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What is a transistor?

Understanding transistors


Reverse current when ON

Reverse current when ON In an NPN transistor, the Base is at positive bias, the Collector at negative bias, and reverse current flows from the Emitter to the Collector. Also, please consider problems arising from usage.

1.It as been determined that no problems, such as degradation or destruction, will arise from use.

2.In the case of an NPN transistor B is symmetrical with C, and E with N. Therefore, C and E can be used as a transistor, even when connected in reverse. In this case current will flow from E to C.

Normal Operation Reverse Operation

3.The following are characteristics of transistors connected in reverse.

  • Low hFE (approx. 10% of the value of the forward direction)
  • Low voltage resistance (around 7-8V, about the same as VEBO) The voltage may even be lower (below 5V) in some standard transistors (Please consider that excessively low voltage resistance may result in breakdown and degradation of characteristics)
  • VCE(sat) and VBE(ON) should not change much
Package power permissible loss

Definition: When voltage is supplied to a transistor the element will begin to generate heat due to power loss based on current flow, particularly when the junction temperature Tj reaches the absolute maximum value (150ºC).

Calculation Method (Where △Tx is the amount of temperature rise when power Px is supplied)

Calculation Method

In this case Pc, Ta, △Tx, and Px can be directly derived from measurement results. Tj is the only value that cannot be directly derived. Therefore, the following shows how to measure VBE, from which we can determine the junction temperature Tj.

In silicon transistors VBE will vary based on the temperature.

Diagram 1: Resistance Temperature Measurement Circuit

Therefore, the junction temperature can be inferred by measuring VBE.
From the measurement circuit at left (Diagram 1) the package power Pc(max) is applied to the transistor.
(In the case of a 1W transistor, the conditions for supply are VCB=10VIE=100mA)

As seen in Diagram 2:

  • VBE1 is measured as the initial value of VBE
  • When power is suppled to the transistor heat saturation will occur at the junction
  • The value of VBE after will be VBE2

From these results: △VBE=VBE2-VBE1

Here a silicon transistor will have a fixed temperature coefficient based on temperature, which is around -2.2mV/ºC. (Darlington transistors: -4.4mV/ºC). Therefore, △VBE from the supplied power can be derived from the rise in junction temperature using the following formula.

Diagram 2: Timing Chart
fT: Gain Bandwidth, Cutoff Frequency

fT: Gain bandwidth indicates the maximum operating frequency of the transistor.
At this time the collector current ratio with the base current is limited to 1 (hFE=1).

Frequency Characteristics

When the base input frequency (operating frequency) is increased the hFE is reduced. At this time when hFE becomes 1, the operating frequency fT is referred to as the gain bandwidth. fT signifies the operating frequency limit. However, in actuality for operation the value will be about 1/5th to 1/10th that of fT.

f: Depends on the measurement equipment. Reference frequency for measurement.
VCE: optional setting - standard value is normally used for ROHM products.
Ic: optional setting - standard value is normally used for ROHM products.


When supplying a flyback voltage to the collector C a voltage greater than Vz is supplied between C and B,
resulting in current flow through the Zener diode and Transistor ON. - ①

This flyback energy is absorbed by DTDS14GP. - ②

At this time a current Iz equivalent to 1/hFE will flow through the Zener diode. In other words, for an Ic (flyback) of 1A and when using the DTDS14GP, which has an hFE of 1000: Iz=1A/1000=1mA.