐵𝐸
𝐼𝐸 = 𝐼𝐸𝑆 (𝑒 𝑉𝑇 − 1)
(4)
𝐼𝐶 = 𝛼𝐹 𝐼𝐸
(5)
𝐼𝐵 = (1 − 𝛼𝐹 )𝐼𝐸
(6)
The relation between 𝛼𝐹 and 𝛽𝐹 is given in equation 7.
𝛽𝐹 =
𝛼𝐹
1−𝛼𝐹
⇔
𝛼𝐹 =
𝛽𝐹
𝛽𝐹 +1
(7)
In saturation region, where both junctions are forward-biased, BJT facilitates high current conduction
from the emitter to the collector and 𝐼𝐶 is independent of 𝐼𝐵 and 𝛽𝐹. Collector-emitter voltage 𝑉𝐶𝐸 is less than
base-emitter voltage 𝑉𝐵𝐸 .
In cut-off region, both junctions are reverse biased and the emitter current 𝐼𝐸 is almost zero, therefore,
𝐼𝐶 is also zero.
The emitter and collector regions switch roles when a BJT is in reverse active region. Since these
transistors are designed to maximize the current gain in forward-active mode, the current gain in reverse
active mode is several times smaller compared to the forward active mode and hence, this mode is seldom
used.
𝑰 − 𝑽 Characteristics
Biasing the BJT typically involves choosing the desired collector current 𝐼𝐶 and collector-emitter
voltage 𝑉𝐶𝐸 , determining what base-current is required to achieve these values and building a circuit that is
consistent with these parameters. Hence, the most important characteristic of BJT is the plot of the collector
current 𝐼𝐶 and collector-emitter voltage 𝑉𝐶𝐸 . Figure 4 shows a common-emitter bias configuration, where base
serves as the input, collector is output and emitter is common to both, and corresponding 𝐼 − 𝑉
characteristics.
(a)
(b)
Figure 4: (a) Common-emitter bias configuration adn (b) corresponding I-V characteristics for a NPN BJT
Each family of curves in Figure 4 is drawn for a different-base current. Note that the collector current
is relatively constant in the forward-active region and the positive slope in this region is due to Early Effect,
which is usually ignored for DC calculations. The boundary between the forward-active and saturation regions
occurs when 𝑉𝐶𝐸 = 𝑉𝐶𝐸,𝑠𝑎𝑡, where 𝑉𝐶𝐸,𝑠𝑎𝑡 is in the range of 0.1 V to 0.3 V.
EE 214 Electronic Circuits Laboratory
Spring 2015
Basic Applications
Transistors are mainly used as amplifiers since they are able to produce a stronger output signal
proportional to a weaker input signal, or as electrically-controlled switches.
Switch:
Amplifier:
Bonus:
For those who are interested, here is the link which shows the 22nm manufacturing technology with
3D transistors. You can also watch this video to see how a CPU is made of billions of transistors.