September 1965 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Some things never change,
and one of them is addressed in this 1965 Electronics World magazine article
entitled, "Understanding the Transistor Data Sheet." Since that time a lot of new
parameters have been added as physical models for precise simulations evolve, but
terms like alpha (collector-emitter current gain), beta (collector-base current
gain, aka hfe), hfT (transit frequency - unity gain point),
etc., have remained the same over many decades. When you compare the fundamental
simple transistor model schematics, they look like ones you would find in a modern
textbook. Accordingly, you can read and remember this article on interpreting transistor
datasheets knowing that it is as useful today as it was in 1965.
Understanding the Transistor Data Sheet
Understanding the Transistor Data Sheet
More than a mere glossary of terms, this article discusses and explains the commonly
used symbols and terms found on data sheets for specifying junction transistors,
FET's, as well as for the junction, tunnel, and zener diodes.
Manufacturers' data sheets are normally consulted when choosing a semiconductor
device for a given application.
On top of the data sheet, alongside the identifying number, will be found a summary
of the general characteristics of the device. The statement includes information
as to device material, e.g., silicon or germanium; type: n-p-n or p-n-p; construction,
e.g., planar; intended service: power, small signal, etc.; operating temperature
range; and a few electrical characteristics such as current gain may be cited.
With information on the general capabilities of the device, the user can then
examine the detailed specs. These will fall into two broad categories: absolute
maximum ratings and electrical parameters. For each value specified it is essential
that the corresponding test conditions be clearly stated on the data sheet in order
to be of real value.
The most commonly encountered specs for junction transistors will be considered
Table 1 - Summary of the h-parameters for three basic transistor
BVCBO is the maximum d.c. reverse (breakdown) voltage that can be
applied between collector and base with the emitter open-circuited.
BVCEO is the maximum d.c. reverse (breakdown) voltage that can be
applied between collector and emitter with the base open-circuited.
BVCES is similar to BVCEO, except that the emitter is connected
to the base.
BVCEX is the collector-emitter breakdown voltage for a reverse-biased
base-emitter junction under specified circuit conditions.
VCE(sat) is the collector saturation voltage when both the emitter-base
and -collector-base junctions are forward biased. This spec has significance for
a transistor operated as a saturated switch.
VRT is the reach-through (or punch-through) voltage. At this voltage,
depletion regions of the emitter-base and collector-base junctions touch each other
and breakdown occurs in the device.
ICBO, ICO is the reverse d.c. saturation collector current
with the emitter open-circuited.
ICEO is the reverse d.c. saturation collector current with the base
ICES is similar to ICEO except the base is connected to
ICEX is the d.c. collector current for a specified base emitter connection.
IEBO, IEO is the d.c. emitter current with the emitter-base
junction reverse-biased and the collector open-circuited. IECS is similar
to IEBO except the collector is connected to the base.
Table 2 - Summary of h- and r-parameter relationships. (It is
assumed that effective collector load is under 5000 ohms.)
Power dissipation is defined as the product of the average operating current
and voltage. For class-A operation, the total power dissipation is equal to the
product of the d.c. quiescent collector voltage and current (collector circuit dissipation)
plus the product of the d.c. quiescent base voltage and current (base circuit dissipation).
Generally, the base circuit dissipation can be neglected and the total taken as
the collector dissipation, Pc. For class-A operation, maximum collector
dissipation occurs when the amplifier is idling. Maximum dissipation for a class-B
push-pull amplifier is slightly more than 20% of the power output.
Device dissipation ratings may be given for free air or case temperature. Case
temperature usually infers that the device is mounted on an infinite heat sink.
In addition, a derating curve (see Fig. 1A) may be included on the data sheet. This
curve tells the user what the maximum allowable dissipation is for a given ambient
Power transistors normally have to be mounted on a heat sink to insure safe operation.
To aid in calculating the required sink size for a given collector dissipation,
a thermal equivalent circuit, as shown in Fig. 1B, may be used. The collector dissipation
Pc, is represented by a current source while thermal loss is indicated
by a thermal resistance, θ. One can write an equation for the thermal circuit
which is analogous to Ohm's Law for an electric circuit: Pc = (Tj
- Ta) / (θjc+θcs+θsa).
For example, let the maximum junction temperature Tj(max) = 100°C,
the ambient temperature Ta = 25°C, and Pc= 10 watts. If
θjc = 0.2°C/w., and θcs = 0.8°C/w., thermal resistance
of the heat sink θsa cannot exceed: θsa = (100 - 25) /10 -
0.2 - 0.8 = 6.5°C/w.
A typical curve relating heat-sink size and θsa is given in Fig. 2A.
For the above example, a heat sink measuring 4" x 4" X 1/4" is required.
For small-signal operation at low frequencies, the transistor may be represented
by the hybrid (h) model shown in Fig. 2B. When the output is short-circuited (V2
= 0), the short-circuit h-parameters are defined:
hi = v1/i1: input resistance (ohms).
hf = i2/i1: forward current gain (dimensionless).
When the input is open-circuited (i1 = 0), the open-circuit parameters
hr = v1/v2: reverse transfer voltage ratio (dimensionless).
ho = i2/v2: output impedance (mhos). A second
subscript is added to the 11- parameters to denote the transistor configuration
being used. For example, hie is the input resistance for the transistor
in the common-emitter (CE) configuration; hfe the forward-current gain
in the CE connection and often referred to as beta (β); hfb, forward
current gain for the transistor in the common-base (CB) configuration, and is also
called alpha (α); and hoc, the output admittance of a transistor
connected in the common-collector (CC) or emitter-follower mode. Table 1 summarizes
Another model for small-signal transistors operated at low frequencies is the
r-T network of Figs. 3A and 3B. The parameters are defined as:
rb = base resistance (ohms).
re = emitter resistance (ohms).
rc = collector resistance (ohms).
