What is a Digital Transistor?

A digital transistor integrates resistors to efficiently control switching.
Unlike conventional transistors that require external resistors, digital transistors integrate the resistors within the package, simplifying circuit design, reducing component count, and saving valuable board space.
They are commonly used in digital signal processing and control circuits, playing a crucial role in everyday electronic devices such as smartphones and home appliances.

Difference between I_O and I_C

I_C: The maximum theoretical current that can flow through the constituent transistor

I_O: The maximum current that can be used by a digital transistor

Explanation

In the case of the DTA/C series, the transistors that make up the digital transistor can handle currents up to 100mA.
This is defined as I_C=100mA. Adding resistors R₁ and R₂ to this configured transistor makes it a digital transistor.
When attempting to flow I_C=100mA through this digital transistor, the base current IB must be appropriately matched, which requires a higher input voltage Vin.
However, the maximum input voltage Vin(max) is defined by the power tolerance of the input resistor R₁ (package power) according to the absolute maximum rating. Therefore, if I_C=100mA is used, the current will exceed the rating, so the maximum current value that can flow through the digital transistor without exceeding Vin(max) is defined as I_O.
As you may know, the absolute maximum ratings are defined as ‘two or more parameters that cannot be supplied simultaneously’, so there is no issue with indicating only I_C, but to align with actual usage conditions, we deliberately include I_O as well.
Based on the above, for circuit design considerations, I_O becomes the absolute maximum rating.

Difference between G_I and h_FE

h_FE: DC current gain of the configured transistor

G_I: DC current gain of the digital transistor

Explanation

Both G_I and h_FE represent the emitter-grounded DC current gain.
A digital transistor is a standard transistor with on or two resistors connected to it.
Here, the DC current gain is the output current divided by the input current, so the amplification factor is not affected by the input resistor R₁. Therefore, models with only an input resistor R₁ expresses the gain using h_FE, which will be equal to the h_FE of the configured transistor.
However, when resistor R₂ is added between E and B, the input current is divided between the current flowing through the configured transistor and current flowing through the E-B resistor R₂.
As a result, the amplification factor decreases compared to when the transistor is used alone. This value is referred to as G_I to distinguish it.

Variation in resistance due to temperature changes

V_BE, h_FE, R₁, and R₂ change with ambient temperature.
The temperature coefficient of h_FE is approx. 0.5%/°C
The temperature coefficient of V_BE is approx. -2mV/°C (with a variation range of -1.8mV/°C to -2.4mV/°C)

The temperature coefficient of R₁ is shown in the following graph.

Regarding the low current region of the output voltage-output current characteristics (in the case of digital transistors)

The output voltage-output current characteristics of digital transistors are measured using the following method.
In the I_O (low current region) no current flows through the Base of the configured transistor, causing the output voltage VO [V_CE(sat)] to rise.

Measurement method: For DTC114EKA, it is measured at I_O/Ii=20/1.
From Ii=I_B+I_R2, (I_R2=V_BE/10k=0.65V/10k=65μA)
I_B=Ii-I_R2=Ii-65μA, in other words, when II becomes less than or equal to 65μA, no current flows through IB and V_O [V_CE(sat)] increases.
As a result, V_O cannot be measured in the low current region.

Switching operation of digital transistors

① Transistor Operation

To operate an NPN transistor, a voltage is applied as shown in Fig. 1.
In this circuit, a forward voltage is applied between the Base (B) and Emitter (E) to inject base current.
In other words, positive holes are injected into the Base (B) region.
When positive holes are injected into the Base (B) region, the free electrons from the Emitter (E) are attracted to the Base (B), but since the Base (B) region is very thin and a Collector voltage is applied, the free electrons pass through the Base (B) region and flow towards the Collector (C). As a result, current flows from the Collector (C) to the Emitter (E).

② Switching Operation

Transistor operation includes both amplification and switching functions.
For amplification, by flowing a Base current IB, the Collector current I_C, which is amplified by a factor of h_FE, flows.
In the active region, the output current is obtained by continuously controlling the Collector current with the input signal.
For switching, the transistor is used in an electrically saturated state when ON (reducing the Collector-Emitter saturation voltage).
The saturation region refers to a state where positive holes are excessively injected. When transitioning from this state to the cutoff region (turning OFF the input pulse), the excess positive holes escape from the Base region to the Base terminal. The Collector current continues to flow until the holes in the Base region are depleted. This time is called tstg (OFF time).
The faster the positive holes in the Base region are removed, the shorter the OFF time (tstg). In the case of a digital transistor, during the OFF state, the escape path for the positive holes in the Base region is through the parallel connection of R₁ and R₂. Here, if R₁ is constant, selecting a smaller R₂ can shorten the OFF time.