De-soldering and replacing a transistor takes very little time.  More time is spent figuring out which one to replace and, sometimes, what to replace it with!

The original transistor might need special ordering, or be discontinued and unavailable.  Knowing how to substitute an alternate transistor type can speed up your repair, or make an otherwise impossible repair possible.

Transistors are given registered type numbers by the Joint Electron Device Engineering Council (JEDEC).  The type number is often printed on the transistor, although product manufacturers sometimes have their own part numbers printed instead.  In that case, the product's Service Manual or Parts List might provide you with the transistor type.

If you don't have that particular type, a transistor cross-reference book can sometimes help you to find a compatible part.  But you might already have a suitable substitute in your parts drawers.

We arrange our transistor stock, not by their type number, but by their specifications  This makes it easy to go right to the drawers holding good candidates for substitution.

You can find transistor specs in data sheets available online.  Use your browser to search for the transistor's JEDEC type, or use a search engine like AllDataSheet.com.

The first spec to consider is the Transistor Outline.  This is the only spec you can actually see.

A Transistor Outline number, or TO, refers to a transistor's physical size, shape, and mounting style.  Some TO numbers are shown here:

The transistor outline doesn't usually tell you which of the three transistor leads connects to its Base terminal (B), its Emitter terminal (E), or its Collector terminal (C).  However, the transistor's data sheet will show you that.

If the lead arrangement of a small replacement transistor differs from that of the original, you can sometimes bend the transistor leads across one another to route them into the correct holes in the circuit board.  Just use a bit of spaghetti tubing on the leads to keep them from touching each other.

The "bipolar transistor", technically a "bipolar junction transistor" or BJT, is a common variety of transistor.

"Bipolar" transistors use both electrons and holes (electron vacancies) as charge carriers.

The following electrical specs are important when choosing a substitute BJT.

Polarity

"Junction" transistors exploit the behavior of junctions between "N-type" and "P-type" semiconductors.  N-type semis contain excess electrons while P-type semis contain excess holes.

"NPN" transistors sandwich a P region between two N regions, creating two junctions.  Similarly, "PNP" transistors sandwich an N region between two P regions.

The central region is called the base (symbol B).  A small current between the emitter (symbol E) and the base controls a larger current between the emitter and the collector (symbol C).

A substitute transistor must have the same polarity (NPN or PNP) as the original.  If it has the wrong polarity, it won't work properly in it's circuit.

Maximum Voltages

If more than a maximum voltage rating is applied to a transistor, it can be permanently damaged.  At the maximum voltage, also called a breakdown voltage (BV), electrons begin to avalanche in the transistor.

During an avalanche, electrons in the P-N transition regions are accelerated to energies so high that they hit bound electrons with enough force to free them, creating additional charge carriers and greatly multiplying the transistor current.

There are three breakdown voltages:

• V_CB - the maximum voltage across the Collector-Base terminals
  
  
• V_CE - the maximum voltage across the Collector-Emitter terminals
  
  
• V_EB - the maximum voltage across the Emitter-Base terminals

In each of these ratings, the 3rd terminal is assumed to be electrically Open (unconnected).  V_CE, for example, may be written as V_CEO, BV_CEO, or most correctly as V_(BR)CEO.

The V_EB rating isn't usually a factor in choosing a substitute transistor.

V_CB is always equal to or greater than V_CE and you can use either of these maximum voltages to compare transistors.  Choose a substitute transistor with a breakdown voltage rating at least as high as the original.

Maximum Current

Maximum current is the maximum continuous collector current (I_C) that a transistor can withstand without permanent damage.

Small, TO-92 or TO-98 transistors, depending on their fabrication, can handle between about 100 and 1000 mA.  A TO-5 package might be rated as high as 5 amps; a TO-220, as high as 25 A; and a TO-3, up to 500 A.

Be sure to choose a substitute transistor with a maximum current rating at least as high as the original.

Maximum Power

Maximum Power, called P_D, is the overall power a transistor can dissipate, through heat, without burning up.

Heat sinks and fans increase the ability of a transistor to dissipate heat.  A TO-5 transistor with a P_D of 3 watts might be able to dissipate 8-10 watts with a heat sink.