β = forward current gain (dimensionless).
Some relations among the r-parameters are:
rd = rc(1 - α) ( ohms)
rm = αrc (ohms).
β = α/(1-α).
Table 2 provides equations relating the h- and r-parameters.
The small-signal parameters are determined with an a.c. signal (usually 1000
cps) applied to the transistor under test. If d.c. or static measurements are made,
the subscripts used are capital letters. For example, hFE = static (d.c.)
forward current gain for the transistor in the common-emitter configuration.
With regard to high-frequency operation of junction transistors, there are a
variety of models from which to choose.
One particular circuit, the hybrid-pi of Fig. 3C is perhaps the most often used
model. The parameters are:
rb' = base-spreading resistance (ohms). This represents the bulk resistance
in the base region of the transistor. gb'e =emitter conductance (mhos).
gcb' = collector-base conductance (mhos). This quantity can usually
be neglected when making calculations.
gce = collector-emitter conductance (mhos) .
gm = transconductance (mhos)
Cb'e=emitter-base diffusion capacitance (picofarads).
Ccb' = collector-base transition capacitance (picofarads).
The upper cut-off frequency denotes the frequency at which the transistor gain
falls -3 db from its mid-frequency value (usually taken as 1000 cps). For the transistor
in the CB mode, the sy-bol used for the cut-off frequency is ƒhfb
or ƒα for the CE connection, ƒhfe or ƒβ.
Other parameters used are ƒT, which is the frequency at which
the CE current gain is unity (or 0 db). Parameter ƒT is also referred
to as the gain-bandwidth-product of the transistor. Generally, ƒα>ƒT>ƒβ.
Another frequently listed spec is the maximum frequency, ƒmax or ƒosc,
at which a transistor can oscillate.
Fig. 1. (A) Typical derating curve. (B) Thermal equivalent circuit.
For a transistor operating as a switch, such as used in digital computers, the
turn-on and turn-off times of the transistor are significant quantities to consider.
Fig. 4A shows a rectangular current input pulse to a CE-connected transistor. The
output pulse, disregarding phase inversion of the transistor, would appear as shown
in Fig. 4B. The delay-time param-eters are defined as:
td = delay time: The time required for the output current to reach
10% of its maximum value.
tr = rise time: The time required for the output current to rise from
10% to 90% of its maximum value.
ts = storage time: The time required for the output current to decrease
from its maximum to 90% of maximum value when the transistor is turned off.
tf =fall time: The time required for the output current to fall from
90% to 10% of its maximum value.
Turn-on time = td+tr; turn-off
time = ts+tƒ.
Occasionally, the manufacturer will include the charge-control parameters in
specifying a switching transistor. These parameters are QT, the total
control charge in the base, and constant SΤR, ΤS,
and ΤF, which refer to rise, storage, and fall time constants, respectively.
Using these parameters, the expected delay times just defined can be calculated.
Fig. 2. (A) Thermal impedance curve of heat sink. (B) The small-signal, low-frequency
hybrid model of a transistor.
Fig. 3. (A) Small-signal, low-frequency r-T models of a transistor in common-emitter
configuration with current source and (B) with voltage source. (C) Hybrid-pi model
for a transistor that is operating in common-emitter circuit configuration.
Fig, 4. (A) Rectangular switching waveform of length T applied to transistor.
(B) Output waveform. (C) Simple model of a field-effect transistor. (D) Tunnel-diode
voltage-current characteristics. (E) Tunnel-diode model.
Some important d.c. ratings used to specify the field-effect transistor (FET)
Vp is the gate-source pinch-off voltage. This is the voltage at which
the channel becomes pinched off and there is no further increase in drain current
for increasing drain voltage.
BVDGO is the drain-gate breakdown voltage with the source open.
BVDSS is the drain-source breakdown voltage with gate connected to
IDSS is the drain current for zero gate voltage.
IGSS is the gate cut-off current deter-mined by connecting the drain
to the source and reverse biasing the gate channel circuit.
A simple small-signal model of an FET is given in Fig. 4C. The parameters are:
gm is the small-signal common-source transconductance.
Cgs is the gate-source capacitance.
Cdg is the drain-gate capacitance.
Cds is the drain-source capacitance.
The v-i characteristics and a small-signal model of a tunnel diode are shown
in Figs. 40 and 4E. The important parameters are:
VP and VV are the peak and valley point voltages, respectively.
IP and IV are the peak and valley point currents, respectively.
VFP is the forward point voltage corresponding to peak current.
IP/IV is the peak-to-valley ratio.
-gd is the negative conductance (mhos) .
C is the total capacitance (picofarads) .
LS is the series inductance (micro-henrys).
RS is series resistance (ohms).
Junction and Zener Diodes
Commonly used parameters for describing junction and zener diodes are:
BVR is the d.c. breakdown voltage for a reverse-biased diode.
VF is the forward voltage drop.
VZ is the nominal zener voltage.
IF is the average forward current.
IZ is the zener current.
Cdep is the depletion or transition capacitance of a reverse-biased
Cdif is the diffusion capacitance of a forward-biased diode.
Two organizations concerned with the standardization of testing procedures and
symbols for semiconductor devices are the Institute of Electronic and Electrical
Engineers and the Electronic Industries Association (JEDEC). It appears that their
efforts in this area have been partially successful.
Posted July 7, 2022