Choose a substitute transistor with a maximum power rating at least as high as the original.

Current Gain

Current gain is only occasionally significant when choosing a substitute transistor.  Actual circuit gain depends on other components.  But if the original transistor has a high gain, try to match it.

Current gain falls off at higher frequencies, so a high-gain transistor can deliver a wider frequency response than a low-gain transistor.

One measure of gain, called h_FE, is often used for comparing transistors.  The capital FE subscript refers to the Forward DC current transfer ratio in a common Emitter circuit.  In other words, I_C / I_B.

Data sheets often specify a minimum or typical value of h_FE, or else a range of values that applies at a certain collector current (I_C).

"Darlington" transistors are made up of two transistors in series and have gains in the thousands, instead of the tens or hundreds.  They also have double the input voltage drop since there are two semiconductor junctions in series.

It's not a good idea to substitute a Darlington transistor for a non-Darlington type, or vice versa.

Field-effect transistors (FETs) use an electric field to control charge flow through a conduction channel.  As with BJTs, the gate (symbol G) is the terminal that controls the transistor current.

However, in a FET, the controlled current flows between the transistor's source (symbol S) and its drain (symbol D).

FETs are unipolar, not bipolar, using either electrons or holes as charge carriers, but not both.

FETs are built into the same transistor outlines as BJTs  but their electrical specs differ:

FET Types

Field-effect transistors come in three basic types:

• Type A - the Junction-gate FET (JFET)
  
  
• Type B - the Insulated-gate FET (IGFET) in
  "depletion mode"
  
  
• Type C - the Insulated-gate FET (IGFET) in
  "enhancement mode"

IGFETs are commonly called MOSFETs (Metal-Oxide-Semiconductor FETs) because, originally, all insulated-gate FETs had metal gates coated with oxide insulation.

FET acronyms now abound but they only describe how a FET is constructed or improved; they don't define a new basic type.

Each type of FET can be fabricated with either an N-type or a P-type conduction channel.  So, overall, there are six types of field-effect transistors.  Following are their circuit symbols:

Be sure to substitute a FET of the same type as the original.

Maximum Voltages

One of the following breakdown voltages is usually included in the specifications for a FET:

• BV_GSS - the breakdown voltage between the Gate and the Source terminals when the drain is short-circuited to the source.  (This rating is used primarily with JFETs.)
  
  
• BV_DSS - the breakdown voltage between the Drain and the Source terminals when the gate is short-circuited to the source.  (This rating is used primarily with Power MOSFETs.)

A breakdown voltage can also be written as V_(BR)GSS or simply V_GSS.

You can use either of the above voltages when comparing FETs.  Just make sure to compare apples with apples.  Pick a substitute FET with a rating at least as high as the original.

Current

I_DSS is the Drain to Source leakage current, often provided for small-signal FETs.  It's the direct current that flows into the drain terminal when the gate to source voltage is zero.

In a depletion type device, I_DSS is an on-state current.  In an enhancement type device, it's an off-state current.  Minimum and maximum values are usually given.  Select a substitute with the same general range of values.

I_D(cont), the Continuous Drain Current, is usually provided for power MOSFETs.  It's a maximum current rating so choose a substitute with a rating at least as high as the original.

Maximum Power

P_D is the overall power the FET can dissipate through heat.  This is the same spec used for bipolar transistors.  Choose a substitute with a rating at least as high as the original.

"On" Resistance

r_DS(on) is the DC resistance between the Drain and Source terminals when a specified gate to source voltage is applied to bias the FET to the on-state.

For a depletion-type FET, the gate to source bias voltage might be 0 V (i.e., a gate to source short).

r_DS(on) could be important when replacing a power MOSFET.

Finding a substitute replacement transistor isn't difficult if you know the specs of the original transistor and organize your stock by specs instead of type numbers.  You'll find that fewer devices need to be stocked, and turnaround time can be reduced.

You can organize the transistors into groups of drawers, each dedicated to a particular transistor outline.  Subdivide the TO groups by electrical ratings like V_CEO, I_C, and P_D, in whatever order you choose.

Then, when you need to pick a substitute for an original transistor, you can quickly home in on all your potential candidates.